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1
Goodbody, Susan Joan. "Heavy fermions and magnetic order." Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47451.
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Ruiz, Femenía Pedro David. "Effective field theories for heavy and light fermions." Doctoral thesis, Universitat de València, 2003. http://hdl.handle.net/10803/9881.
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In this thesis, various aspects of different effective field theory (EFT) approximations to the Standard Model of elementary particle physics have been elaborated. The EFT's treated range from non-relativistic approximations to quantum electrodynamics (QED) and quantum chromodynamics (QCD), which can be applied if the quanta under consideration moves at velocities small compared with the speed of light, to chiral perturbation theory as well as the effective theory which emerges in the limit of a large number of colour degrees of freedom in QCD. The latter two constitute expansions in the energy of the process under investigation, and are applicable if this energy is small compared to a typical hadronic scale like, for example, the mass of the -meson. The first type of EFT's, namely NRQED and NRQCD, has been applied in chapter 2 and 3 to study the production cross section of tau lepton pairs as well as quark-antiquark pairs in electron-positron annihilation close to the production threshold, where the system is non-relativistic. In chapter 2 previous computations on tau threshold production are extended to the next-to-next-to-leading order in the velocity expansion and the full analytic form of the two-photon box amplitude, which had not been known explicitly before, is given. As an aside, in chapter 3 we discuss contributions to the total cross section of production of a heavy quark-antiquark pair which arise from intermediate states composed of light QCD degrees of freedom like light mesons or gluonia. These novel contributions constitute an advancement in the field of QCD sum rules for heavy quarks, since they point to the crucial necessity to exactly specify what is actually measured when one speaks about heavy quark production out of e+e- collisions. The second part of the thesis, chapters 4 and 5, tackles with several issues of low-energyhadronic physics. The effective approximation of QCD, chiral perturbation theory, as initially pioneered by Gasser and Leutwyler is only applicable up to energies roughly equal to the -meson mass. However, in recent years it proofed possible to extend this energy range by explicitly including higher resonance states into the effective Lagrangian. This so-called resonance chiral effective theory has been extended in chapter 3 to include a minimal odd-intrinsic parity sector constructed in such a way that it is fully compatible with short-distance QCD constraints. This is an important result for incorporating resonance fields in a chiral framework, and corrects a claim to the contrary put forward recently by other authors. As a by-product, predictions for odd-intrinsic-parity radiative decays of vector mesons in very good agreement with experiment are obtained. In the last chapter of this thesis, we move to the more crowded 1-2 Gev region in electron-positron annihilation where many more resonances need to be included in the field theoretic description. We outline the general strategy to follow in order to obtain a QCD-based parameterization of the total hadronic cross section at these energies, mainly focusing in the technical part of the analysis. A closed formula for the QCD vector-vector current correlator is proposed including contributions from resonance chiral theory at 1-loop with the odd-intrinsic-parity sector introduced before and resummation techniques. Although the practical implementation of the results shall require further investigations, this study shows that the use of EFT's of QCD in the intermediate energy region, populated by resonances, provides a powerful tool to endow the basic information of the underlying theory into the hadron phenomenology in an essentially model-independent way.Bajo el título de "Effective field theories for heavy and light fermions" se han englobado diversos temas relacionados con la aplicación de las teorías efectivas derivadas de la Electrodinámica y la Cromodinámica Cuántica a la producción de leptones y quarks a partir de la aniquilación de un par electrón-positrón. En el primer bloque, que se ocupa de la creación de sabores pesados, se estudia el cálculo de la sección eficaz de producción de leptones tau en la región umbral, y se hace una revisión profunda de la técnica de las reglas de suma para quarks pesados. La producción electromagnética de hadrones formados por quark ligeros se aborda en la segunda parte de esta tesis. En primer lugar se presenta el procedimiento para inferir relaciones entre las constantes del Lagrangiano efectivo con resonancias y QCD para el sector de paridad-intrínseca negativa, y a continuación se deriva una expresión teórica para el correlador de dos corrientes vectoriales en la región de las resonancias, cuya parte imaginaria se relaciona con la sección eficaz hadrónica.
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Wilson, Neil James William. "Heavy fermions : systems for exploring exotic quantum order." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627430.
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Khamseh, Ava. "Lattice phenomenology of heavy quarks using dynamical fermions." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28855.
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The Standard Model of particle physics is believed to be only the low energy limit of a more fundamental theory. In order to determine its range of validity, a major part of theoretical and experimental efforts in physics is dedicated to precision tests of the Standard Model. Lattice QCD is a non-perturbative, first-principles approach to Quantum Field Theory. It plays an important role in flavor physics by providing calculations of non-perturbative strong interaction contributions to weak processes involving quarks. Measurements of hadronic quantities can be used to constrain the Standard Model as well as theories Beyond the Standard Model. The first part of this thesis contains theoretical developments regarding non-perturbative renormalization. A new renormalization scheme, RI/mSMOM, for fermion bilinear operators in QCD at non-vanishing quark mass is presented. In order to investigate the properties of the mSMOM scheme, an explicit one-loop computation in perturbation theory using dimensional regularization is performed. Numerically, vertex functions are generated on the lattice, with an appropriate projector, based on the RI/SMOM scheme and the renormalization factors are extracted. Quantities measured include renormalization of the axial current ZA, required to renormalize the axial current entering the computation of the decay constant and the renormalization of the bag parameter. The second part of this report focuses on flavor physics phenomenology on the lattice. It presents results of the first run of the RBC/UKQCD charm project with (2+1)-flavor Domain Wall fermions. Observables and matrix elements are measured on lattices with Iwasaki gauge action. There are two ensembles at the physical point with inverse lattice spacings 1.73 and 2.36 GeV and a third finer ensemble at 2.76 GeV as well as four other auxiliary ensembles with smaller volumes and heavier pion masses which are used to perform the continuum extrapolations. The quantities measured in the region of the charm quark mass are meson masses, decay constants, the matrix element of the OV V +AA operator, the neutral D-meson mixing parameter B and the SU(3) breaking ratio ξ.
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Brammall, M. I. "Stoner criteria in transition metal oxides and heavy fermions." Thesis, University of Birmingham, 2011. http://etheses.bham.ac.uk//id/eprint/1648/.
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This thesis is an examination of the uses of mean-field theory in problems of the theory of strongly-correlated electronic systems, particularly to the problem of orbital ordering in transition metal oxides. We will apply mean-field theory to various models for orbital ordering of transition metal oxides, and also show that mean-field theory is not as bad an approximation as it might initially seem. We are also interested modelling superconductivity in heavy fermion systems. We conclude from our modelling on transition metal oxides that the mean-field theory we use based on the Stoner criterion will not be adequate to model such complicated phenomena. We propose an alternate mean-field theory based on non-linear fermionic transformations which we introduce. We suggest further improvements in the form of a non-orthogonal transformation, which we also introduce. As a diversion, we model frustrated antiferromagnetism on a pyrochlore lattice. The particular material is Gd\(_2\)Ti\(_2\)O\(_7\). We show that there are many effects in competition with each other. We conclude with a proposed magnetic structure which appears to be a better fit to experimental data than previous suggestions.
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Amaricci, Adriano. "Mottness scenario for non-Fermi liquid phase in heavy fermions." Paris 11, 2009. http://www.theses.fr/2009PA112037.
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La thèse porte sur la construction d'un scénario pour la phase non-liquide de Fermi (NLF) observé dans certains matériaux à forte corrélation électronique, généralement appelés "fermions lourds". Un des modèles le plus utilisé pour étudier ces matériaux c'est le réseau d'Anderson. Il décrit des électrons localisés pour effet de la forte corrélation et hybridant avec des électrons délocalisés dans tout le matériau. On c'est concentrés sur l'étude des propriétés du métal corrélé obtenu en dopant par trous l'isolant de Mott. Cette phase métallique est associée à des excitations électroniques incohérentes. Cet état NLF à origine des forts fluctuations quantiques des moments magnétique locaux, qui, interagissant avec les électrons de conduction, préviens la formation des porteurs à durée de vie moyenne infinie. Nous avons donc montré que les fluctuations locales (quantiques) fournissent une source suffisante de diffusion pour détruire le liquide de Fermi, au lieu de l'idée générale supposant que les fluctuations magnétiques doivent être à longue-portée.
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Michal, Vincent P. "Zeeman effects in heavy electron superconductors." Thesis, Grenoble, 2012. http://www.theses.fr/2012GRENY043/document.
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Comprendre les propriétés des composés à électrons fortement corrélés nouvellement découverts est un important défi à la fois pour des raisons fondamentales et un impact industriel à long terme. Une activité expérimentale sur les métaux et supraconducteurs à électrons lourds a mis en évidence des effets qui se démarquent clairement de notre compréhension actuelle. Le but de cette thèse est de modéliser les effets de spin spéciaux qui ont été observés en réponse à un champ magnétique dans le supraconducteur CeCoIn(5). Elle est composée de deux parties. Dans un premier temps nous avons à faire à la distribution anormale du champ magnétique local dans le réseau de vortex révélé par les expériences de diffraction de neutrons à petits angles et rotation de spin muonique. Sur la base de a théorie de Ginzburg-Landau avec prise en compte de l'effet de spin, nous analysons l'inhomogénéité du champ local dans le réseau de vortex et calculons des expressions pour les facteurs de forme en diffraction neutronique et la largeur de raie statique en rotation de spin muonique. Nous montrons que les données expérimentales anormales sont le résultat de supercourants générés par le spin circulant autour du cœur du vortex et donnent une augmentation de l'inhomogénéité du champ sur une distance de l'ordre de la longueur de corrélation du supraconducteur à partir de l'axe du vortex. L'importance de l'effet est contrôlée par une seule quantité (le paramètre de Maki) qui permet la détermination de propriétés physiques du système à partir de données expérimentales. La seconde partie traite d'une transition d'onde de densité de spin presque commensurable dans un supraconducteur non-conventionnel. Elle est motivée par l'observation du confinement d'un ordre d'onde de densité de spin dans la phase supraconductrice de CeCoIn(5) dans un champ magnétique. Dans le cadre de la formulation spin-fermion nous proposons un mécanisme pour la transition de l'état fondamental qui consiste du ralentissement du mode collectif de fluctuation de densité de spin induit par le champ (exciton de spin) vers un ordre statique. Cela représente un scénario par lequel la transition vers l'ordre de spin est reliée intrinsèquement au supraconducteurUnderstanding the properties of newly discovered strongly correlated electron compounds is a considerable challenge for both fundamental matters and long-term industrial impact. Experimental activity on heavy electron metals and superconductors has lead to highlighting effects that depart from current knowledge. The thesis is aimed at modelling effects that have been observed in response to magnetic field in the heavy electron superconductor CeCoIn$_5$. This consists of two parts. In the first time we deal with the vortex lattice state anomalous local magnetic field space variations as highlighted by small angle neutron scattering and muon spin rotation experiment. On the basis of the Ginzburg-Landau theory with account of spin effect, we analyse the local field inhomogeneity in the vortex lattice and derive expressions for the neutron scattering form factors and muon spin rotation static linewidth. The anomalous experimental data are shown to be result of spin driven supercurrents which circulate around the vortex cores and lead to an increase with external field in the internal field inhomogeneity on a distance of the order of the superconducting coherence length from the vortex axis. The importance of the effect is controlled by a single quantity (the Maki parameter). The second part is on nearly commensurate spin density wave transition in a quasi two-dimensional superconductor. It is motivated by observation of the confinement of spin density wave ordering inside the superconducting state of CeCoIn$_5$ in magnetic field. In the frame of the spin-fermion formulation we propose a mechanism for the ground state transition consisting in the field-induced slowing down of a collective spin density fluctuation mode (spin-exciton) to static ordering. This represents a scenario by which the transition to spin ordering is intrinsically related to superconductivity
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AMARICCI, ADRIANO. "Mottness scenario for the non-Fermi liquid phase in heavy fermions." Doctoral thesis, Università degli Studi di Roma "Tor Vergata", 2009. http://hdl.handle.net/2108/879.
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In this thesis we describe a new theoretical approach to the problem of non-Fermi liquid
behavior in heavy fermion systems.
For almost forty years our comprehension of the metallic behavior in many materials has
been founded on the Landau physical intuition about interacting Fermi systems [6, 69]. The
idea of Landau, that interacting fermions could be regarded as free particles with renormalized
parameters, is at the basis of the standard picture of the solids in terms of single independent
electrons delocalized throughout the systems. The extraordinary success of this scenario is
demonstrated by the impressive number of predictions and results, on which has root a large
part of the actual technology.
The Fermi-liquid concept has been also extended to systems showing a strong electronelectron
interaction, i.e. strongly correlated electrons systems. Under suitable conditions the
low temperature metallic properties of these systems can be interpreted in terms of renormalized
quasiparticles.
Nevertheless, recent experiments on some strongly correlated materials have shown remarkable
deviations from the Fermi liquid predictions concerning different physical observables,
such as specific heat C, resistivity ρ or susceptibility χ [94].
The theoretical understanding of the breakdown of the Fermi liquid paradigm observed
heavy fermion systems or in high Tc superconductors is one of the open challenges in the correlated
electrons physics. In this thesiswe shall showthat a new approach to the heavy fermions physics can be based
on the DMFT solution of one of the canonical model of this area, namely the periodic Anderson
model. In particular we demonstrate that, contrary to conventional expectations, a non-Fermi
liquid state is readily obtained from this model within the DMFT framework. In agreement
with the quantum criticality scenario, this novel NFL state is located in the neighborhood of
a quantum phase transition, but unlike the standard quantum criticality scenario sketched before,
the relevant quantum transition here is a Mott transition. Thus, the present study sheds a
different light onto the NFL problem, showing that the coupling to long wavelength magnetic
fluctuations (absent in DMFT) is not a prerequisite for the realization of a NFL scenario. Local
temporalmagnetic fluctuations alone can provide sufficient scattering to produce an incoherent metallic state. The presence of such large local magnetic fluctuations in ourmodel has origin in
the competition between magnetic interactions, namely the super-exchange antiferromagnetic
interaction between the correlated electrons and the ferromagnetic interaction indirectly driven
by the delocalization of the doped charges.
Thus, we are able to obtain a DMFT description of the non-Fermi liquid phase in heavy
fermion systems which is based on the proximity to a Mott point, i.e. Mottness scenario. Our
study shows that the PAM, solved within DMFT, may be considered as a “bare bones” or minimal
approach able to capture the physical scenario for the formation of a NFL state and that is
in qualitative agreement with some observed phenomenology in heavy fermion systems. The entire thesis is structured as follows.
• In the first chapter we review briefly the Landau theory of Fermi liquid and we give
an overview on the heavy fermion phenomenology under both the theoretical and the
experimental point of view. In particular, we present briefly some experimental evidences
for the non-Fermi liquid phase in heavy fermions materials. Finally we discuss the main
theoretical ideas proposed to explain the breakdown of the Fermi liquid paradigm.
• In the second chapterwe introduce the theoretical tool used throughout this thesis, namely
the DMFT. We derive the basic equations of the theory, step by step comparing with the
well known classical mean-field theory (reviewed in the first part of the same chapter).
An overview of the main techniques used to solve the DMFT equations is presented at
the end of the chapter.
• In the third chapter we introduce the periodic Anderson model. After having studied
the solvable limits of the model we proceed by deriving the related DMFT equations
and briefly discuss the algorithm used to obtain their solution. Next, we illustrate the
derivation of the internal energy expression for the PAMin the framework of DMFT. This
will be used later on to obtain the thermodynamical properties of the model. Finally, we
extend the DMFT equations to deal with long range magnetically ordered phases.
• In the fourth chapter we discuss the various parameter regimes of the model respect to
the so-called ZSA diagram. We will demonstrate numerically the existence of a Mott insulating
state for large values of the correlation, in two physically relevant regimes. Then
we will study the Mott metal-insulator transitions driven by doping. In particular we
show the existence of two different transition scenarios, each of which will be discussed.
One of the two transitions will constitute the framework for the derivation of the Mottness
scenario, that is the subject of the next chapter.
• In the fifth chapter we shall demonstrate the existence of a non-Fermi liquid state in the neighborhood of a Mott point. Then,we showthe existence of a crossover regime towards
a Fermi liquid phase for temperatures exponentially small in the doping. This coherence
temperature scale is estimated and discussed in terms of the exhaustion problem [68, 67].
Next, we study the magnetic properties of the non-Fermi liquid regime. The main finding
being the stability of this phase respect to the magnetic order. We present a study of
the thermodynamical properties of the system, such as entropy of specific heat, finding
for this latter remarkable evidences of logarithmic divergences, in agreement with experimental
observations [94]. Finally we shall summarize the results in two phase diagrams
that are in qualitatively accord with the observed phenomenology of heavy fermions systems.
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McDonough, Johnathan. "F electrons in CePbâ†3." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338253.
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Hathurusinghe, Dewage Prabuddha Madusanka. "The Application of Two Fluid Model to IR Spectra of Heavy Fermions." University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1541588399996827.
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Pokharel, Mani Raj. "Thermoelectric transport properties of nanostructured FeSb2 and Ce-based heavy-fermions CeCu6 and CeAl3." Thesis, Boston College, 2015. http://hdl.handle.net/2345/bc-ir:104129.
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Thesis advisor: Cyril P. OpeilThermoelectric (TE) energy conversion is an all-solid-state technology which can convert waste thermal energy into useful electric power and cool ambience without using harmful gases like CFC. Due to their several advantages over traditional energy conversion technologies, thermoelectric generators (TEG) and coolers (TEC) have drawn enormous research efforts. The objective of this work is to find promising materials for thermoelectric cooling applications and optimize their thermoelectric performances. Finding a material with a good value for the thermoelectric figure-of-merit (ZT) at cryogenic temperatures, specifically below 77 K, has been of great interest. This work demonstrates that FeSb2, CeCu6 and CeAl3, all belonging to a class of materials with strongly correlated electron behavior; exhibit promising thermoelectric properties below 77 K. In general, ZT of a TE material can be increased using two basic approaches: lattice thermal conductivity reduction and power factor (PF) enhancement. The results of this study indicate that nanostructuring effectively decreases the thermal conductivity of FeSb2, CeCu6 and CeAl3 leading to improved ZT. The approach of introducing point-defect scattering to further reduce the thermal conductivity is successfully implemented for Te-substituted FeSb2 nanostructured samples. A semiconductor/metal interface has long been proposed to exhibit enhanced thermoelectric properties. We use this technique by introducing Ag-nanoparticles in the host FeSb2 which further increases ZT by 70%. Additionally, a detailed investigation is made on the phonon-drag effect as a possible mechanism responsible for the large value of the Seebeck coefficient of FeSb2. We show that the phonon-drag mechanism contributes significantly to the large Seebeck effect in FeSb2 and hence this effect cannot be minor as was proposed in literatures previously. A model based on Kapitza-resistance and effective medium approach (EMA) is used to analyze the thermal conductivities of nanostructured FeSb2 samples. We find a notably large value for Kapitza length at low temperatures indicating the dominance of inter-grain thermal resistance over bulk thermal resistance in determining the thermal properties of FeSb2
Thesis (PhD) — Boston College, 2015
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
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Naritsuka, Masahiro. "Controlling unconventional superconductivity in artificially engineered heavy-fermion superlattices." Kyoto University, 2020. http://hdl.handle.net/2433/253073.
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Singh, Yogesh Pratap. "Quantum Criticality and Unconventional Properties of Heavy Fermion Superconductor Ce1-xYbxCoIn5." Kent State University / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=kent1437492491.
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Bach, Alexandra P. R. "Instrumentation and thermometry for the study of heavy fermion compounds." Thesis, University of Bristol, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.340154.
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Brunton, Rosalind Elizabeth. "Strong correlation effects in heavy fermion and double exchange systems." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.300345.
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Adhikari, Ram Bahadur. "Superconductivity and Magnetism in Selected Filled Skutterudites and Heavy Fermion Systems." Kent State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=kent16173874994732.
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Hüsges, Anna Zita. "Probing magnetic fluctuations close to quantum critical points by neutron scattering." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-204647.
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Second-order phase transitions involve critical fluctuations just below and above the transition temperature. Macroscopically, they manifest in the power-law behaviour of many physical properties such as the susceptibility and the specific heat. The power-laws are predicted to be universal, i.e. the same exponents are expected for a certain class of transitions irrespective of the microscopic details of the system. The underlying commonality of such transitions is the divergence of the correlation length ξ and the correlation time ξ_τ of the critical fluctuations at the transition temperature. Both ξ and ξ_τ can be directly observed by neutron scattering experiments, making them an ideal tool for the study of critical phenomena.
At classical phase transitions, the critical fluctuations will be thermal in nature. However, if a second-order transition occurs at T = 0, thermal fluctuations are frozen, and the transition is driven by quantum fluctuations instead. This is called a quantum critical point. The quantum nature of the fluctuations influences observable properties, also at finite temperatures, and causes unusual behaviour in the vicinity of the quantum critical point or the existence of exotic phases, e.g. unconventional superconductivity. Heavy-fermion compounds are a class of materials that is well suited for the study of quantum criticality. They frequently show second-order transitions into a magnetically ordered state at very low temperatures, which can easily be tuned to T = 0 by the application of pressure, magnetic fields or element substitution.
In this thesis, fluctuations near a quantum critical point are investigated for three heavy-fermion systems. CeCu2Si2 shows unconventional superconductivity close to an antiferromagnetic quantum critical point. Results from single-crystal neutron spectroscopy and thermodynamic measurements are discussed and some details are also given about the synthesis of large single crystals. The focus of the study is the comparison of the inelastic response of magnetic and superconducting samples, which are found to be very similar for ΔE > 0.2 meV.
CePdAl has an antiferromagnetic state with partial magnetic frustration. The ordering temperature can be suppressed by Ni substitution towards a quantum critical point. Single-crystal neutron diffraction experiments of three members of the substitution series were analysed. They revealed several unusual effects of the frustrated state in the pure sample, and show that magnetic order and frustration persist in the substituted samples.
YbNi4P2 is a rare example of a compound with ferromagnetic quantum criticality, which has only been studied in the last few years. The aim of the powder neutron spectroscopy experiments presented here was to obtain an overview of the relevant energy scales, i.e. the crystal electric field, local magnetic fluctuations and ferromagnetic fluctuations. Simulations using the program McPhase were performed for a thorough understanding of the crystal electric field.
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Fernandez-Panyella, Amalia. "Systèmes à fermions lourds à base d'Ytterbium : une instrumentation novatrice avec un enjeux fondamental." Thesis, Grenoble, 2012. http://www.theses.fr/2012GRENY050/document.
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Cette thèse a pour objet l'étude de 3 systèmes de fermions lourds à base d'Ytterbium, YbCu2Si2, YbCo2Zn20 et YbRh2Si2, dans des conditions extrêmes de hautes pressions, basses températures et hauts champs magnétiques. Une partie très importante dans ce travail a été le développement d'un dispositif de génération de force à très basses températures permettant une modulation de la pression in-situ adapté à des cellules à enclumes diamant. Pour YbCu2Si2, les mesures de ac-susceptibilité sous pression et sous champ magnétiques ainsi que les mesures d'aimantation sous pression montrent que l'ordre magnétique qui apparaît pour P> 8 GPa est ferromagnétique. L'étude du changement de la valence de l'ion Yb par des mesures de diffusion inélastique en conditions résonantes de rayons X ont permis de clarifier son interaction avec le magnétisme. Même si l'état trivalent est clairement favorisé sous pression, la valence reste inférieure à 3, proche de l'ordre magnétique, même dans les plus hautes pressions et plus basses températures. Nous avons également procédé à une recherche détaillée pour les signatures de metamagnétisme avec des mesures à très haut champ. Dans la deuxième partie de ma thèse, les mesures de ac-calorimétrie et ac-susceptibilité sous pression ont permit d'établir le diagramme de phase (P-T) dans YbCo2Zn20 jusqu'à 14 GPa. En plus, nos résultats suggèrent que l'ordre magnétique qui apparaît pour P>1 GPa est antiferromagnétique. Les forts effets du champ magnétique observées dans les propriétés physiques dans YbCo2Zn20 sont probablement liés à l'interaction entre le champ magnétique et les spins des électrons 4f et non à leur charge car la valence de l'Yb s'avère insensible à l'application d'un champ magnétique de 10 T. La partie finale de cette thèse est consacré à l'étude du diagramme de phase (H-T) sous pression dans YbRh2Si2 pour déterminer l'évolution de la température de l'ordre magnétique en fonctionne de H et P et mieux comprendre les effets liés à ce deux paramètres de contrôleIn this thesis, three Yb-based heavy fermion compounds, YbCu2Si2, YbCo2Zn20 and YbRh2Si2, are studied under extreme conditions, i.e, high pressure, low temperatures and high magnetic fields. An important part of the work has been the set up of an in-situ tuning pressure device to measure diamond anvil cells at dilution fridge temperatures. This has enable most of the experimental results presented here. In YbCu2Si2 the nature of the pressure-induced magnetic order that arises for P> 8 GPa has been clarified to be ferromagnetic by ac-susceptibility and magnetization measurements under pressure. The interplay of magnetism and valence change has been investigated by measuring the valence of the title compound at high pressures and low temperatures using resonant inelastic x-ray scattering (RIXS). As expected, pressure favors the trivalent state but the Yb ion valence remains below 3 even at the highest pressure and at low temperatures very close to the onset of the magnetic order. We have also performed a detailed search for signatures of metamagnetism. The second part of my thesis focuses on YbCo2Zn20. We have extended the (P-T) phase diagram up to 14 GPa by ac-calorimetry and ac-susceptibility measurements. Our results shed some light on the nature of the magnetic order that arises for P>1 GPa which is antiferromangetic. The strong field effects observed in the physical properties in YbCo2Zn20 are probably related to the interaction between the magnetic field and the spin of the 4f electrons rather than to their charge as the valence of the Yb ion is insensitive to the application of a magnetic field of 10 T. The final part of this thesis is devoted to the detailed study of the (H-T) phase diagram under pressure in YbRh2Si2 to determine the evolution of the magnetic ordering temperatures as a function of H and P to better understand the interplay of the two control parameters
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Combier, Tristan. "Criticalité quantique ferromagnétique dans les composés ternaires à base d'uranium URhSi, URhAl et UCoAl." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY062/document.
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Dans cette thèse, on étudie la criticalité quantique ferromagnétique dans trois composés ternaires à base d'uranium, par des mesures thermodynamiques et de transport sur des échantillons monocristallins, à basse température et sous haute pression. URhSi et URhAl sont des systèmes ferromagnétiques itinérants, tandis que UCoAl est un système paramagnétique étant proche d'une instabilité ferromagnétique. Tous ont une phase ordonnée de type Ising. Dans le composé orthorhombique URhSi, on montre que la température de Curie diminue lorsqu'un champ magnétique est appliqué perpendiculairement à l'axe facile d'aimantation, et une transition de phase quantique est attendue autour de 40~T. Dans le système hexagonal URhAl, on établit le diagramme de phase pression--température pour la première fois, lequel indique une transition de phase quantique vers 5~GPa. Dans le composé isostructural UCoAl, on étudie la transition métamagnétique par des mesures d'aimantation, d'effet Hall, de résistivité et de dichroïsme circulaire magnétique des rayons X. On observe des phénomènes de relaxation magnétique intrigants, avec des sortes de marches. L'effet Hall et la résistivité ont été mesurés à des températures de réfrigérateur à dilution, sous pression hydrostatique jusqu'à 2,2~GPa, et sous champ magnétique jusqu'à 16~T. La transition métamagnétique se termine sous pression et champ magnétique au niveau d'un point critique quantique terminal. Dans cette région, il se produit une forte augmentation de la masse effective, et une différence intrigante entre champ montant et descendant apparaît dans la résistivité transverse. Ce pourrait être la signature d'une nouvelle phase, éventuellement reliée aux phénomènes de relaxation observés dans les mesures d'aimantation, et résultant de frustrations au sein du réseau quasi-Kagomé que forment les atomes d'uranium dans cette structure cristallineIn this thesis we explore the ferromagnetic quantum criticality in three uranium-based ternary compounds, by means of thermodynamical and transport measurements on single crystal samples, at low temperature and high pressure. URhSi and URhAl are itinerant ferromagnets, while UCoAl is a paramagnet being close to a ferromagnetic instability. All of them have Ising-type magnetic ordering. In the orthorhombic compound URhSi, we show that the Curie temperature decreases upon applying a magnetic field perpendicular to the easy magnetization axis, and a quantum phase transition is expected around 40~T. In the hexagonal system URhAl, we establish the pressure--temperature phase diagram for the first time, indicating a quantum phase transition around 5~GPa. In the isostructural compound UCoAl, we investigate the metamagnetic transition with measurements of magnetization, Hall effect, resistivity and X-ray magnetic circular dichroism. Some intriguing magnetic relaxation phenomena are observed, with step-like features. Hall effect and resistivity have been measured at dilution temperatures, under hydrostatic pressure up to 2.2 GPa and magnetic field up to 16~T. The metamagnetic transition terminates under pressure and magnetic field at a quantum critical endpoint. In this region, a strong effective mass enhancement occurs, and an intriguing difference between up and down field sweeps appears in transverse resistivity. This may be the signature of a new phase, supposedly linked to the relaxation phenomena observed in magnetic measurements, arising from frustration on the quasi-Kagome lattice of uranium atoms in this crystal structure
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Bastien, Gaël. "Interplay between magnetic quantum criticality, Fermi surface and unconventional superconductivity in UCoGe, URhGe and URu2Si2." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY005/document.
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Cette thèse montre de nouveaux résultats sur les supraconducteurs ferromagnétiques UCoGe et URhGe et sur l’ordre caché dans URu2Si2. Le diagramme de phase pression température d’UCoGe a été étudié jusqu’à 10.5 GPa. L’ordre ferromagnétique subsiste jusqu’à la pression critique pc≈1 GPa et la supraconductivité non conventionnelle jusqu’à p = 4 GPa. Les fluctuations magnétiques responsables de la supraconductivité peuvent être réduites par l’application d’un champ magnétique. Les surfaces de Fermi d’UCoGe et d’URhGe ont été mesurées grace aux oscillations quantiques. Quatre poches ont été détectées dans UCoGe, elles subissent une succession de transition de Lifshitz de la surface de Fermi sous champ magnétique. Les poches détectés évoluent continument avec la pression jusqu’à 2.5 GPa, sans montrer de reconstruction de la surface de Fermi à la pression critique pc. Dans URhGe, trois poches lourdes de la surface de Fermi ont aussi été découvertes. Enfin dans la phase d’ordre caché d’URu2Si2, les oscillations quantiques ont révélé une forte anisotropie du facteur gyromagnétique g pour deux poches de la surface de Fermi, qui est comparable à l’anisotropie macroscopique. Cette dernière a été étudiée à partir du champcritique supérieur de la supraconductivitéThis thesis is concentrated on the ferromagnetic superconductors UCoGe and URhGe andon the hidden order state in URu2Si2. In the first part the pressure temperature phase diagram of UCoGe was studied up to 10.5 GPa. Ferromagnetism vanishes at the critical pressure pc≈1 GPa. Unconventional superconductivity and non Fermi liquid behavior can be observed in a broad pressure range around pc. The superconducting upper critical field properties were explained by the suppression of the magnetic fluctuations under field. In the second part the Fermi surfaces of UCoGe and URhGe were investigated by quantum oscillations. In UCoGe four Fermi surface pockets were observed. Under magnetic field successive Lifshitz transitions of the Fermi surface have been detected. The observed Fermi surface pockets in UCoGe evolve smoothly with pressure up to 2.5 GPa and do not show any Fermi surface reconstruction at the critical pressure pc. In URhGe, three heavy Fermi surface pockets were detected by quantum oscillations. In the last part the quantum oscillation study in the hidden order state of URu2Si2 shows a strong g factor anisotropy for two Fermi surface pockets, which is compared to the macroscopic g factor anisotropy extractedfrom the upper critical field study
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Holanda, Junior Lino Martins de 1984. "Ressonância de spin eletrônico (ESR) em compostos intermetálicos." [s.n.], 2014. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277002.
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Orientador: Pascoal José Giglio PagliusoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Nesta Tese de Doutorado desenvolvemos estudos de Ressonância de Spin Eletrônico (ESR) em compostos intermetálicos, incluindo os férmions pesados ?-YbAlB4, ?-YbAlB4 and ? -YbAlxFe1-xB4, os metais ?-LuAlB4, AlB2 e o isolante Kondo, FeSi. Nossas medidas foram feitas em um espectrômetro modelo ELEXSYS-CW Bruker usando uma cavidade ressonante do tipo TE102 para a faixa de frequência em banda X (?~ 9.4GHz). A técnica de Ressonância de Spin Eletrônico aplicada aos compostos Férmions pesados foi sempre um desafio devido a presença de fortes correlações eletrônicas que tendem a alargar as linhas de ESR. Porem, nos últimos anos essa técnica ganhou renovado interesse após a descoberta do sinal de ESR na rede Kondo YbRh2Si2. Após esse trabalho, ESR voltou a ser considerada uma técnica importante para explorar os elétrons 4f e suas interações microscópicas em compostos de terras raras, uma vez que ela sonda diretamente o íon no sitio da rede. A motivação desse trabalho e, portanto, usar a técnica de ESR para entender a dinâmica dos elétrons 4f pesados em uma rede de Kondo, para os novos compostos férmions pesados ?-YbAlB4, -YbAlB4 and ?-YbAlxFe1-xB4. Neste trabalho buscou-se utilizar a técnica de ESR nesses compostos para alcançar o entendimento microscópio dos critérios que permitem a observação de um sinal de ESR em férmios pesados. Alem do mais, quando o sinal e observado, a sonda de ESR e propícia para acompanhar a evolução dos elétrons 4f em altas temperaturas para quase-partículas pesadas em baixas temperaturas. Nossas observações nos permitiram propor um cenário qualitativo baseado na existência de um modo ressonante acoplado entre ons Kondo e os elétrons de condução. Nos discutimos as características físicas gerais para que compostos férmios pesados sejam candidatos a apresentarem tal sinal de ESR. Ainda neste trabalho, estudamos os compostos intermetálicos ?-LuAlB4, AlB2 e FeSi com propriedades estruturais ou eletrônicas similares as dos compostos ?-YbAlB4, ?-YbAlB4 and ?-YbAlxFe1-xB4 em busca de uma generalizac~ao mais abrangente do cenário proposto neste trabalho
Abstract: In this work, we have performed an Electron Spin Resonance (ESR) study in intermetallic compounds including the heavy fermions ?-YbAlB4, ?-YbAlB4 and ? -YbAlxFe1-xB4, the Fermi liquid metals ?-LuAlB4, AlB2 and the Kondo insulator, FeSi. Our measurements were made on a spectrometer Bruker CW-model ELEXSYS using a resonant cavity TE102 in X Band ( ?~ 9.4GHz). The Electron Spin Resonance technique applied to heavy fermions compounds was little explored due to difficulty in finding ESR signal of Kondo ions due to the large coupling between the resonating spins and conduction electrons. However, in recent years this technique has become focus of great attention especially after the discovery of the ESR signal in a Kondo lattice YbRh2Si2. In this context, ESR was brought to the scene as one of the main techniques to bring insights to this problem since it could probe directly the f electrons of Kondo ions and their interaction with the conduction electrons. The motivation of this work is to use the ESR technique to investigate new heavy fermions compounds and to understand when these systems can present an ESR signal. In these cases ESR can help to understand more deeply how localized f electrons at high-T evolve to itinerant heavy quasi-particles in a low-T metallic state. Our observations for ?-YbAlB4, -YbAlB4 and ?-YbAlxFe1-xB4, allowed us to make some qualitative speculations about this phenomenon and within this scenario, we propose some general characteristics for heavy fermions compounds to become candidates to present ESR signal. In this work we have also measured ?-LuAlB4, AlB2 and the Kondo insulator, FeSi, in an attempt to generalize the ESR results found for ?-YbAlB4, ?-YbAlB4 and ?-YbAlxFe1-xB4 to a more broad family of compounds
Doutorado
Física
Doutor em Ciências
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Palacio, Morales Alexandra. "Thermoélectricité des composés fortement corrélés sous conditions extrêmes." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY042/document.
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Cette thèse porte sous l'étude sous conditions extrêmes (basse température, fort champ magnétique et haute pression) des composés fortement corrélés du type fermions lourds. Trois composés ont été analysés UCoAl, UGe$_2$ et CeRh$_2$Si$_2$, en utilisant principalement de mesures thermoélectriques; une technique récente et très sensible dans le domaine des fermions lourds. À cette fin, de nouvelles dispositives de mesures de pouvoir thermoélectricité sous pression ont été développés au cours de cette thèse.Concernant le composé d'UCoAl, notre étude a permis d'analyser précisément la transition metamagnétique, induite par le champ magnétique, entre la phase paramagnétique (PM) et la phase ferromagnétique (FM) ainsi que, son évolution sous pression. Ainsi, nos mesures ont permis de compléter le diagramme de phase $(T,P,H)$ et notamment, de mettre en évidence la structure magnétique originale qui apparaît sous pression en forme de ```wings" pas des mesures thermoélectriques.Une fine analyse de la surface de Fermi de la phase FM$2$ d'UGe$_2$ a été réalisée grace à l'observation des oscillations quantiques du pouvoir thermoélectrique. Les résultats obtenues ont été comparés aux études conventionnelles des oscillations quantiques comme de ``de Haas-van Alphen" (dHvA) et de ``Suhbnikov-de Hass" (SdH) effets. Une très bonne accord entre les trois techniques a été constatée. % et montre les avantages d'utilisation des mesures du pouvoir thermoélectrique pour analyser les paramètres microscopiques des fermions lourds.% Les inconvénients de cette technique sont aussi présentés.Finalement, dans le système CeRh$_2$Si$_2$, la suppression du domaine antiferromagnétique (AF) sous champ magnétique $H_c sim 26$T et sous pression $Psim 1$GPa a été étudiée. Un très fort changement de la surface de Fermi à $H_c$ correspondant à la transition de l'ordre AF vers une phase paramagnétique polarisée (PPM), a été observé. Sous pression, des fluctuations magnétiques et une reconstruction de la surface de Fermi apparaissent autour de $P_c$. Ces fluctuations cachent la nature de la suppression de l'ordre AF vers un ordre paramagnétique (PM). L'étude du diagramme de phase $(T,H,P)$ révèle que les phases PM et PPM sont différentes, cependant des points en commun demeurentThermopower is a technique whose importance is related to the possibility of directly measuring electronic properties of the systems, as it is sensitive to the derivative of the density of states. In this work, the low temperature regime of strongly correlated electron systems has been studied using this technique. For that, a new pressure-field thermopower device was developed, and used, to determine $(T,P,H)$ phase diagrams of the itinerant ferromagnets UCoAl and UGe$_2$, and of the weak antiferromagnet CeRh$_2$Si$_2$.For example, in the case of UCoAl, this same technique was used to analyze the metamagnetic transition from paramagnetic (PM) to ferromagnetic (FM) phases and to study its evolution towards the quantum critical end point. The existence of exotic magnetic excitations in the ground state and around the critical end point were also evidenced.On the compound CeRh$_2$Si$_2$, the suppression of the antiferromagnetic (AF) order by magnetic fields and pressures was explored. A strong change of the Fermi surface at $H_c$, the field at which the suppression of the AF into the paramagnetic polarized (PPM) phase, was observed. We show that under pressure, the magnetic fluctuations around the critical pressure $P_c$ masked the Fermi surface reconstruction of the AF phase into the PM phase. The analysis of the $(T,P,H)$ phase diagram revealed that the non-ordered phases of this compound (PM and PPM) are different, therefore pressure and field behave as different suppressor mechanisms.In the UGe$_2$ compound, the analysis of its Fermi surface by thermopower quantum oscillations was performed as a last example of the utility and of the importance of this technique. To the best of the author knowledge, this is the first time that this technique was used in heavy fermion systems. A comparison to traditional probes such as de Haas-van Alphen and Shubnikov–de Haas effects was done. We observed a good agreement between them and we explain the advantages and the disadvantages of thermopower quantum oscillations technique over the traditional probes
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Gourgout, Adrien. "Instabilités de surface de Fermi avec et sans transitions magnétiques : étude de URhGe, UPd2AI3, UCoGe et CeIrIn5." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY001/document.
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Dans cette thèse, j'ai étudié l'évolution de la surface de Fermi sous l'influence d'un champ magnétique dans des systèmes massifs facilement polarisables à basse température. La première partie est dévouée aux cas du supraconducteur ferromagnétique UCoGe et du supraconducteur paramagnétique CeIrIn5, où la surface de Fermi peut être modifiée sans transition magnétique. Dans UCoGe, plusieurs anomalies successives ont été détectées dans l'effet Seebeck, la résistivité et l'effet Hall, sans transition nette dans l'aimantation. L'observation d'oscillations quantiques montre que ces anomalies sont reliées à des changements de topologie de la surface de Fermi, aussi appelés transitions de Lifshitz. Dans CeIrIn5, une anomalie est détectée dans l'effet Seebeck à HM = 28 T et les oscillations quantiques observées en magnétométrie torque montrent qu'une transition de Lifshitz à lieu à ce champ.Dans la deuxième partie, j'ai étudié comment varie la surface de Fermi à travers une transition magnétique du premier ordre induite par le champ magnétique dans le supraconducteur ferromagnétique URhGe avec le champ selon l'axe de difficile aimantation b et le supraconducteur antiferromagnétique UPd2Al3 avec le champ dans le plan basal. Dans URhGe, l'effet Seebeck permet d'observer un changement de la surface de Fermi à la transition de réorientation des spins à HR = 11.75 T et avec la résistivité confirme le caractère premier ordre de la transition en plus de fournir la localisation dans le diagramme de phase du point tricritique. Dans UPd2Al3, une nouvelle branche de la surface de Fermi est observée dans les oscillations quantiques de de Haas-van Alphen dans l'état antiferromagnétique et l'effet Seebeck montre que la surface de Fermi change à la transition métamagnétique à HM = 18 T. En outre, quatre nouvelles branches sont observées dans la phase polarisée au delà de HM et qui ne peuvent être associées à celles calculées dans les états paramagnétique et antiferromagnétiqueIn this thesis, we have studied the evolution of the Fermi surface under the influence of a magnetic field in bulk materials that can be easily polarized at low temperature. The first part was devoted to the cases of the ferromagnetic superconductor UCoGe with a magnetic field applied along the easy magnetization c-axis and the paramagnetic superconductor CeIrIn5 with the field along the c-axis. In UCoGe, several successive anomalies were detected in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition being detected in magnetization. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In CeIrIn5, the thermoelectric power detected an anomaly at HM = 28 T and the quantum oscillations observed in torque magnetometry showed that a Lifshitz transition occurs at this field.In the second part of this thesis, we studied the evolution of the Fermi surface through first order magnetic transitions induced by magnetic field. In the ferromagnetic superconductor URhGe with the field applied along the hard magnetization b-axis and the antiferromagnetic superconductor UPd2Al3 with the field in the basal plane. In URhGe, the thermoelectric power allowed to observe a change in the Fermi surface at the spin reorientation transition at HR = 11.75 T defining the ferromagnetic state and along with resistivity confirmed the first order character of the transition as well as give a location of the tricritical point. In UPd2Al3, a new branch was observed in de Haas-van Alphen experiment in the antiferromagnetic phase and the thermoelectric power showed that the Fermi surface is reconstructed at the metamagnetic transition at HM = 18 T where the antiferromagnetic state is suppressed and could suggest that the Fermi surface changes before this transition. Additionally, four new branches were observed in the polarized paramagnetic phase, above HM, that cannot be associated with calculated branches in the paramagnetic of antiferromagnetic states
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Pépin, Catherine. "Phenomenes critiques autour de la transition magnetique dans les fermions lourds." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10135.
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Nous etudions differents aspects de la competition entre magnetisme localise et effet kondo dans les fermions lourds. Dans le cadre du modele du reseau kondo, nous sommes parvenus a traiter les fluctuations de spin sur le meme plan que les effets d'ecrantage kondo, grace a une approche alternative au developpement en 1/n qui met en uvre une transformation de hubbard-stratonovich generalisee. Nous retrouvons, a partir d'un modele microscopique, la nature duale des excitations magnetiques observees par diffusion inelastique de neutrons: la susceptibilite magnetique est la somme d'une contribution quasielastique typique du liquide de fermi et d'une composante inelastique caracteristique des interactions rkky. Poussant le developpement perturbatif a un niveau self-consistent dans les champs de soin et de charge, nous donnons ensuite une description des phenomenes critiques autour du point de transition vers l'etat ordonne. Ce point critique decrit qualitativement le passage d'un liquide de fermi a un regime de magnetisme localise. Deux lignes de cross-over sont determinees, qui distinguent les variations avec la temperature de la susceptibilite statique, de la chaleur specifique et de la longueur de correlation. L'exposant dynamique depend de la constante de couplage j entre impuretes magnetiques et electrons de conduction. Afin de prendre en compte profondement les effets du magnetisme localise, nous introduisons dans la deuxieme partie une nouvelle representation du spin de l'impurete localisee, dans laquelle une fraction du spin est representee par des fermions appropries a la descrption de l'effet kondo, la fraction restante est representee par des bosons adaptes a la description du magnetisme localise. La thermodynamique s'ecrit alors dans une formulation supersymetrique de l'integrale fonctionnelle. Directement appliquee au modele du reseau kondo, cette representation donne de facon asymptotique les proprietes de magnetisme localise. Enfin, nous proposons dans la troisieme partie un calcul numerique de la susceptibilite magnetique sur des structures de bande realistes, qui nous permet de discuter les effets de nesting dans les fermions lourds
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Wu, Beilun. "Unconventional superconductivity in the ferromagnetic superconductor UCoGe." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY010/document.
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Cette thèse discute essentiellement sur le champ critique supérieur du supraconducteur ferromagnétique UCoGe. La conductivité thermique et d'autres méthodes expérimentales ont été utilisées pour confirmer les nombreux comportements particuliers de Hc2 dans UCoGe, précédemment observés dans des études de résistivité. Ces caractéristiques, y compris une anisotropie forte et des courbures anormales, ne peuvent pas être interprétées en termes de théories classiques pour Hc2. Au lieu de cela, un phénomène spécifique aux supraconducteurs ferromagnétiques - la dépendance en champ de l'interaction d'appariement doit être considéré. Nous montrons que cet effet peut être analysé de façon cohérente avec des propriétés de la phase normales et peut être aussi comparé quantitativement avec une théorie existante. Ceci conduit à une clarification nette pour le cas de H//c dans UCoGe, et explique en même temps le comportement différent de Hc2 dans UCoGe et URhGe. Ces résultats soutiennent fortement l'origine magnétique de la supraconductivité dans ces systèmes. Pour H//b, nous montrons que certaines observations expérimentales convergentes suggèrent un possible changement d'état supraconducteur induit par le champ magnétique transversal dans UCoGe. Indépendamment du reste de l'étude, le dernier chapitre présente quelques résultats expérimentaux sur la phase normale de UCoGe et sur l'autre système de fermions lourds UBe13This thesis mainly discuss the upper critical field of the ferromagnetic superconductor UCoGe.Thermal conductivity and other experimental methods have been used to confirm the numerous particularbehaviors of Hc2 in UCoGe, previously observed in resistivity studies. These features, including the stronganisotropy and the anomalous curvatures, cannot be interpreted in terms of classical theories for Hc2.Instead, a phenomenon specific to the ferromagnetic superconductors - the field dependence of the pairinginteraction, needs to be considered. We show that this effect can be consistently analyzed with normalphase properties, and is quantitatively compared with existing theory. This leads to a net clarificationfor the case of H//c in UCoGe, and at the same timeexplains the different behavior of Hc2 in UCoGe and URhGe. These resultsstrongly support the magnetic origin of superconductivity in these systems. For H//b, we showconvergent experimental observations that suggest a possible change of the superconducting state inducedby the transverse magnetic field in UCoGe. Independent from the rest of the study, the last chapter presents someexperimental results on the normal phase of UCoGe and on the other heavy-fermion system UBe13
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Madeira, Lucas 1991. "Many-body systems : heavy rare-gases adsorbed on graphene substrates and ultracold Fermi gases = Sistemas de muitos corpos: gases nobres pesados adsorvidos em substratos de grafeno e gases de Fermi ultrafrios." [s.n.], 2015. http://repositorio.unicamp.br/jspui/handle/REPOSIP/276942.
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Orientador: Silvio Antonio Sachetto VitielloDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Nessa dissertação nós investigamos dois sistemas de muitos corpos. Na primeira parte nós escolhemos uma abordagem clássica para estudar a adsorção de gases nobres pesados, Ne, Ar, Kr, Xe e Rn, em substratos de grafeno. Nós apresentamos evidências de camadas adsorvidas comensuradas, as quais dependem fortemente da simetria do substrato, para duas estruturas: camadas de Ne na rede sqrt{7} X sqrt{7} e Kr na rede sqrt{3} X sqrt{3}. Para estudar o derretimento nós introduzimos um parâmetro de ordem e sua susceptibilidade. O calor específico e a susceptibilidade em função da temperatura foram calculados para os gases nobres pesados em diversas densidades. A posição e largura característica dos picos do calor específico e da susceptibilidade foram determinadas. Finalmente, nós investigamos a distância dos primeiros vizinhos e a distância entre a camada e o substrato, identificando contribuições relacionadas aos picos do calor específico e da susceptibilidade. A segunda parte da dissertação trata de uma linha de vórtice no gás unitário de Fermi. Gases fermiônicos ultrafrios são notáveis devido à possibilidade experimental de variar as interações interpartículas através de ressonâncias de Feshbach, o que possibilita a observação do crossover BCS-BEC. No meio do crossover encontra-se um estado fortemente interagente, o gás unitário de Fermi. Uma linha de vórtice corresponde a uma excitação desse sistema com unidades de circulação quantizadas. Nós construímos funções de onda, inspiradas na função BCS, para descrever o estado fundamental e também o sistema com uma linha de vórtice. Nossos resultados para o estado fundamental elucidam aspectos da geometria cilíndrica do problema. O perfil de densidade é constante no centro do cilindro e vai a zero suavemente na borda. Nós separamos a contribuição devido à parede da energia do estado fundamental e determinamos a energia por partícula do bulk, epsilon_0=(0.42 +- 0.01) E_{FG}. Nós também calculamos o gap superfluído para essa geometria, Delta=(0.76 +- 0.01) E_{FG}. Para o sistema com a linha de vórtice nós obtivemos o perfil de densidade, o qual corresponde a uma densidade não nula no centro do vórtice, e a energia de excitação por partícula, epsilon_{ex}=(0.0058 +- 0.0003) E_{FG}. Os métodos empregados nessa dissertação, Dinâmica Molecular, Monte Carlo Variacional e Monte Carlo de Difusão, nos dão uma base sólida para a investigação de sistemas relacionados, e outros sistemas, de muitos corpos no futuro
Abstract: In this dissertation we investigated two many-body systems. For the first part we chose a classical approach to study the adsorption of heavy rare-gases, Ne, Ar, Kr, Xe and Rn, on graphene substrates. We presented evidences of commensurate adlayers, which depend strongly on the symmetry of the substrate, for two structures: Ne adlayers in the sqrt{7} X sqrt{7} superlattice and Kr in the sqrt{3} X sqrt{3} lattice. In order to study the melting of the system we introduced an order parameter, and its susceptibility. The specific heat and susceptibility as a function of the temperature were calculated for the heavy noble gases at various densities. The position and characteristic width of the specific heat and susceptibility peaks of these systems were determined. Finally, we investigated the first neighbor distance and the distance between the adlayer and the substrate, identifying contributions related to specific heat and melting peaks. The second part of the dissertation deals with a vortex line in the unitary Fermi gas. Ultracold Fermi gases are remarkable due to the experimental possibility to tune interparticle interactions through Feshbach resonances, which allows the observation of the BCS-BEC crossover. Right in the middle of the crossover lies a strongly interacting state, the unitary Fermi gas. A vortex line corresponds to an excitation of this system with quantized units of circulation. We developed wavefunctions, inspired by the BCS wavefunction, to describe the ground state and also for a system with a vortex line. Our results for the ground state elucidate aspects of the cylindrical geometry of the problem. The density profile is flat in the center of the cylinder and vanishes smoothly at the wall. We were able to separate from the ground state of the system the wall contribution and we have determined the bulk energy as epsilon_0=(0.42 +- 0.01) E_{FG} per particle. We also calculated the superfluid pairing gap for this geometry, Delta=(0.76 +- 0.01) E_{FG}. For the system with a vortex line we obtained the density profile, which corresponds to a non-zero density at the core, and the excitation energy, epsilon_{ex}=(0.0058 +- 0.0003) E_{FG} per particle. The methods employed in this dissertation, Molecular Dynamics, Variational Monte Carlo and Diffusion Monte Carlo, give us a solid basis for the investigation of related and other many-body systems in the future
Mestrado
Física
Mestre em Física
2012/24195-2
FAPESP
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Villar, Arribi Pablo. "Fermions lourds et métaux de Hund dans les supraconducteurs à base de fer." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY070/document.
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Matériaux dans lesquels les électrons responsables des propriétés de basse énergie son soumis à fortes corrélations sont aujourd'hui très étudiés à la recherche de nouvelles phases émergentes aux propriétés surprenantes et/ou utiles.Les supraconducteurs à base de fer (IBSC) sont maintenant considérés dans cette classe de composés. En utilissant des techniques multi-corps nécessaires pour le traitement théorique de ces corrélations (théorie du champ moyen de spin esclave - SSMFT et théorie du champ moyen dynamique - DMFT - en conjonction avec la théorie du fonctionnelle de la densité, DFT), dans cette thèse, j'etudie plusieurs propriétés d'IBSC.D’abord, j'analyse les composés très dopés de la famille de IBSC, qui montrent expérimentalement certains comportements typiques des ``fermions lourds'', des composés typiquement des terres rares ou des actinides, où des électrons extrêmement corrélés coexistent avec des électrons moins corrélés. En particulier je me concentre sur la chaleur spécifique et le pouvoir thermoélectrique et je montre comment ces propriétés peuvent être comprises dans le paradigme récemment développé ``métaux de Hund''. En effet, l’échange intra-atomique (le ``couplage de Hund'') est responsable de ces matériaux à éléments métal de transition en montrant la physique des fermions lourds. Je montre aussi que les caractéristiques typiquement fermions-lourds du spectre d’excitation, connues car les singularités de Van Hove sont bien capturées par notre modélisation au sein de DFT+SSMFT. J'utilise ensuite DMFT dans un modèle afin d'étudier l'impact direct des singularités de Van Hove sur la force des corrélations.Dans une seconde partie, je montre comment FeSe, le IBSC actuellement le plus étudié, se trouve également dans une phase métal de Hund, mais il est amené à la frontière de cette phase par la pression. Cette frontière est liée à une augmentation de la compressibilité électronique qui est positivement corrélée à l’augmentation de la supraconductivité trouvée dans les expériences.Je réalise une étude analogue sur le détenteur du record pour la température supraconductrice critique la plus élevée, la monocouche FeSe où je trouve également une compressibilité augmentée. Cela appuie la récente proposition selon laquelle la frontière du métal de Hund favorise la supraconductivité à haute température.Enfin, j'étudie la nature du magnétisme dans une autre famille de IBSC, les germanides de fer. J'explore différents ordres magnétiques possibles avec des simulations DFT et leur concurrence (ce qui peut en principe favoriser la supraconductivité) dans plusieurs composés où différents substitutions sont appliquées au composé parent YFe2Ge2. J'étudie également l'effet de la pression chimique sur ce composéMaterials where the electrons responsible for the low-energy properties experience strong correlations are today very investigated in search of emerging new phases with surprising and/or useful properties. Iron-based superconductors (IBSC) are now considered in this class of compounds. Using the many-body techniques necessary for the theoretical treatment of these correlations (slave-spin mean field theory - SSMFT- and dynamical mean field theory - DMFT- in conjunction with density functional theory, DFT), in this thesis I address several properties of IBSC.First I analyze the very hole-doped compounds in the IBSC family, that show experimentally some behaviors typical of the so-called “heavy fermions”, compounds typically of rare earth or actinides, where extremely correlated electrons coexist with others less correlated. In particular I focus on the specific heat and the thermoelectric power and show how these properties can be understood in the recently developed paradigm of “Hund’s metals”. Indeed the intra-atomic exchange (the “Hund’s coupling”) is responsible for these materials of transition metal elements showing heavy-fermionic physics. I show also that typical heavy-fermionic features of the excitation spectrum, known as Van Hove singularities are well captured by our modelization within DFT+SSMFT. I then use DMFT in a model in order to study the direct impact of the Van Hove singularities on the strength of correlations.In a second part I show how FeSe, the presently most studied IBSC, is also in a Hund’s metal phase, but it is brought to the frontier of this phase by pressure. This frontier is connected to an enhancement of the electronic compressibility which correlates positively then with the enhancement of superconductivity found in experiments. I perform an analogous study on the record holder for the highest critical superconducting temperature, the monolayer FeSe where I also find an enhanced compressibility. This supports the recent proposal that the frontier of a Hund's metal favors high-temperature superconductivity.Finally I study the nature of magnetism in another family of IBSC, the iron-germanides. I explore different possible magnetic orders with DFT simulations and study their competition (which can in principle favor superconductivity) in several compounds where different chemical substitutions are applied to the parent compound YFe2Ge2. I also study the effect of chemical pressure on this compound
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Guillaume, Alexandre. "Transitions de phases dans les fermions lourds sous contrainte uniaxiale." Université Joseph Fourier (Grenoble), 1999. http://www.theses.fr/1999GRE10060.
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Nous presentons les premiers resultats obtenus avec un dispositif experimental permettant de changer la contrainte uniaxiale continument a tres basse temperature. Le principe de l'appareil consiste a controler la pression d'helium appliquee sur un soufflet metallique connecte thermiquement a la boite de melange d'un refrigerateur a dilution. Nous etudions l'effet de la contrainte uniaxiale, appliquee selon l'axe c du compose tetragonal uru#2si#2. Nous trouvons que la temperature de transition supraconductrice t#c varie de facon parabolique en fonction de la contrainte #c avec une pente de 76 mk/kbar a #c = 0 kbar et une pente nulle a 6. 5 kbar. La temperature de transition antiferromagnetique t#n varie lineairement en fonction de la contrainte avec une pente egale a 32 mk/kbar. Nous rapportons les premieres mesures de l'evolution du gap des excitations de la phase antiferromagnetique en fonction de la contrainte uniaxiale selon l'axe c. Il decroit lineairement avec une pente de 0. 63 k/kbar. Grace aux mesures de champ critique, nous donnons la premiere estimation de la croissance de la masse effective avec la contrainte selon l'axe c, m*(6. 5 kbar) = 1. 07 m*(0 kbar). Pour le compose hexagonal upt#3, nous confirmons que la temperature de transition supraconductrice t#+#c decroit, d'abord avec une pente de 14. 71. 5 mk/kbar, pour une contrainte appliquee suivant l'axe c inferieure a 2 kbar puis decroit, avec une pente de 5. 20. 7 mk/kbar, pour des contraintes superieures. En variant de 0 a 8. 4 kbar la contrainte #a appliquee selon l'axe a, nous trouvons que la temperature t#+#c decroit regulierement avec une pente de 2. 00. 7 mk/kbar. A partir de l'evaluation du coefficient a de la loi = #0+at#2 et de la pente du second champ critique a bas champ, nous trouvons que la masse effective augmente avec la contrainte suivant l'axe c et diminue avec la contrainte suivant l'axe a:m*(#c = 6. 5 kbar) = 1. 26 m*(#c = 0 kbar) ; m*(#a = 6. 5 kbar) = 0. 86 m*(#a = 0 kbar).
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Thomas, Frédéric. "Diagramme de phase sous pression hydrostatique du supraconducteur à fermions lourds CeCu2Si2." Grenoble 1, 1993. http://www.theses.fr/1993GRE10184.
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Le diagramme de phase donnant l'evolution en pression de la temperature de transition supraconductrice du compose cecu#2si#2 est reexamine. Les mesures sont realisees entre 0 et 10 gpa et une attention particuliere est accordee a l'obtention de conditions hydrostatiques. Apres un rappel des proprietes generales des systemes a fermion lourds et de l'interet de la pression pour ces systemes (chap. 1), les techniques de production et de controle de pressions hydrostatiques sont decrites (chap. 2): emploi de cellules a enclumes de diamant naturel, utilisation de l'helium comme milieu transmetteur de pression, fluorescence du rubis pour la mesure de la pression. La geometrie de mesure de susceptibilite alternative servant a detecter la temperature de transition supraconductrice dans un refrigerateur a #3he est ensuite decrite. Chap. 3, les principales proprietes de cecu#2si#2 sont exposees avec un rappel des problemes qui subsistent pour ce materiau et quelques experiences complementaires realisees sur des poly- ou des monocristaux. Une etude tentant d'expliquer l'anisotropie de la resistivite est fournie. Le chap. 4 presente les resultats centraux de ce travail: le diagramme de phase t#c(p) entre 0 et 10 gpa ainsi qu'une explication theorique de celui-ci. Le diagramme a une allure singuliere avec deux pressions caracteristiques p#1=3,1 gpa et p#2=7,6 gpa et des regions de dependance lineaire particulierement marquees. L'analyse theorique repose sur deux principes de base: l'augmentation avec le pression de la largeur de la bande etroite sur laquelle est construite la supraconductivite et l'existence de modifications topologiques dans la serie des surfaces equi-energies
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Zoghbi, Bilal. "Spin Fluctuations and non-Fermi Liquid Behavior Close to a Quantum Critical Point in CeNi2Ge2." Kent State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=kent1255375705.
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Holanda, Junior Lino Martins de 1984. "Ressonância de spin eletrônico (ERS) em compostos tipo férmions pesadas a base de Itérbio (Yb)." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278529.
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Orientador: Pascoal José Giglio PagliusoDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-14T11:11:25Z (GMT). No. of bitstreams: 1 HolandaJunior_LinoMartinsde_M.pdf: 2170608 bytes, checksum: 3abed7533be911b023d90094d4aa28f4 (MD5) Previous issue date: 2009
Resumo: Nesta Dissertação desenvolvemos estudos de Ressonância de Spin Eletrônico (ESR) em monocristais dos compostos tipo férmions pesados YbR h2Si2 e YbAlB4. No caso do sistem YbRh2Si2, exploraramos os experimentos de ESR para três bandas de frequência existentes em nosso laboratório (n = 4,1 GHz (Banda-S), n = 9,4 GHz (Banda-X) e n = 34,0 GHz (Banda-Q)), o que situa o campo de ressonância da linha de ESR em diferentes regimes do diagrama de fase do YbRh2Si2, onde as fases antiferromagnética (AFM), líquido de Fermi (FL) e não-líquido de Fermi (NFL) podem ser encontradas. Foram investigados efeitos de campo cristalino e mudanças da dinâmica de spin dos elétrons 4¦ do Yb, para as diferentes bandas, e também para monocristais de YbR h2Si2 dopados com Lu e crescidos em uxo de Zn. Nossos resultados sugerem que o sinal de ESR observado em YbRh2Si2 consiste em um modo acoplado entre os 4f do Yb3+ e os elétrons de condução, em um regime tipo bottleneck com presença de interações antiferromagnéticas. Para o sistema YbAlB4, realizamos experimentos de ESR em banda-X nas duas fases iso-estequiométricas b -YbAl B4 e - a -YbAlB4. Para as amostras de b - YbAlB4 encontramos um comportamento interessante do sinal de ESR que apresenta características de uma ressonância de elétrons de condução (CESR) a altas temperaturas e adquire propriedades do íon de Yb3+ a baixas temperaturas. Tal dualidade não foi observada na fase a - YbAlB4. Nós discutiremos uma possível correlação entre o espectro de ESR observado nos dois sistemas férmions pesados que se situam em lados opostos de um ponto crítico quântico (Quantum Critical Point - QCP) em seus diagramas de fase. Essa correlação permitiu uma especulação sobre origem desse sinal de ESR em compostos férmions pesados, na qual a proximidade ao QCP desenvolve um papel crucial para o comportamento do espectro de ESR observado nessa classe de compostos.
Abstract: In this work we have performed Electron Spin Resonance (ESR) experiments on single crystals of YbR h2Si2 and YbAlB4 heavy fermion compounds. For YbRh2Si2, we explore the ESR measurements at three frequency bands (n = 4,1 GHz (S-Band), n = 9,4 GHz (X-Band) e n = 34,0 GHz (Q-Band)) which places the ESR resonance field within different regimes in the phase diagram of YbRh2Si2 where antiferromagnetic (AFM), Fermi liquid (FL) and non Fermi liquid (NFL) phases could be found. We have also explored the effects of crystal field and spin dynamics of Yb3+ for these different bands and also as a function of Lu-doping and single crystal growth methods. Our results indicate that the ESR signal found YbRh2Si2 behave such a Kondo coupled mode in a bottleneck-like regime with the presence of antiferromagnetic interactions. For YbAlB4, we have performed X-band experiments for single crystals the two different phases b -YbAl B4 and a -YbAlB4. For b -YbAlB4, we found a remarkable ESR signal that behaves as a conduction electron spin resonance (CESR) at high temperatures and acquires characteristics of the Yb3+ local moment ESR at low temperature. This behavior was not found in the a -YbAlB4. The striking and unique dual behavior observed in the same ESR spectra of b -YbAl B4 - a -YbAlB4 associated to the ESR results found for a -YbAlB4, YbRh2Si2 allow us to propose a qualitative scenario that may explain the origin of the ESR signal in HF systems. We speculate that for HF systems a strongly coupled 4¦ and ce give origin to Kondo coupled ESR modes with may behave as CESR or LM ESR, depending on the strength of Jfs. Moreover, HF systems near a QCP may have propitious conditions to present such a signal.
Mestrado
Física da Matéria Condensada
Mestre em Física
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Faulhaber, Enrico. "Untersuchung der magnetischen Eigenschaften von CeCu2(Si1-xGex)2 mittels Neutronenstreuung." Doctoral thesis, Göttingen Cuvillier, 2008. http://d-nb.info/98903660X/04.
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Raymond, Stéphane. "Excitations de basse énergie dans les fermions lourds par diffusion inélastique des neutrons." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10103.
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La technique de diffusion inelastique des neutrons a ete utilisee pour caracteriser le spectre des excitations magnetiques de basse energie dans les composes a electrons fortement correles cenisn, ce#1#-#xla#xru#2si#2 et ceru#2ge#2. Cenisn est un compose kondo semimetallique dont le spectre des excitations magnetiques est compose de deux modes d'energie 2 et 4 mev et respectivement piques au centre et bord de zone de brillouin. L'application d'un champ magnetique met en evidence la nature differente de ces modes : le premier se deplace vers les plus hautes energies et la reponse du second s'elargit. Les modeles de gap d'hybridation particularises pour ce compose permettent de relier le mode a 4 mev aux proprietes macroscopiques observees. La dynamique des moments magnetiques pres d'une instabilite magnetique a ete etudiee dans le compose ce#1##xla#xru#2si#2 avec x = 0. 075. De facon surprenante, un ordre magnetique non conventionnel a ete mis en evidence (m = 0. 02 #b et t#n = 1. 8 k). La dynamique de spin par rapport au compose pur a ete reliee, grace a la theorie des fluctuations de spin de moriya, aux mesures macroscopiques. L'application d'un champ magnetique sur le compose ceru#2si#2 met en evidence le changement de la reponse dynamique du systeme de part et d'autre de la transition pseudometamagnetique. La reponse quasielastique et a courte portee a bas champ est remplacee par un ordre statique a longue distance en champ fort (h = 12 t). Dans ce contexte, la phase antiferromagnetique du compose ferromagnetique ceru#2ge#2 a ete caracterisee. Il existe alors une analogie entre ce compose sous sa temperature de curie et ceru#2si#2 au dessus du champ metamagnetique.
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Griveau, Jean-Christophe. "Etude de composés fermions lourds supraconducteurs sous hautes pressions : CECU2SI2 et UPD2AL3." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10252.
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L'objet de cette these est l'etude des composes fermions lourds supraconducteurs cecu#2si#2 et upd#2al#3 sous pression dans leur phase normale et supraconductrice par resistivite electrique (p#a#c) et par susceptibilite magnetique alternative (#a#c). Les mesures ont ete effectuees en conditions quasi-hydrostatiques (steatite) et hydrostatiques (#4he). Les mesures par resistivite electrique se sont effectuees sous champ magnetique : un diagramme hc#2(t) a ete etabli sous pression pour les deux composes. Pour cecu#2si#2, l'etude sur monocristal en resistivite electrique sous champ magnetique et sous pression (b,t)#- a montre une augmentation drastique du champ critique dans le regime ii avec hc#2(o)5 t contre 2,1 t a pression nulle. Les ajustements du champ critique dans un modele de supraconducteur en limite propre et couplage faible indiquent une anisotropie liee sans doute a la surface de fermi non spherique (coefficient gyromagnetique <1) tandis que ceux en limite sale et couplage fort indiquent une changement de comportement net entre les regimes i et ii. La pente varie fortement sous pression : 25 k/t dans le regime i et 12 k/t dans le regime ii. Pour upd#2al#3, une etude en susceptibilite magnetique sous pression #a#c- sur polycristaux caracterises en transport a permis d'etablir : un diagramme t#c(p) montrant une forte diminution de la supraconductivite apres 6 gpa. On constate en resistivite sous champ et sous pression le maintient du regime de liquide de fermi. On constate peu de variation du champ critique sous pression : la masse effective liee a la pente a t#c presente un faible maximum autour de 5 gpa avant de diminuer fortement a partir de 6 gpa. Ces deux composes presentent deux aspects de l'interaction entre supraconductivite non conventionnelle et magnetisme : competition pour cecu#2si#2 et coexistence pour upd#2al#3.
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Dionicio, Gabriel Alejandro. "Electrical resistivity of YbRh2Si2 and EuT2Ge2 (T = Co, Cu) at extreme conditions of pressure and temperature." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1170255664129-64812.
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This investigation address the effect that pressure, p, and temperature, T, have on 4f-states of the rare-earth elements in the isostructural YbRh2Si2, EuCo2Ge2, and EuCu2Ge2 compounds. Upon applying pressure, the volume of the unit cell reduces, enforcing either the enhancement of the hybridization of the 4f-localized electrons with the ligand or a change in the valence state of the rare-earth ions. Here, we probe the effect of a pressure-induced lattice contraction on these system by means of electrical resistivity, from room temperature down to 100 mK. At ambient pressure, the electrical resistivity of YbRh2Si2 shows a broad peak at 130 K related to the incoherent scattering on the ground state and the excited crystalline electrical field (CEF) levels. At T_N = 70 mK, YbRh2Si2 undergoes a magnetic phase transition. Upon applying pressure up to p_1 = 4 GPa , T_N increases monotonously while the peak in the electrical resistivity is shifted to lower temperatures. For p < p_1 a different behavior is observed; namely, T_N depends weakly on the applied pressure and a decomposition of the single peak in the electrical resistivity into several shoulders and peaks occurs. Above p_2 = 9 GPa, the electrical resistivity is significantly reduced for T < 50 K and this process is accompanied by a sudden enhancement of T_N. Thus, our results confirm the unexpected behavior of the magnetization as function of pressure reported by Plessel et al. The small value of the magnetic ordering temperature for p < p_2 and the strength of the mechanism that leads to the peaks and shoulders in the electrical resistivity suggest that the f-electrons are still screened by the conduction electrons. Therefore, the observed behavior for pressures lower than p_2 might be a consequence of the competition of two different types of magnetic fluctuations (seemingly AFM and FM). Furthermore, the results suggest that a sudden change of the CEF scheme occurs at pressures higher than p_1, which would have an influence on the ground state. Additionally, a comparison of the pressure dependent features in the electrical resistivity of YbRh2Si2 with similar maxima in other isostructural YbT2X2 (T = transition metal; X = Si or Ge) compounds was performed. For the comparison, a simple relation that considers the Coqblin-Schrieffer model and the hypothesis of Lavagna et al. is proposed. A systematic behavior is observed depending on the transition metal; namely, it seems that the higher the atomic radii of the T-atom the smallest the pressure dependence of the maximum in the electrical resistivity, suggesting a weaker coupling of localized- and conduction-electrons. It is also observed that an increase in the density of conduction electrons reduces the pressure dependence of the characteristic Kondo temperature. The mechanism responsible for the sudden enhancement of T_N in YbRh2Si2 at about p_2 is still unknown. However two plausible scenarios are discussed. The Eu-ions in EuCo2Ge2 and EuCu2Ge2 have a divalent character in the range 100 mK < T < 300 K. Therefore, these systems order magnetically at T_N = 23 K and T_N = 12 K, respectively. The studies performed on EuCo2Ge2 and EuCu2Ge2 as a function of pressure suggest that a change to a non-magnetic trivalent state of the Eu-ions might occur at zero temperature for pressures higher than 3 GPa and 7 GPa, respectively. A common and characteristic feature on EuCo2Ge2 and EuCu2Ge2 is the absence of a clear first order transition from the divalent to the trivalent state of the Eu-ions at finite temperature for p > 3 GPa and for p > 7 GPa, respectively. In other isostructural Eu-based compounds, a discontinuous and abrupt change in the thermodynamic and transport properties associated to the valence transition of the Eu-ions is typically observed at finite temperatures. In contrast, the electrical resistivity of EuCo2Ge2 and EuCu2Ge2 changes smoothly as a function of pressure and temperature. The analysis of the the electrical resistivity of EuCo2Ge2 suggest that a classical critical point might be close to the AFM-ordered phase, being a hallmark of this compound. The overall temperature dependence of the the electrical resistivity of EuCo2Ge2 changes significantly at 3 GPa; therefore, it seems that the system suddenly enters to a T-dependent valence-fluctuating regime. Additionally, the pressure-dependent electrical-resistivity isotherms show a step-like behavior. Thus, it is concluded that discontinuous change of the ground state might occur at 3 GPa. The electrical resistivity of EuCu2Ge2 at high pressure is characterized by a negative logarithmic T-dependence in the pressure range 5 GPa < p < 7 GPa for T > T_N and by a broad peak in the pressure dependent residual resistivity, whose maximum is located at 7.3 GPa. The first behavior resembles the incoherent scattering process typical for an exchange coupling mechanism between the localized electrons and the ligand. This and the peak effect in the local 4f susceptibility observed in NMR measurements are consistent with such a coupling mechanism. Thus, it would be for the first time that a dense Eu-based compound like EuCu2Ge2 show such a behavior. Combining the results of the experiment performed at high pressures on EuCu2Ge2 with the studies performed in the EuCu2(Ge1-xSix)2 series, a crossover from an antiferromagnetically ordered state into a Fermi-liquid state for pressures higher than 7.3 GPa may be inferred from the analysis. Therefore, it may be possible that the sudden depopulation of 4f-level occur mediated by quantum fluctuation of the charge due to a strong Coulomb repulsion between the localized-electrons and the ligand. This phenomenon would explain the broad peak in the residual resistivity. To our knowledge, this would be the first Eu-based compound, isostructural to ThCr2Si2, that show such a transition as function of pressure at very low temperatures.
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Bastide, Christophe. "Etude de quelques problèmes théoriques dans les phases normales et supraconductrices des fermions lourds." Grenoble 1, 1987. http://www.theses.fr/1987GRE10155.
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Etude de la dilatation thermique negative de ceal::(3) associee au comportement de la chaleur specifique sous pression, en utilisant des resultats de methodes variationnelles appliquees au reseau d'anderson. Etude des proprietes de la phase supraconductrice des fermions lourds en relation avec les differentes symetries possibles pour le parametre d'ordre. Calcul de l'effet d'impuretes non magnetiques sur la temperature critique des phases de symetrie s-etendu et d. Etude du reseau d'anderson dans la limite d'une faible amplitude de saut
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Wykhoff, Jan. "Elektronenspinresonanz in Yb-basierten Kondogitter-Systemen." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-39395.
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Die Elektronenspinresonanz (ESR) untersucht die im quasistatischen Magnetfeld resonante Absorption eines an die Probe angelegten Mikrowellenmagnetfeldes.
Es wurde das System Yb1-w A1-w (Rh1-x Cox)2 (Si1-y Gey) 2 mit A=La, bzw. Lu, sowie das System YbIr2Si2 mittels ESR untersucht.
Unter Kondo-Wechselwirkung vieler Leitungselektronen mit einem lokalen 4f-Moment des Kondo-Ions bildet sich ein nicht-magnetisches Grundzustands-Singlett, was zur Abschirmung des magnetischen Moments führt.
YbRh2Si2 ist das erste Schwere-Fermionen-System mit Kondo-Ionen, das ohne Dotierung zusätzlicher ESR-Sonden ein ESR-Signal unterhalb der Kondo-Temperatur aufweist.
Es zeigt sich, dass das ESR-Signal nicht mittels gängiger ESR-Theorien konsistent beschrieben werden kann.
Die Messungen, die im Rahmen dieser Arbeit angestellt wurden, flossen in die Entwicklung von weiterführenden Theorien (z.B. [1], [2]) ein.
Die Temperaturabhängigkeit des ESR-g-Faktors konnte damit erfolgreich beschrieben werden, womit erstmals der Nachweis einer Kondo-Wechselwirkung in Kondo-Gitter-Systemen mittels ESR gelang. Ferner konnte die Bedeutung von ferromagnetischen Fluktuationen für eine kleine, beobachtbare Linienbreite beschrieben werden. Der ESR-Methode ist somit die Kondo-Spindynamik direkt zugänglich. Dieser Zugang ist neu und einzigartig, denn andere Methoden (NMR, inelastische Neutronenstreuung) charakterisieren die Kondo-Spindynamik auf indirekte Weise.
[1] P. Wölfle und E. Abrahams. Phenomenology of esr in heavy-fermion systems: Fermi-liquid und nicht-fermi-liquid regimes Phys. Rev. B, 80(23): 235112, 2009.
[2] B. I. Kochelaev, S. I. Belov, A. M. Skvortsova, A. S. Kutusov, J. Sichelschmidt, J. Wykhoff, C. Geibel und F. Steglich. Why could electron spin resonance be observed in a heavy fermion kondo lattice? Eur. Phys. J. B, 72(4): 485, 2009.
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Faulhaber, Enrico. "Untersuchung der magnetischen Eigenschaften von CeCu2(Si1-xGex)2 mittels Neutronenstreuung." Doctoral thesis, Technische Universität Dresden, 2007. https://tud.qucosa.de/id/qucosa%3A23704.
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1979 wurde mit CeCu2Si2 erstmalig ein Schwere-Fermionen-Supraleiter entdeckt. Diese Verbindung, entdeckt von Steglich und Mitarbeitern, befindet sich nahe an einem quantenkritischen Punkt, an dem die magnetische Ordnung gerade unterdrückt wird. Der Abstand zu diesem Punkt kann sowohl durch Druck als auch durch Germaniumsubstitution auf dem Siliziumplatz variiert werden. Dabei treten neben der Supraleitung in CeCu2Si2 auch verschiedene magnetische Phasen bei höherem Germaniumgehalt auf. CeCu2Si2 ordnet magnetisch unterhalb von TN = 0.8 K in einer Spindichtewelle, während das Schwere-Fermionen-System CeCu2Ge2 unterhalb von TN = 4.1 K antiferromagnetisch ordnet. In dieser Arbeit wurde die Substitutionsreihe CeCu2(Si1-xGex)2 mittels Neutronendiffraktion untersucht. Ausgehend von Proben mit hohem Germaniumgehalt von x = 0.45, deren magnetische Struktur detailliert untersucht wurde, wurden schrittweise die Eigenschaften von Proben mit kleinerem x erschlossen, um schließlich die (bis dato unbekannte) magnetische Struktur in CeCu2Si2 aufzuklären. Weiterhin wurden Untersuchungen zumWechselspiel zwischenMagnetismus und Supraleitung durchgeführt. Hierzu wurde mit einem selbstentwickelten Aufbau dieWechselfeldsuszeptibilität simultan zu den Diffraktionsexperimenten aufgezeichnet. Durch die direkte Korrelation konnte nachgewiesen werden, dass in CeCu2Si2 keine mikroskopische Koexistenz von Supraleitung und magnetischer Ordnung vorliegt, sondern mikroskopische Phasenseparation. - Die Arbeit ist auch über den Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen mit der ISBN 978-3-86727-587-3 erhältlich.In 1979 the first heavy-fermion superconductor CeCu2Si2 was discovered by Steglich et al. The system is near a quantum critical point (QCP), where the magnetic order is just suppressed. The distance to the QCP can be variied with hydrostatic pressure as well as by germanium substitution on the silicon site. Next to the superconductivity in CeCu2Si2 one finds distinct magnetic phases while increasing the germanium content. CeCu2Si2 shows a magnetic order of a spin-density-type below TN = 0.8 K, whereas the heavy fermion system CeCu2Ge2 orders below TN = 4.1 K as an antiferromagnet. The focus of this thesis is on neutron-diffraction in the system CeCu2(Si1-xGex)2. Starting with a sample with a high germanium content of x = 0.45, the magnetic structures are investigated in detail. Following a step-by-step approach, samples with reduced x are investigated subsequently to figure out the properties of pure CeCu2Si2, which were not accessible before. Furthermore, the complex interaction between magnetism and superconductivity is investigated in detail. Using a specially designed setup, the ac-susceptibility could be recorded simultaneously during the neutron diffraction experiments. Due to the direct correlation between antiferromagnetic signals and diamagnetic features, the microscopic coexistence of superconductivity and magnetic order can be ruled out. Instead, a phase separation on the microscopic scale is found. - The thesis is also available from the publisher Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen under the ISBN 978-3-86727-587-3.
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Adriano, Cris 1980. "Efeitos da substituição química nos diagramas de fases dos férmions pesados Ce2MIn8 (M = Rh e Ir)." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278513.
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Orientador: Pascoal José Giglio PagliusoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Este trabalho apresenta estudos sistemáticos das propriedades magnéticas dos compostos Ce2MIn8 (M = Rh, Ir) dopados com Cd, Sn e Ga no sítio cristalográfico do In. Foram utilizadas as técnicas de medidas macroscópicas de susceptibilidade magnética, calor específico e resistividade elétrica, além de medidas de caracterização estrutural pelo método de difração de pó de raios-x e medidas microscópicas da determinação da estrutura magnética usando a técnica de espalhamento magnético de raios-x e nêutrons para os compostos dopados com Cd. Os compostos Ce2MIn8 (chamados de 218) são tetragonais (grupo espacial P4/mmm) e são a versão bi-camada da família de férmions pesados supercondutores Cem MnIn3m+2n(M = Co, Rh, Ir; m = 1,2; n = 0,1), na qual os compostos do tipo CeMIn5 (chamados de 115) foram muito estudados por apresentarem uma variedade de propriedades físicas interessantes, tais como efeito Kondo, supercondutividade não convencional, ordenamento magnético, comportamento non-Fermi-Liquid, e ocorrência de pontos críticos quânticos. Mais interessante ainda é o fato de que estas propriedades podem ser controladas por dopagem, pressão externa e/ou campo magnético gerando diagramas de fases bastante interessantes. O objetivo de se estudar os parentes Ce2MIn8 é entender o papel da dimensionalidade nas propriedades físicas destes compostos e comparar os resultados encontrados com aqueles observados para os 115. Foram construídos os diagramas de fase inéditos para as séries de Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, C e2Rh0.5Ir0.5In8-xC dx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax a partir de dados obtidos utilizando as técnicas mencionadas acima. Foi visto que a dopagem de Cd nos compostos Ce2MIn8 favorece o AFM através do aumento da TN em função da concentração de Cd. Os resultados sugerem que o Cd está agindo de modo a diminuir a densidade de carga local e isso está favorecendo o ordenamento magnético. Porém, resultados da variação da temperatura do máximo da resistividade elétrica em função da concentração de Cd, sugerem que efeitos de campo cristalino também estão presentes nestes compostos. A estrutura magnética dos compostos de Ce2RhIn8-xCdx e C e2IrIn8-xCdx dopados com x = 2.6 % de Cd foram estudados em baixas-T e encontrou-se um vetor de propagação do tipo (½ ½ 0) consistente com ordenamento AFM abaixo de TN. Verificou-se que a orientação do momento magnético tende a ir na direção do plano-ab com dopagem de Cd nestes compostos. A substituição de In por Sn nos compostos de Ce2MIn8 mostrou que esta dopagem está desfavorecendo o ordenamento magnético através da diminuição do valor da TN em função da concentração de Sn. Os resultados sugerem que o Sn está favorecendo o efeito Kondo e que efeitos de campo cristalino são menos observáveis nos compostos dopados com Sn. Foi visto que a dopagem com Ga no sítio do In igualmente diminui a temperatura de ordenamento magnético e favorece o efeito Kondo. Porém, os resultados sugerem que efeitos de campo cristalino estão presentes pela variação não usual da temperatura do máximo das curvas de resistividade elétrica em função da dopagem com Ga. Neste trabalho também são apresentados estudos da evolução do magnetismo dos elétrons 4f, em compostos relacionados que não são férmions pesados para R2CoGa8, para R = Gd, Tb, Dy, Ho, Er e Tm. Nestes compostos podemos observar a evolução do magnetismo e dos efeitos de campo cristalino sem a presença do efeito Kondo
Abstract: This thesis describes the investigation of the physical properties of the (Cd, Sn, Ga)-doped Ce2MIn8 (M = Rh, Ir) intermetallic compounds. Temperature dependent magnetic susceptibility, heat capacity and electrical resistivity measurements were performed to characterize the macroscopic properties of the synthesized compounds. X-ray powder diffraction were used to determine their phase purity and crystal structure and X-ray and neutron magnetic scattering techniques were used to study the microscopic magnetic structure in low temperature. The Ce2MIn8 (M = Rh, Ir) compounds belong to the family of the heavy-fermions Ce-based compounds CenMIn3n+2 (M = Co, Rh, Ir; n = 1, 2), where the occurrence of unconventional superconductivity (USC) in many of its members has been motivating intense work in past few years. In particular these compounds present a wide range of interesting properties as Kondo effect, USC, magnetism, non Fermi liquid and Fermi liquid behavior and the occurrence of quantum critical points. Particularly interesting is the tunability of their ground state by pressure and chemical doping that revealed very interesting and rich phase diagrams with properties that are unprecedented for this class of compounds. Indeed the study of the 218 parent compounds is a great opportunity to further study the relationship between magnetism, USC and crystal structure in this family. We report in this work five unpublished phase diagrams of this family: Ce2RhIn8-xCdx, Ce2IrIn8-xCdx, Ce2Rh0.5Ir0.5 In8-xCdx, Ce2RhIn8-xSnx e Ce2RhIn8-xGax. Our results revealed that Cd-doping Ce2MIn8 enhances the antiferromagnetic ordering temperature as a function of the Cd concentration. The results suggest that the Cd-doping in these compounds is inducing an electronic tuning by locally decreasing the Ce 3+ density of states and this is favoring the AFM ordering according com a Doniach-like scenario. However the evolution of the maximum in the resistivity as a function of the Cd concentration and the small variation of the magnetic moment orientation of the doped compounds suggest that Cd-doping is also changing the crystal field (CEF) parameters in these compounds. The magnetic structure studied by X-ray and neutron magnetic scattering showed the Cd-doped samples present just below TN a commensurate antiferromagnetic structure with a propagation vector (½ ½ 0). The magnetic structure determination indicates that the magnetic moment orientation of the Cd doped samples tend going to the ab-plane. The Sn chemical substitution in Ce2MI n8 (M = Rh, Ir) showed Sn-doping is decreasing the magnetic order by favoring the Kondo effect. This is the opposite of the Cd-doping effect because Sn is now increasing the local density of states of the Ce3+ions. As Sn-doping drives 4f to a more itinerant character the CEF effect are less important for this compounds. On the other hand, the Ga-doped Ce2RhI n8 similarly show a decreasing of the magnetic order due to the enhancement of the Kondo effect by chemical pressure. However the unexpected evolution of the maximum in the resistivity as a function of the Ga suggests in this dopant is changing the crystal field is still an important effect in this compound. Finally we present the 4f magnetism evolution studies on the series of the Ga-based R2CoGa8 for R = Gd ¿ Tm compounds, where one can follow the evolution of the magnetism and the CEF parameters without the presence of the Kondo effect
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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Holtmeier, Stefan. "Effets magnétovolumiques dans des composés à fermions lourds CeRu2Si2, CeCu6 et CeNiSn." Université Joseph Fourier (Grenoble), 1994. http://www.theses.fr/1994GRE10117.
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Ce memoire decrit des etudes de dilatation thermique et de magnetostriction effectuees au moyen d'un dilatometre capacitif sur des monocristaux de composes a fermions lourds, entre 0. 2 et 300 k et sous des champs magnetiques allant jusqu'a 11 t. Dans une premiere partie, sont decrits des resultats sur l'alliage ce#0#. #9#8#5y#0#. #0#1#5ru#2si#2, qui completent une etude precedente du compose ceru#2si#2 pur et de l'alliage ce#0#. #9#5la#0#. #0#5ru#2si#2. Leur energie caracteristique obeit a une loi d'echelle. Aucun des de ces trois composes ne presente d'ordre magnetique a longue distance, mais des correlations antiferromagnetiques a courte portee, lesquelles disparaissent lors d'une transition induite sous champ, qualifiee de pseudometamagnetique. Les effets d'alliage sur la dilatation volumique et sur la largeur de cette transition sont discutes, et une explication basee sur un effet kondo en presence des correlations est presentee. Le compose cecu#6 montre des effets magnetovolumiques tres anisotropes. A tres basse temperature, une faible transition pseudometamagnetique a pu etre detectee. Le comportement sous champ est discute en terme d'effet kondo a un site. La formation d'un gap semiconducteur dans cenisn n'a pas de forte influence sur les proprietes magnetovolumiques. Par contre, nos experiences confirment l'existence d'un pseudogap de spin tres anisotrope. Les modeles theoriques qui tentent d'expliquer ce comportement nouveau dans les semiconducteurs magnetiques sont discutes
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Panarin, Justin. "Dynamique de spin dans le supraconducteur non conventionnel CeCoIn¥." Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00767160.
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Cette thèse porte sur l'étude de la dynamique de spin dans CeCoIn5 et plus précisément sur l'étude de la "résonance de spin" dans ce composé. CeCoIn5 est le composé à fermions lourds à base de cérium ayant la température de supraconductivité la plus élevé ( Tc = 2.3 K) et présente une supraconductivité non-conventionnelle avec un gap de type d_{x^2-y^2}. Dans l'état supraconducteur, le spectre des excitations magnétiques est radicalement modifié avec l'apparition d'un excitation particulièrement intense appelé "Résonance de spin". Ce type d'excitations a déjà été découvert dans les supraconducteurs haute-températures, dans d'autres composés à fermions lourds et également dans les nouveaux supraconducteurs au Fer. Dans cette thèse, nous étudions l'évolution de la résonance de spin en présence d'un champ magnétique et avec l'introduction d'impuretés magnétiques et non-magnétiques. D'après notre étude, les impuretés vont influer sur la résonance de spin via le gap supraconducteur, en effet la présence d'impuretés diminue le gap supraconducteur et l'énergie de la résonance de spin va diminuer de manière proportionnelle. L'influence du champ magnétique est délicate à considérer mais nos recherches porteraient vers un splitting Zeeman de la résonance de spin en accord avec les modèles développés pour les cuprates.
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Zemirli, Saliha. "Etude sous haute pression de quelques fermions lourds : effets de cohérence et ordre magnétique." Grenoble 1, 1986. http://www.theses.fr/1986GRE10148.
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Mesures de conductivite sous haute pression a differentes temperatures pour ceal::(2), cecu::(2) et cecu::(6). Etudes par diffraction de neutrons, resistivite thermique et electrique, pouvoir thermoelectrique interpretees a l'aide de differents modeles
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Arndt, Julia. "Wechselspiel von Magnetismus und Supraleitung im Schwere-Fermionen-System CeCu2Si2." Doctoral thesis, Max-Planck-Institut für Chemische Physik fester Stoffe, 2009. https://tud.qucosa.de/id/qucosa%3A25298.
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Das Auftreten von Supraleitung in Systemen mit schweren Fermionen, erstmals entdeckt in CeCu_2Si_2, wird mit der Nähe zu einem quantenkritischen Punkt in Verbindung gebracht. Daraus ergibt sich ein komplexes Zusammenspiel von Magnetismus und Supraleitung, das in der vorliegenden Arbeit durch Messungen der spezifischen Wärme, der Wechselfeldsuszeptibilität und durch inelastische Neutronenstreuexperimente an verschiedenen Einkristallen von CeCu_2(Si_{1-x}Ge_x)_2 untersucht wird. Der Schwerpunkt liegt auf der genauen Charakterisierung des magnetischen Anregungsspektrums von CeCu_2Si_2 des S-Typs. Die Ergebnisse der Neutronenstreumessungen implizieren stark, dass die Kopplung der supraleitenden Cooper-Paare durch überdämpfte Spinfluktuationen vermittelt wird, die in der Umgebung eines Quantenphasenübergangs gehäuft auftreten. Unter Substitution einiger Si- durch Ge-Atome in CeCu_2Si_2 stabilisiert sich die magnetische Ordnung, und die Supraleitung wird zunehmend unterdrückt. Neutronenstreumessungen ergeben, dass dies bei 2 % Ge-Substitution dazu führt, dass sich Magnetismus und Supraleitung gegenseitig verdrängen, während sie bei 10 % Ge-Substitution mikroskopisch koexistieren. - (Die Dissertation ist veröffentlicht im Logos Verlag Berlin GmbH, Berlin, Deutschland, http://www.logos-verlag.de, ISBN: 978-3-8325-2456-2)The occurrence of superconductivity in systems with heavy fermions, discovered for the first time in CeCu_2Si_2, is often linked to the vicinity of a quantum critical point. This results in a complex interplay of magnetism and superconductivity, which is studied by means of specific heat and ac susceptibility measurements as well as neutron scattering experiments on different single crystals of CeCu_2(Si_{1-x}Ge_x)_2 in the present thesis. The focus is put on the detailed characterisation of the magnetic excitation spectrum in S-type CeCu_2Si_2. Neutron scattering results strongly imply that the coupling of superconducting Cooper pairs is mediated by overdamped spin fluctuations, which accumulate in the vicinity of a quantum phase transition. By substituting Si by Ge atoms in CeCu_2Si_2 magnetic order is stabilised and superconductivity successively suppressed. Neutron scattering experiments demonstrate that 2 % Ge substitution leads to magnetic order being displaced by superconductivity on decreasing temperature, whereas both coexist microscopically in the case of 10 % Ge substitution.
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Dionicio, Gabriel Alejandro. "Electrical resistivity of YbRh2Si2 and EuT2Ge2 (T = Co, Cu) at extreme conditions of pressure and temperature." Doctoral thesis, Technische Universität Dresden, 2006. https://tud.qucosa.de/id/qucosa%3A25011.
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This investigation address the effect that pressure, p, and temperature, T, have on 4f-states of the rare-earth elements in the isostructural YbRh2Si2, EuCo2Ge2, and EuCu2Ge2 compounds. Upon applying pressure, the volume of the unit cell reduces, enforcing either the enhancement of the hybridization of the 4f-localized electrons with the ligand or a change in the valence state of the rare-earth ions. Here, we probe the effect of a pressure-induced lattice contraction on these system by means of electrical resistivity, from room temperature down to 100 mK. At ambient pressure, the electrical resistivity of YbRh2Si2 shows a broad peak at 130 K related to the incoherent scattering on the ground state and the excited crystalline electrical field (CEF) levels. At T_N = 70 mK, YbRh2Si2 undergoes a magnetic phase transition. Upon applying pressure up to p_1 = 4 GPa , T_N increases monotonously while the peak in the electrical resistivity is shifted to lower temperatures. For p < p_1 a different behavior is observed; namely, T_N depends weakly on the applied pressure and a decomposition of the single peak in the electrical resistivity into several shoulders and peaks occurs. Above p_2 = 9 GPa, the electrical resistivity is significantly reduced for T < 50 K and this process is accompanied by a sudden enhancement of T_N. Thus, our results confirm the unexpected behavior of the magnetization as function of pressure reported by Plessel et al. The small value of the magnetic ordering temperature for p < p_2 and the strength of the mechanism that leads to the peaks and shoulders in the electrical resistivity suggest that the f-electrons are still screened by the conduction electrons. Therefore, the observed behavior for pressures lower than p_2 might be a consequence of the competition of two different types of magnetic fluctuations (seemingly AFM and FM). Furthermore, the results suggest that a sudden change of the CEF scheme occurs at pressures higher than p_1, which would have an influence on the ground state. Additionally, a comparison of the pressure dependent features in the electrical resistivity of YbRh2Si2 with similar maxima in other isostructural YbT2X2 (T = transition metal; X = Si or Ge) compounds was performed. For the comparison, a simple relation that considers the Coqblin-Schrieffer model and the hypothesis of Lavagna et al. is proposed. A systematic behavior is observed depending on the transition metal; namely, it seems that the higher the atomic radii of the T-atom the smallest the pressure dependence of the maximum in the electrical resistivity, suggesting a weaker coupling of localized- and conduction-electrons. It is also observed that an increase in the density of conduction electrons reduces the pressure dependence of the characteristic Kondo temperature. The mechanism responsible for the sudden enhancement of T_N in YbRh2Si2 at about p_2 is still unknown. However two plausible scenarios are discussed. The Eu-ions in EuCo2Ge2 and EuCu2Ge2 have a divalent character in the range 100 mK < T < 300 K. Therefore, these systems order magnetically at T_N = 23 K and T_N = 12 K, respectively. The studies performed on EuCo2Ge2 and EuCu2Ge2 as a function of pressure suggest that a change to a non-magnetic trivalent state of the Eu-ions might occur at zero temperature for pressures higher than 3 GPa and 7 GPa, respectively. A common and characteristic feature on EuCo2Ge2 and EuCu2Ge2 is the absence of a clear first order transition from the divalent to the trivalent state of the Eu-ions at finite temperature for p > 3 GPa and for p > 7 GPa, respectively. In other isostructural Eu-based compounds, a discontinuous and abrupt change in the thermodynamic and transport properties associated to the valence transition of the Eu-ions is typically observed at finite temperatures. In contrast, the electrical resistivity of EuCo2Ge2 and EuCu2Ge2 changes smoothly as a function of pressure and temperature. The analysis of the the electrical resistivity of EuCo2Ge2 suggest that a classical critical point might be close to the AFM-ordered phase, being a hallmark of this compound. The overall temperature dependence of the the electrical resistivity of EuCo2Ge2 changes significantly at 3 GPa; therefore, it seems that the system suddenly enters to a T-dependent valence-fluctuating regime. Additionally, the pressure-dependent electrical-resistivity isotherms show a step-like behavior. Thus, it is concluded that discontinuous change of the ground state might occur at 3 GPa. The electrical resistivity of EuCu2Ge2 at high pressure is characterized by a negative logarithmic T-dependence in the pressure range 5 GPa < p < 7 GPa for T > T_N and by a broad peak in the pressure dependent residual resistivity, whose maximum is located at 7.3 GPa. The first behavior resembles the incoherent scattering process typical for an exchange coupling mechanism between the localized electrons and the ligand. This and the peak effect in the local 4f susceptibility observed in NMR measurements are consistent with such a coupling mechanism. Thus, it would be for the first time that a dense Eu-based compound like EuCu2Ge2 show such a behavior. Combining the results of the experiment performed at high pressures on EuCu2Ge2 with the studies performed in the EuCu2(Ge1-xSix)2 series, a crossover from an antiferromagnetically ordered state into a Fermi-liquid state for pressures higher than 7.3 GPa may be inferred from the analysis. Therefore, it may be possible that the sudden depopulation of 4f-level occur mediated by quantum fluctuation of the charge due to a strong Coulomb repulsion between the localized-electrons and the ligand. This phenomenon would explain the broad peak in the residual resistivity. To our knowledge, this would be the first Eu-based compound, isostructural to ThCr2Si2, that show such a transition as function of pressure at very low temperatures.
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Howald, Ludovic. "Interactions entre la supraconductivité et la criticité quantique, dans les composés CeCoIn5, URhGe et UCoGe." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00584598.
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Le sujet de cette thèse est l'analyse du second champ critique supraconducteur (Hc2) ainsi que l'interaction entre la supraconductivité et les points critiques quantiques (PCQ), pour les composés CeCoIn5, URhGe et UCoGe. Dans le composé CeCoIn5, l'étude par résistivité du domaine de liquide de Fermi a permis la localisation précise du PCQ a pression ambiante. Cette analyse permet d'invalider l'hypothèse d'une coïncidence entre Hc2(0) et le PCQ. Dans une deuxième partie, l'évolution sous pression de Hc2 est analysée. Le dôme supraconducteur de ce composé est non-conventionnel avec deux pressions caractéristiques différentes: à ~1.6GPa, la température de transition supraconductrice est maximum alors que c'est à ~0.4GPa que la plupart des grandeurs physiques (maximum de Hc2(0), maximum de la pente dHc2/dT, maximum du saut de chaleur spécifique DC/C, ...) suggèrent la présence d'un PCQ. Nous expliquons cet antagonisme par l'importance des processus de brisure de pairs liés a la proximité du PCQ. Ces deux observations nous permettent de proposer un nouveau diagramme de phase pour CeCoIn5. Dans une troisième partie, les mesures de conduction thermique sur les composés URhGe et UCoGe sont présentées. Elles nous permettent dans un premier temps d'obtenir la transition "bulk" supraconductrice et de confirmer la forme in-habituelle de Hc2 observée en résistivité. La dépendance en températures et en champs de la conduction thermique nous permet d'identifier une contribution non-électronique au transport de chaleur jusqu'aux plus basses températures. D'autre part, nous identifions deux différents domaines supraconducteurs a bas et hauts champs appliqués selon l'axe b. Ces deux domaines sont compatibles avec un modèle de supraconductivité multigaps. Suivant ces observations et des mesures de pouvoir thermoélectrique, nous proposons un modèle de transition de Lifshitz pour ces deux composés.
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Rodière, Pierre. "Supraconductivité et magnétisme dans le composé à électrons fortement corrélés UPt3." Université Joseph Fourier (Grenoble), 2001. http://www.theses.fr/2001GRE10025.
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Najib, Abdelkrim. "Etude de quelques composés de cérium du type fermions lourds : corrélations, cohérence, diagrammes de phases sous pression." Grenoble 1, 1988. http://www.theses.fr/1988GRE10064.
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Etude des composes ce in cu::(2), ce mg et ce cu::(6). Les etudes de susceptibilite magnetique, resistivite electrique et chaleur specifique sur les solutions solides(ce-y)incu::(2) et (ce-la)incu::(2) mettent en evidence des correlations antiferromagnetiques dans le compose ceincu::(2). Influnce des impuretes sur la coherence du reseau. Determination pour ce compose du coefficient de grueneisen a partir des proprietes de transport sous pression hydrostatique. Etude pour les autres composes des diagrammes de phase sous pression hydrostatique jusqu'a 130 kbar
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Keller, Niels. "Étude angulaire du deuxième champ critique dans les supraconducteurs à fermions lourds UPt3, URu2Si2 et CeCu2Si2." Université Joseph Fourier (Grenoble ; 1971-2015), 1994. http://www.theses.fr/1994GRE10119.
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Cette these est consacree a l'etude experimentale du deuxieme champ critique des supraconducteurs a fermions lourds upt#3, uru#2si#2 et cecu#2si#2. La supraconductivite de ces composes, qui est presumee de nature non conventionnelle, est supposee influencer fortement la dependance angulaire du deuxieme champ critique h#c#2 dans le plan basal du compose hexagonal upt#3 et des composes tetragonaux uru#2si#2 et cecu#2si#2. Ainsi, la mesure de la variation angulaire de h#c#2 peut aider a distinguer le type de parametre d'ordre supraconducteur. La variation angulaire de h#c#2 dans le plan basal et le diagramme de phase anisotrope h#c#2(t) ont ete determines pour les trois composes au moyen des mesures de resistivite sous champ magnetique (orientale dans toutes les directions spatiales) et a tres basses temperatures. En ce qui concerne le compose upt#3, une contribution de la supraconductivite de surface liee a la geometrie a ete mise en evidence, dans les mesures de resistivite pour des whiskers. L'etude de nouveaux monocristaux d'upt#3 de bonne qualite, dont les transitions supraconductrices mesurees par chaleur specifique et par resistivite coincident, a permis de detecter pour la premiere fois une modulation intrinseque d'ordre six (periodicite de 60) du deuxieme champ critique dans le plan hexagonal. Cette modulation change le signe au point tetracritique du diagramme de phase. Ce resultat est une evidence directe que le parametre d'ordre supraconducteur change entre les differentes phases supraconductrices. Le deuxieme champ critique h#c#2, mesure dans le plan tetragonal des composes uru#2si#2 et cecu#2si#2, presente une modulation de h#c#2 de symetrie quatre qui n'avait jamais ete observee jusqu'a present. A premiere vue, une interpretation selon un parametre d'ordre de type d se propose, mais des efforts supplementaires sont necessaires afin d'avoir la certitude que les effets de surface ne sont pas presents dans ces faibles modulations du h#c#2
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Arndt, Julia. "Wechselspiel von Magnetismus und Supraleitung im Schwere-Fermionen-System CeCu2Si2." Doctoral thesis, Berlin Logos, 2010. http://d-nb.info/1003247911/04.
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Bittar, Eduardo Matzenbacher. "Estudo das propriedades físicas locais e globais de compostos intermetálicos cúbicos de terras raras (R) : RT3 (T=Al, Cd, In, Sn)." [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278539.
Full textAbstract:
Orientador: Pascoal Jose Giglio PagliusoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Séries de compostos que apresentam a ocorrência de fenômenos físicos interessantes em materiais estruturalmente simples constituem uma excelente oportunidade para se estudar sistematicamente a inter-relação entre tais fenômenos e suas origens microscópicas, sem se ater a efeitos associados com estruturas cristalinas complexas, de mais baixa simetria. Nesse sentido, esta Tese se propôs a estudar três temas de investigação: o sistema CeIn3-xSnx e o seu análogo não magnético LaIn3-xSnx, o sistema CeIn3-xCdx e o composto YbAl3 e o seu composto de referência LuAl3. O sistema CeIn3-xSnx apresenta um ponto crítico quântico (QCP) em x ? 0.7, onde a ordem antiferromagnética (TN = 10 K no CeIn3) é totalmente suprimida. Para 0.7 < x < 2 os compostos são líquidos de Fermi e a partir de x = 2 apresentam valência intermediária. Nesse sistema, dopado com Gd3+, medidas de ressonância de spin eletrônico (ESR) indicaram que o elétron 4f do Ce passa de um comportamento com momento localizado para itinerante com a dopagem de Sn. Os resultados de ESR do Gd3+ no sistema LaIn3-xSnx foram analisados com um modelo de uma única banda (tipo s) e sem dependência com a transferência de momento. Também investigamos o sistema CeIn3-xCdx através de medidas de susceptibilidade magnética, calor específico e resistividade elétrica. A substituição do In por Cd diminui a TN do CeIn3 e igualmente faz o coeficiente do calor específico eletrônico decrescer, indicando que o sistema não está na direção de um QCP. Os compostos YbAl3 e LuAl3, dopados com Gd3+, também foram estudados por ESR, porém, na análise, utilizou-se um modelo de múltiplas bandas para explicar os resultados. Medidas de calor específico e susceptibilidade magnética, nessas amostras, mostraram que interações de troca elétron-elétron estão presentes em ambos os compostos. Esses resultados microscópicos podem ser importantes em modelos teóricos mais realistas para materiais férmions pesados de valência intermedi'aria a base de Yb
Abstract: Series of compounds which have interesting physical phenomena, occurring in structurally simple materials, provide an excellent opportunity to systematically study the interrelationship between these phenomena and their microscopic origins, without worrying about effects associated with other complex crystal structures of lower symmetry. Thus, this thesis set out to study three research themes: the CeIn3-xSnx system and its nonmagnetic analogue LaIn3-xSnx, the CeIn3-xCdx system and the YbAl3 compound and its reference compound LuAl3. The CeIn3-xSnx system has a quantum critical point (QCP) at x ? 0.7, where the antiferromagnetic order (TN = 10 K in CeIn3) is totally suppressed. For 0.7 < x < 2 the compounds are Fermi liquids and from x = 2 the materials show intermediate valence behavior. In this system, doped with Gd3+, measurements of electron spin resonance (ESR) indicated that the Ce 4f electron presents a crossover between a localized to itinerant behavior with Sn doping. The ESR results of Gd3+ in the LaIn3-xSnx system were analyzed with a single band model (s type) and no q dependence. We also investigated the CeIn3-xCdx system by measuring magnetic susceptibility, specific heat and electrical resistivity proprieties. The substitution of In by Cd decreases the TN of CeIn3 and also decreases the electronic specific heat coefficient, indicating that the system is not being tuned to a QCP. The YbAl3 and LuAl3 compounds, doped with Gd3+, were also studied by ESR, but in the analysis, we used multiband model to explain the results. Specific heat and magnetic susceptibility measurements in these samples showed that electron-electron exchange interactions are present in both compounds. These ESR results may be relevant to construct more realistic models of the microscopic behavior of other heavy fermion-based intermediate valence Yb compounds
Doutorado
Física da Matéria Condensada
Doutor em Ciências