Dissertations / Theses on the topic 'Non-centrosymmetric'

Non-centrosymmetric superconductors (NCS) and related compounds have been studied using magnetic, specific heat and transport measurements as well as by neutron scattering and muon spin relaxation/rotation (μSR). The crystal structures of NCS lack inversion symmetry and in the presence of a finite antisymmetric spin orbit coupling, the Cooper pairs are a mixture of spin-singlet and spin-triplet states. In particular, the cerium based NCS have been reported to display unconventional superconductivity. Two different approaches for studying NCS are used. Firstly, the ground states of materials in the CeTX3 (T = transition metal, X = Si or Ge) family have been studied. CeCoGe3 is an antiferromagnet at ambient pressures and becomes superconducting at p > 4.3 GPa and was studied using inelastic neutron scattering (INS), muon spin relaxation/rotation (μSR), neutron diffraction and magnetic susceptibility measurements. The crystal electric fields (CEF) were studied using INS and magnetic susceptibility and the CEF scheme was evaluated. From this a ground state magnetic moment of 1.01 μB/Ce along the c axis was predicted. However, a magnetic moment of 0.405 μB/Ce along the c axis was observed in single crystal neutron diffraction measurements, indicating a reduced magnetic moment due to hybridization between the cerium f-electrons and the conduction band. The INS response was compared to the isostructural CePdSi3, CePtSi3 and CeRuSi3. The former two order antiferromagnetically and the Kondo temperatures were evaluated from the quasielastic scattering. CeRuSi3 is non-magnetic and there is a broad peak in the magnetic scattering at 59 meV. Another approach is to study weakly correlated NCS to look for evidence of unconventional behaviour. In particular, systems where the spin-orbit coupling can be varied by the substitution of heavier atoms into non-centrosymmetric positions were considered. LaPdSi3 and LaPtSi3 are superconductors with Tc = 2.65 and 1.52 K respectively and crystallize in the same crystal structure as the CeTX3 compounds. Magnetization, specific heat and μSR measurements reveal that both compounds are weakly coupled, fully gapped s-wave superconductors but LaPdSi3 is a type-I material while LaPtSi3 is type-II with a Ginzburg-Landau parameter of 2.49. The superconducting properties of single crystals of Nb0.18Re0.82 have been investigated and are discussed.

Terahertz (THz) spectroscopy is an emerging technology with promising applications in imaging, homeland security, and material detection and quantification. Frequencies in the THz region can be generated by optical rectification of ultrafast near-infrared laser pulses in the presence of a nonlinear optical (NLO) materials such as organic crystals. Non-centrosymmetric organic THz generating crystals such as DAST, HMQ-TMS, and OH1 have received special attention due to the strong generated fields on the order of MV/cm. The cation of these organic salts is designed by connecting electron-donating with electron-accepting groups via a highly planar aromatic system. To improve the performance of organic crystals for THz generation, the molecular hyperpolarizability (β) can be optimized by introducing modifications in the architecture of these push-pull chromophores. However, the large dipole moments associated with molecules that have a large β promote the formation of NLO inactive centrosymmetric molecular alignments in the crystal state. This dissertation provides important insights into the design of new push-pull chromophores that feature a) higher β values compared with state-of-the-art organic crystals, and b) non-centrosymmetric molecular packing in the crystalline state. The first strategy presented on this dissertation relates to the introduction of a triple bond instead of a double bond in the cation of DAST to improve the β parameter. The newly designed 4DEP core was combined with different anions to promote non-centrosymmetric molecular packing with almost ideal arrangements for THz generation. However, large single crystals were difficult to obtain and high THz generation was not achieved. The second strategy presented in this dissertation raises the value of β by extending the π-conjugation length in different cations with dimethylamino and methoxy electron-donating groups. A new molecular cation, 6MNEP, was found to have large β value combined with ideal non-centrosymmetric molecular packing. Combining these two factors, a ~ 75% higher performance for THz generation is expected for 6MNEP compared with DAST. Currently, we are testing different crystallization techniques to grow large single crystals of 6MNEP. In addition to the strategies developed to increase the β parameter value, we also introduce a new molecular modification to induce non-centrosymmetric packing in organic salt THz generating crystals. This is achieved by substituting a methyl by an ethyl group in the quaternary nitrogen of hydrogen-bonded crystals. We showed the applicability of this method for changing molecular packing in the crystal state from centrosymmetric to non-centrosymmetric in two different molecular cations. We also demonstrated the generation of strong THz fields in the novel NLO crystal EHPSI-4NBS.

Recentemente, os compostos não-centrossimétricos têm sido intensamente estudados devido à grande variedade de comportamentos magnéticos, os quais são induzidos pela ausência de centro de simetria na sua estrutura cristalográfica. Esta assimetria induz uma modificação na estrutura de bandas, causando um acoplamento spin-orbita antissimétrico quem é responsável por mecanismos de interação magnética inusuais. A supercondutividade em este tipo de compostos apresenta propriedades que divergem do comportamento esperado pela teoria BCS. Neste trabalho será apresentado um estudo sobre a investigação de supercondutividade em dois sistemas ternários, Y-Co-C e Lu-Ni-C particularmente nos compostos YCo1-xC2 e Lu1-xThNiC2. Estes compostos cristalizam na estrutura CeNiC2 de simetria ortorrômbica é do grupo espacial 38 Amm2 que não possui centro de inversão. As técnicas de caracterização usadas neste trabalho incluem medidas de magnetização, resistividade e calor especifico como funções da temperatura, assim como magnetização como função do campo magnético aplicado. O composto YCo0.7C2 é supercondutor com Tc = 4 K e exibe um comportamento que diverge da teoria BCS. As medidas realizadas neste trabalho sugerem que este material é um forte candidato como supercondutor não convencional, onde poderia existir uma mistura de contribuições nos canais singleto e tripleto. No caso do sistema Lu1-xThxNiC2 os resultados preliminares indicam claramente supercondutividade nos compostos dopados com Th, onde a composição Lu0.6Th0.4NiC2 apresenta uma Tc= 8 K.
In recent years, non-centrosymmetric compounds have attracted a great interest because of their wealth variety of topical behaviors, induced by the lack of the inversion center on the crystallographic structure. This asymmetry leads to a strong modification in the band structures, causing an antisymmetric spin-orbit coupling which is responsible for unusual magnetic interaction mechanisms. Superconductivity in compounds whose crystal structure lacks inversion symmetry are known to display intriguing properties that deviate from conventional BCS superconducting behavior. Here we report the analysis of the ternary systems Y-Co-C and Lu-Ni-C, We focused our analysis in the intermetallic compounds YCo1-xC2 and Lu1-xThxNiC2. Magnetization, resistivity, and heat capacity measurements on polycrystalline samples of noncentrosymmetric YCo0.7C2, showing clear evidence of bulk superconductivity with a critical temperature of Tc =4 K. Interestingly the specific heat of the superconducting state deviates from conventional exponential temperature dependence, which is suggestive of possible unconventional superconducting behavior in YCo0.7C2, similar to that seen in the isostructural and isoelectronic superconductor LaNiC2. Besides, these results strongly suggest that this material is a strong candidate of multiband superconductivity. In the case of the system Lu1- xThxNiC2 our results showed a clear superconducting signal in the Th doped samples, where the composition Lu0.6Th0.4NiC2 has a Tc= 8 K.

Ces travaux de thèse traitent de l'exploitation des nombreuses propriétés chimiques, physiques et structurales qu'offre le macrocycle porphyrinique.Le premier chapitre présente la sensibilisation de lanthanides émettant dans le proche-infrarouge par un chromophore porphyrinique. La synthèse d'une porphyrine substituée par deux 8-hydroxyquinoléines, sa métallation avec des lanthanides (La, Nd, Yb et Gd) ainsi que les propriétés photophysiques de ces complexes sont présentées. La synthèse de porphyrines fonctionnalisées par des oxamates orientés de façon convergente dans le but de diminuer les désactivations non radiatives du lanthanide par vibrations des liaisons est également étudiée. Le second chapitre décrit la synthèse de porphyrines substituées par deux groupements pyridyl et éthynylpyridyl en position méso-trans. La métallation du macrocyclique porphyrinique par du Zn(II) conduit à la formation de réseaux de coordination mono- ou bi-dimensionnels dont la topologie est dépendante de la nature de la porphyrine. Le troisième chapitre présente la fonctionnalisation de surface par chimi- ou par physi-sorption d'un réseau de coordination directionnel. Dans un premier temps la synthèse de porphyrines substituées par des pyridines et des terpyridines est présentée. Puis l'adsorption de ces tectons sur une surface de graphite est décrite. Enfin une dernière partie traite de la synthèse de porphyrines fonctionnalisées par deux ou quatre bras possédant des substituants soufrés, qui après chimisorption sur une surface d'or, pourraient servir de point d'ancrage à la construction d'un réseau de coordination directionnel perpendiculaire à la surface.

Thesis (Ph. D.)--York University, 2001. Graduate Programme in Chemistry.
Typescript. Includes bibliographical references (leaves 134-141). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pNQ67891.

碩士
國立交通大學
物理研究所
101
According to theoretical calculations, the asymmetric spin-orbit coupling from non-centrosymmetric structure may lead to an order parameter of two gaps or an anisotropic gap. Low temperature specific heat C(T,H) of Re24Ti5, Re21.75Ta7.25 and Re24.9Hf4.1 were investigated, and we fitted their superconducting electronic specific heat by various order parameter symmetries. These compounds are the of non-centrosymmetric superconductors with α-Mn structure. The specific heat can be well described within the context of an isotropic s-wave for all three compounds, and the 2∆⁄(k_B T_c ) is equal to 3.68 ± 0.01 for Re24Ti5, 3.70 ± 0.07 for Re21.75Ta7.25 and 3.69 for Re24.9Hf4.1. The results of ∆C/(γ_n T_c), 2∆⁄(k_B T_c ) and electron-phonon coupling λ_ep indicate a moderate coupling in these compounds. For Re24.9Hf4.1, the upper critical field H_c2 (0)=9.01±0.22 T was estimated from the Werthamer-Helfand-Hohenberg (WHH), consistent with H_c2 (0)=9.84±0.60 T from γ(H). For two-gap fitting results, the results of Re24Ti5 are (2∆_L)⁄(k_B T_c=) 3.87±.07 and (2∆_S)⁄(k_B T_c=) 3.50±0.06, and those of Re24.9Hf4.1 are (2∆_L)⁄(k_B T_c=) 3.97±0.04 and (2∆_S)⁄(k_B T_c=) 3.41±0.04. As for the fitting of the anisotropic gap, all values of 2∆⁄(k_B T_c ) are the same as those with s-wave, but the individual gap anisotropic α is 0.070 ± 0.025 for Re24Ti5, 0.149 ± 0.033 for Re21.75Ta7.25 and 0.106 ± 0.015 for Re24.9Hf4.1, respectively. Although we found these superconductors are within at the dirty limit, the existence of p-wave order parameter in Re21.75Ta7.25 and Re24.9Hf4.1 cannot be totally excluded considering the case of the two-gap superconductor MgB2 which is also at the dirty limit.

Despite its history of more than one hundred years, the phenomenon of superconductivity has not lost any of its allure. During that time the concept and perception of the superconducting state - both from an experimental and theoretical point of view - has evolved in way that has triggered increasing interest. What was initially believed to simply be the disappearance of electrical resistivity, turned out to be a universal and inevitable result of quantum statistics, characterized by many more aspects apart from its zero resistivity. The insights of BCS-theory eventually helped to uncover its deep connection to particle physics and consequently led to the formulation of the Anderson-Higgs-mechanism. The very core of this theory is the concept of gauge symmetry (breaking). Within the framework of condensed-matter theory, gauge invariance is only one of several symmetry groups which are crucial for the description and classification of superconducting states. \\ In this thesis, we employ time-reversal, inversion, point group and spin symmetries to investigate and derive possible Hamiltonians featuring spin-orbit interaction in two and three spatial dimensions. In particular, this thesis aims at a generalization of existing numerical concepts to open up the path to spin-orbit coupled (non)centrosymmetric superconductors in multi-orbital models. This is done in a two-fold way: On the one hand, we formulate - based on the Kohn-Luttinger effect - the perturbative renormalization group in the weak-coupling limit. On the other hand, we define the spinful flow equations of the effective action in the framework of functional renormalization, which is valid for finite interaction strength as well. Both perturbative and functional renormalization groups produce a low-energy effective (spinful) theory that eventually gives rise to a particular superconducting state, which is investigated on the level of the irreducible two-particle vertex. The symbiotic relationship between both perturbative and functional renormalization can be traced back to the fact that, while the perturbative renormalization at infinitesimal coupling is only capable of dealing with the Cooper instability, the functional renormalization can investigate a plethora of instabilities both in the particle-particle and particle-hole channels. \\ Time-reversal and inversion are the two key symmetries, which are being used to discriminate between two scenarios. If both time-reversal and inversion symmetry are present, the Fermi surface will be two-fold degenerate and characterized by a pseudospin degree of freedom. In contrast, if inversion symmetry is broken, the Fermi surface will be spin-split and labeled by helicity. In both cases, we construct the symmetry allowed states in the particle-particle as well as the particle-hole channel. The methods presented are formally unified and implemented in a modern object-oriented reusable and extendable C++ code. This methodological implementation is employed to one member of both families of pseudospin and helicity characterized systems. For the pseudospin case, we choose the intriguing matter of strontium ruthenate, which has been heavily investigated for already twenty-four years, but still keeps puzzling researchers. Finally, as the helicity based application, we consider the oxide heterostructure LaAlO$_{3}$/SrTiO$_{3}$, which became famous for its highly mobile two- dimensional electron gas and is suspected to host topological superconductivity
Trotz seiner über hundertjährigen Geschichte seit seiner Entdeckung hat das Phänomen der Supraleitung nichts von seiner ursprünglichen Faszination eingebüßt. Vielmehr hat sich in der Zwischenzeit der Begriff und das Verständnis des supraleitenden Zustandes in einer Weise weiterentwickelt, die das Interesse daran eher hat zunehmen lassen. Was anfänglich ausschließlich für ein Verschwinden des elektrischen Widerstands gehalten wurde, ist tatsächlich ein universelles und unvermeidliches Resultat der Quantenstatistik und besitzt viel mehr bemerkenswerte Eigenschaften als nur den widerstandslosen elektrischen Transport. Die Erkenntnisse der BCS-Theorie haben schließlich dazu geführt die tiefe Verbindung zur Teilchenphysik zu offenbaren und trugen entscheidend zur Formulierung des Anderson-Higgs-Mechanismus bei. Der wichtigste Baustein dieser Theorie ist das Konzept der (Brechung der) Eichsymmetrie. Im Rahmen der Festkörperphysik ist die Eichsymmetrie nur eine von mehreren Symmetrien, die eine essentielle Rolle für die Beschreibung und Einordnung von Phänomenen der Supraleitung spielen. \\ In dieser Arbeit wenden wir Zeitumkehr-, (räumliche) Inversions-, Punktgruppen- und Spin-Symmetrien an, um mögliche Hamilton-Operatoren in zwei und drei räumlichen Dimensionen, welche Spin-Bahn-Kopplung enthalten, herzuleiten und zu untersuchen. Diese Arbeit zielt auf eine Verallgemeinerung von existierenden numerischen Konzepten ab und erschließt den Weg die supraleitenden Eigenschaften von Modellen mit starker Spin-Bahn-Kopplung und mit oder ohne Inversionszentrum zu untersuchen. Dies geschieht mit Hilfe zweier methodischer Ansätze. Erstens formulieren wir aufbauend auf dem Kohn-Luttinger Effekt die störungstheoretische Renormierungsgruppe im Limes schwacher Kopplung. Zweitens verwenden wir die spinaufgelösten Flussgleichungen der effektiven Wirkung im Rahmen der funktionalen Renormierungsgruppe, die auch für endliche Wechselwirkungsstärke gültig sind. Die symbiotische Ergänzung der perturbativen und funktionalen Renormierungsgruppen ist darauf zurückzuführen, dass es mit der perturbativen Methode zwar möglich ist die Cooper Instabilität bei infinitesimaler Wechselwirkung numerisch exakt zu berechnen, aber nur die funktionale Renormierungsgruppe auch Teilchen-Loch Kondensate zugänglich macht. \\ Zeitumkehr- und Inversionssymmetrie sind die beiden Schlüsselsymmetrien, die verwendet werden, um zwei Szenarien zu unterscheiden. Falls sowohl Zeitumkehr- als auch Inversionssymmetrie gültig sind, sind die Fermiflächen zweifach entartet und durch einen Pseudospin-Freiheitsgrad charakterisiert. Im Gegensatz dazu führt der Verlust der Inversionssymmetrie zur Spinaufspaltung der Fermiflächen, die dann durch die sogenannte Helizität gekennzeichnet sind. In beiden Fällen leiten wir alle symmetrie-erlaubten Zustände her, welche die entsprechenden Teilchen-Teilchen und Teilchen-Loch Kondensate beschreiben. Die vorstellten und verallgemeinerten Methoden sind im Rahmen dieser Arbeit formal miteinander verbunden und in einem modernen objektorientierten C++ Quellcode implementiert worden. \\ Als erste vorläufige Anwendungen für diese methodische Implementierung betrachten wir zwei Systeme, die jeweils einer der beiden Familien zugeordnet werden können. Zum einen berechnen wir in der Pseudospin-Formulierung der perturbativen und funktionalen Renormierungsgruppen die Instabilitäten eines Dreiorbital-Modells für Strontiumruthenat, das seit seiner erstmaligen Synthese trotz intensiver Forschung immer noch Rätsel aufgibt. Zum anderen betrachten wir das zweidimensionale Elektronengas, das sich an der Schnittstelle zwischen LaAlO$_{3}$ und SrTiO$_{3}$ bildet und welches durch seine hohe Ladungsträgermobilität bekannt geworden ist

Title: Magnetism in the non-centrosymmetric uranium compound: UIrSi3 Author: RNDr. Jaroslav Valenta Department: Department of Condensed Matter Physics Supervisor of the doctoral thesis: RNDr. Jiří Prchal, Ph.D., DCMP Abstract: The study presented in this thesis is focused on UIrSi3 which is one of two known uranium compounds that crystallize in a non-centrosymmetric structure. This first study of UIrSi3 in single-crystalline form confirms antiferromagnetic order below TN = 41.7 K and reveals strong uniaxial anisotropy. The antiferromagnetic order is suppressed by application of a magnetic field along the c axis (0HC = 7.3 T at 2 K). In contrast, application of a magnetic field up to 14 T along the a axis gives paramagnetic response. The high TN, contrasting with the low 0HC, together with the opposite signs of the paramagnetic Curie temperatures of the a-axis and c-axis paramagnetic susceptibility indicate a competition between antiferromagnetic and ferromagnetic interactions. The first-order metamagnetic transition at HC shows asymmetric hysteresis which reflects a complex antiferromagnetic ground state. With increasing temperature, the hysteresis becomes gradually smaller and vanishes at 28 K where the first-order transition changes to second-order transition which remains up to TN. The point where the...

碩士
國立臺灣大學
電子工程學研究所
100
The thesis discussed the higher order modes enhancement on non-centrosymmetric, double apertures and bowtie-like hole array by experimental and simulation results. Considering the extraordinary transmission on these two types hole array, higher order modes were measured and calculated directly. In non-centrosymmetric hole array, trumpet shape hole array was investigated in the relation between main peaks and length of X. By varying the periodic parameter Px, the interaction between Wood’s anomaly and LSRs were shown clearly. Also, double LSRs are shown in MIM structure in shorter and longer wavelength. Double apertures sample perforated with Ag film were investigated. Besides the strong enhancement in bowtie structure [86], the experiment provide a novel way to stimulate higher order modes enhancement in different geometric figures such as double circles and double pentagons. Within double apertures structure such as circles and square, experiment results show the strong enhancement on (3,0) and (4,0) Ag/Si mode. Bowtie-like samples also provide higher order mode transmission. By replacing one of the bowtie triangles with circle, square, and rotation triangle, experiments show that the second order mode still being stimulated highly than fundamental mode. Also, double bowtie samples in orthogonal direction were investigated. Experiment results show that (2,2) Ag/Si mode stimulated more than fundamental mode and the (2,0) Ag/Si mode.

Dans la première partie, nous présentons les résultats de l'étude du supraconducteur sans inversion de symétrie LaRhSi3 par spectroscopie muonique. En champ nul, nous n'avons pas détecté de champ interne. Ceci indique que la fonction d'onde de l'état supraconducteur n'est pas dominée par l'état triplet. Les mesures en champ transverse de 35G présentent une transition en accord avec la transition de phase attendue sous le champ critique Hc1. Nous avons répété ces mesures pour un champ entre Hc1 et Hc2, 150G. Le spectre obtenu pour ces mesures conserve l'asymétrie et relaxe rapidement à basse température tel que prédit pour un supraconducteur dans la phase d'Abrikosov. Néanmoins, les relaxations produites par ce balayage en température présentent une transition à près de 2 fois la température critique attendue. Dans la deuxième partie de ce mémoire, nous donnons l'interprétation des résultats de la diffraction neutronique inélastique par l'étude des champs électriques cristallins. Ces mesures ont été effectuées sur des aimants frustrés SrHo2O4 et SrDy2O4 sous la forme de poudre. L'étude des niveaux produits par les champs cristallins par la méthode des opérateurs de Stevens indique une perte du moment cinétique dans les deux matériaux. Pour le SrDy2O4, l'état fondamental serait constitué de quatre états dégénérés quasi accidentellement qui portent un moment magnétique total non-nul. Toute fois, nos mesures de susceptibilité magnétique ne montrent aucun ordre au-dessus de 50mK. Pour le SrHo2O4, le fondamental est formé d'une paire accidentelle. Nous obtenons un moment magnétique de 6.94(8)$\mu_B$ ce qui s'accorde avec les données expérimentales.
In the first part of this thesis, we present our muon spectroscopy results of the non-centrosymmetric superconductor LaRhSi3. Zero magnetic field measurements showed no internal field, suggesting at most a very small triplet component in the superconducting wave function. A temperature scan taken in 35G transverse-field geometry showed a phase transition at Hc1. The asymmetry of the muon relaxation spectrum measured at 20mK and field of 150G, which is between the lower and upper critical fields, shows a faster relaxation rate compared to the spectrum obtained at 4K, a behaviour expected in the presence of vortices in the type II superconductor. However, the temperature dependence of the relaxation rate begins to increase all the way to a temperature twice Tc of LaRhSi3 in a field of 150G. In the second part of this document, we present the interpretation of inelastic neutron scattering by crystalline electric fields. These measurements were carried out on powder samples of frustrated quantum magnets SrHo2O4 and SrDy2O4. Using Stevens operators method, we determined the level scheme du to crystalline electric field effects using a point charge model and fitted it to the experimental data. For the SrDy2O4 we found a ground state that appears to be composed of four quasi-accidental degenerate states that carry a non zero magnetic moment even though susceptibility measurements did not indicate magnetic order above 50mK. For the SrHo2O4, the ground state is a pair with a 6.98(8)$\mu_B$ magnetic moment. This lat value fots with experimental data.