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1
Mottaghizadeh, Alireza. "Non-conventional insulators : metal-insulator transition and topological protection." Electronic Thesis or Diss., Paris 6, 2014. http://www.theses.fr/2014PA066652.
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Ce manuscrit présente une étude expérimentale de phase isolante non-conventionnelle, l'isolant d'Anderson, induit par le désordre, l'isolant de Mott, induit par les interactions de Coulomb, et les isolants topologiques.Dans une première partie du manuscrit, je décrirais le développement d'une méthode pour étudier la réponse de charge de nanoparticules par Microscopie à Force Electrostatique (EFM). Cette méthode a été appliquée à des nanoparticules de magnétite (Fe3O4), un matériau qui présente une transition métal-isolant, i.e. la transition de Verwey, lors de son refroidissement en dessous d'une température TV~120 K.Dans une seconde partie, ce manuscrit présente une étude détaillée de l'évolution de la densité d'états au travers de la transition métal-isolant entre un isolant de type Anderson-Mott et une phase métallique dans le matériau SrTiO3, et ceci, en fonction de la concentration de dopants, les lacunes d'oxygènes. Nous avons trouvé que dans un dispositif memoresistif de type Au-SrTiO3-Au, la concentration de dopants pouvait être ajustée par migration des lacunes d'oxygènes à l'aide d'un champ. Dans cette jonction tunnel, l'évolution de la densités d'états au travers de la transition métal-isolant peut être étudiée de façon continue. Finalement, dans une troisième partie, le manuscrit présente le développement d'une méthode pour la microfabrication d'anneaux de Aharonov-Bohm avec l'isolant topologique, Bi2Se3, déposée par épitaxie à jet moléculaire. Des résultats préliminaires sur les propriétés de transport quantique de ces dispositifs seront présentés<br>This manuscript presents an experimental study of unconventional insulating phases, which are the Anderson insulator, induced by disorder, the Mott insulator, induced by Coulomb interactions, and topological insulators.In a first part of the manuscript, I will describe the development of a method to study the charge response of nanoparticles through Electrostatic Force Microscopy (EFM). This method has been applied to magnetite Fe3O4 nanoparticles, a material that presents a metal-insulator transition, i.e. the Verwey transition, upon cooling the system below a temperature Tv=120K. In a second part, this manuscript presents a detailed study of the evolution of the Density Of States (DOS) across the metal-insulator transition between an Anderson-Mott insulator and a metallic phase in the material SrTiO3 and this, as function of dopant concentration, i.e. oxygen vacancies. We found that in this memristive type device Au-SrTiO3-Au, the dopant concentration could be fine-tuned through electric-field migration of oxygen vacancies. In this tunnel junction device, the evolution of the DOS can be followed continuously across the metal-insulator transition. Finally, in a third part, the manuscript presents the development of a method for the microfabrication of Aharonov-Bohm rings with the topological insulator material, Bi2Se3, grown by molecular beam epitaxy. Preliminary results on the quantum transport properties of these devices will be presented
2
Mottaghizadeh, Alireza. "Non-conventional insulators : metal-insulator transition and topological protection." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066652/document.
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Ce manuscrit présente une étude expérimentale de phase isolante non-conventionnelle, l'isolant d'Anderson, induit par le désordre, l'isolant de Mott, induit par les interactions de Coulomb, et les isolants topologiques.Dans une première partie du manuscrit, je décrirais le développement d'une méthode pour étudier la réponse de charge de nanoparticules par Microscopie à Force Electrostatique (EFM). Cette méthode a été appliquée à des nanoparticules de magnétite (Fe3O4), un matériau qui présente une transition métal-isolant, i.e. la transition de Verwey, lors de son refroidissement en dessous d'une température TV~120 K.Dans une seconde partie, ce manuscrit présente une étude détaillée de l'évolution de la densité d'états au travers de la transition métal-isolant entre un isolant de type Anderson-Mott et une phase métallique dans le matériau SrTiO3, et ceci, en fonction de la concentration de dopants, les lacunes d'oxygènes. Nous avons trouvé que dans un dispositif memoresistif de type Au-SrTiO3-Au, la concentration de dopants pouvait être ajustée par migration des lacunes d'oxygènes à l'aide d'un champ. Dans cette jonction tunnel, l'évolution de la densités d'états au travers de la transition métal-isolant peut être étudiée de façon continue. Finalement, dans une troisième partie, le manuscrit présente le développement d'une méthode pour la microfabrication d'anneaux de Aharonov-Bohm avec l'isolant topologique, Bi2Se3, déposée par épitaxie à jet moléculaire. Des résultats préliminaires sur les propriétés de transport quantique de ces dispositifs seront présentés<br>This manuscript presents an experimental study of unconventional insulating phases, which are the Anderson insulator, induced by disorder, the Mott insulator, induced by Coulomb interactions, and topological insulators.In a first part of the manuscript, I will describe the development of a method to study the charge response of nanoparticles through Electrostatic Force Microscopy (EFM). This method has been applied to magnetite Fe3O4 nanoparticles, a material that presents a metal-insulator transition, i.e. the Verwey transition, upon cooling the system below a temperature Tv=120K. In a second part, this manuscript presents a detailed study of the evolution of the Density Of States (DOS) across the metal-insulator transition between an Anderson-Mott insulator and a metallic phase in the material SrTiO3 and this, as function of dopant concentration, i.e. oxygen vacancies. We found that in this memristive type device Au-SrTiO3-Au, the dopant concentration could be fine-tuned through electric-field migration of oxygen vacancies. In this tunnel junction device, the evolution of the DOS can be followed continuously across the metal-insulator transition. Finally, in a third part, the manuscript presents the development of a method for the microfabrication of Aharonov-Bohm rings with the topological insulator material, Bi2Se3, grown by molecular beam epitaxy. Preliminary results on the quantum transport properties of these devices will be presented
3
Vale, J. G. "The nature of the metal-insulator transition in 5d transition metal oxides." Thesis, University College London (University of London), 2017. http://discovery.ucl.ac.uk/1538695/.
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A number of 5d transition metal oxides (TMOs) either undergo, or lie proximate to, a metal-insulator transition (MIT). However these MITs frequently depart from a Mott-Hubbard picture, in which the interactions are dominated by the interplay between the on-site Coulomb repulsion and electronic bandwidth. In 5d TMOs the sizeable intrinsic spin-orbit coupling plays an important role, and gives rise to electronic and magnetic ground states -- at both sides of the MIT -- that cannot be adequately described within a purely L-S coupling scenario. In this thesis the aim is to understand the role of spin-orbit coupling in determining the electronic and magnetic properties of 5d TMOs. There has been a large amount of recent interest within this field (both experimentally and theoretically), however thus far has mostly been limited to the 5d5, j =1/2 limit. The perovskite iridates Sr2IrO4 and Sr3Ir2O7 lie within this limit. Theoretical predictions suggest a significant easy-plane anisotropy is present for the single layer Sr2IrO4. I show that this anisotropy can be observed and quantified, using magnetic critical scattering and previously published resonant inelastic X-ray scattering (RIXS) data. This differs from previous results that suggest purely 2D Heisenberg behaviour. Meanwhile the critical fluctuations in bilayer Sr3Ir2O7 have a three-dimensional nature, which can be directly related to the intra-bilayer coupling and significant anisotropy previously probed by RIXS. I also demonstrate that resonant X-ray scattering techniques can be successfully applied to other 5d systems, especially the d3 osmates. Both NaOsO3 and Cd2Os2O7 undergo MITs directly linked to the onset of antiferromagnetic order (Slater or Lifshitz mechanisms). The first ever high-resolution RIXS measurements at the Os L3 absorption edge reveal that there is a correlated evolution of the electronic and magnetic excitations through the respective MITs. The behaviour is consistent with a scenario in which the effect of spin-orbit coupling and electron correlations are reduced with respect to the iridates, yet still manifests through a strong spin wave anisotropy. Finally I show that the study of 5d TMOs can be extended into the time domain. Through the development of new instrumentation, the transient dynamics of photo-doped Sr2IrO4 were probed by time-resolved resonant (in)elastic X-ray scattering. The relevant time scales can be directly compared to the interaction strengths and anisotropies in the undoped state. Moreover, there seems to be an effective mapping of the transient behaviour in the photo-doped state to an equivalent level of bulk electron doping in (Sr_{1-x}La_x)2IrO4.
4
Milde, Frank. "Disorder induced metal insulator transition in anisotropic systems." Doctoral thesis, [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=963658441.
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Villagonzalo, Cristine. "Thermoelectric Transport at the Metal-Insulator Transition in Disordered Systems." Doctoral thesis, Universitätsbibliothek Chemnitz, 2001. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200100602.
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This dissertation demonstrates the behavior of the electronic
transport properties in the presence of a temperature gradient in
disordered systems near the metal-insulator transition. In
particular, we first determine the d.c. conductivity, the
thermopower, the thermal conductivity, the Lorenz number, the figure
of merit, and the specific heat of a three-dimensional Anderson model
of localization by two phenomenological approaches. Then we also compute
the d.c. conductivity, the localization length and the Peltier
coefficient in one dimension by a new microscopic approach based on
the recursive Green's functions method. A fully analytic study is
difficult, if not impossible, due to the problem of treating the
intrinsic disorder in the model, as well as, incorporating a
temperature gradient in the Hamiltonian. Therefore, we resort to
various numerical methods to investigate the problem.
6
Asal, Rasool Abid. "The metal-insulator transition in the amorphous silicon-nickel system." Thesis, University of Leicester, 1993. http://hdl.handle.net/2381/35586.
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Amorphous thin films of Si1-yNiy:H have been prepared over a wide range of compositions by radio-frequency sputtering in an argon/hydrogen plasma and their properties studied by various techniques. Transmission electron microscope investigations confirmed that the films were amorphous and the composition of the films was determined by EDAX. The principal object of the study is to investigate the nature of the semiconductor-metal transition in the a-Si1-yNiy:H system. The system has been shown to exhibit a semiconductor-to-metal transition as a function of concentration at approximately y = 0.26 at which value the optical gap shrinks to zero and beyond which the reflectivity falls with increasing photon energy in the region 0.5 - 2 eV, i.e becomes Drude-like. D.C. electrical conductivity measurements as a function of temperature show an increase in conductivity and a decrease in activation energy with increasing nickel content which is close to zero for y 0.26. The optical joint density of states (OJDOS) is finite at all energies for y ~ 0.26, confirming the existence of overlap between the conduction and valence bands. Pressure-induced transitions from semiconductor-to-metallic behaviour of the a-Si1-yNiy:H films have been investigated by measurements of the optical absorption edge as a function of pressure in a diamond anvil cell and by measurements of the electrical conductivity in a Bridgman opposed-anvil apparatus, both at room temperature. The optical gap decreases with increasing pressure, becoming zero at pressures that are lower the higher the nickel content. The electrical conductivity increases with applied pressure for all samples studied, reaching a saturation value close to the Mott's minimum metallic conductivity; this also occurs at lower values of the pressure for films with higher nickel content. Information on the structure and the local bonding configurations for the a-Si1-yNiy:H films was obtained from EXAFS and IR measurements. The results indicate that there is a significant change in the local environment of the Ni atoms as their concentration is changed but the system appears to favour chemical ordering.
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Madaras, Scott. "Insulator To Metal Transition Dynamics Of Vanadium Dioxide Thin Films." W&M ScholarWorks, 2020. https://scholarworks.wm.edu/etd/1616444322.
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Vanadium Dioxide (VO2) is a strongly correlated material which has been studied for many decades. VO2 has been proposed for uses in technologies such as optical modulators, IR modulators, optical switches and Mott memory devices. These technologies are taking advantage of VO2’s insulator to metal transition (IMT) and the corresponding changes to the optical and material properties. The insulator to metal transition in VO2 can be accessed by thermal heating, applied electric field, or ultra-fast photo induced processes. Recently, thin films of VO2 grown on Titanium Dioxide doped with Niobium (TiO2:Nb), have shown promise as a possible UV photo detector with high quantum efficiency which utilizes a heterostructure between these two materials. In this work, the dynamics of the IMT on thin films of VO2 is explored. We show that surface plasmons generated in an Au thin film can induce the insulator to metal transition in a thin film of VO2 due to the enhanced electric field as well as help detect the IMT via changes in its resonance condition. Time resolved pump probe studies were also done on thin films of VO2 grown on TiO2 and TiO2:Nb, using UV photon energy of 3.1 eV (400nm wavelength). The fluence threshold of the IMT at 3.1 eV was significantly lower than published values for the 1.55 eV pump fluence. The time response of the IMT shows uncommon reflectivity dynamics in these samples. The response was partially attributed to internal interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2, and can be elucidated by a diffusion model with respect to its optical properties. Finally, the photocurrent generation time constants for the sample with highest quantum efficiency are given and compared to its ultrafast photo induced IMT time constants.
8
Collins-McIntyre, Liam James. "Transition-metal doped Bi2Se3 and Bi2Te3 topological insulator thin films." Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:480ea55a-5cac-4bab-a992-a3201f10f4c5.
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Topological insulators (TIs) are recently predicted, and much studied, new quantum materials. These materials are characterised by their unique surface electronic properties; namely, behaving as band insulators within their bulk, but with spin-momentum locked surface or edge states at their interface. These surface/edge crossing states are protected by the underlying time-reversal symmetry (TRS) of the bulk band structure, leading to a robust topological surface state (TSS) that is resistant to scattering from impurities which do not break TRS. Their surface band dispersion has a characteristic crossing at time reversal invariant momenta (TRIM) called a Dirac cone. It has been predicted that the introduction of a TRS breaking effect, through ferromagnetic order for instance, will open a band-gap in this Dirac cone. It can be seen that magnetic fields are not time reversal invariant by considering a solenoid. If time is reversed, the current will also reverse in the solenoid and so the magnetic field will also be reversed. So it can be seen that magnetic fields transform as odd under time reversal, the same will be true of internal magnetisation. By manipulating this gapped surface state a wide range of new physical phenomena are predicted, or in some cases, already experimentally observed. Of particular interest is the recently observed quantum anomalous Hall effect (QAHE) as well as, e.g., topological magneto-electric effect, surface Majorana Fermions and image magnetic monopoles. Building on these novel physical effects, it is hoped to open new pathways and device applications within the emerging fields of spintronics and quantum computation. This thesis presents an investigation of the nature of magnetic doping of the chalcogenide TIs Bi<sub>2</sub>Se<sub>3</sub> and Bi<sub>2</sub>Te<sub>3</sub> using 3d transition-metal dopants (Mn and Cr). Samples were grown by molecular beam epitaxy (MBE), an ideal growth method for the creation of high-quality thin film TI samples with very low defect densities. The grown films were investigated using a range of complementary lab-based and synchrotron-based techniques to fully resolve their physical structure, as well as their magnetic and electronic properties. The ultimate aim being to form a ferromagnetic ground state in the insulating material, which may be expanded into device applications. Samples of bulk Mn-doped Bi<sub>2</sub>Te<sub>3</sub> are presented and it is shown that a ferromagnetic ground state is formed below a measured T<sub>C</sub> of 9-13 K as determined by a range of experimental methodologies. These samples are found to have significant inhomogeneities within the crystal, a problem that is reduced in MBE-grown crystals. Mn-doped Bi<sub>2</sub>Se<sub>3</sub> thin films were grown by MBE and their magnetic properties investigated by superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). These reveal a saturation magnetisation of 5.1 μ<sub>B</sub>/Mn and show the formation of short-range magnetic order at 2.5 K (from XMCD) with indication of a ferromagnetic ground state forming below 1.5 K. Thin films of Cr-doped Bi<sub>2</sub>Se<sub>3</sub> were grown by MBE, driven by the recent observation of the QAHE in Cr-doped (Bi<sub>1−x</sub>Sb<sub>x</sub>)<sub>2</sub>Te<sub>3</sub>. Investigation by SQUID shows a ferromagnetic ground state below 8.5 K with a saturation magnetisation of 2.1 μ<sub>B</sub>/Cr. Polarised neutron reflectometry shows a uniform magnetisation profile with no indication of surface enhancement or of a magnetic dead layer. Further studies by extended x-ray absorption fine structure (EXAFS) and XMCD elucidate the electronic nature of the magnetic ground state of these materials. It is found that hybridisation between the Cr d and Se p orbitals leads to the Cr being divalent when doping on the Bi<sup>3+</sup> site. This covalent character to the electronic structure runs counter to the previously held belief that divalent Cr would originate from Cr clusters within the van der Waals gap of this material. The work overall demonstrates the formation of a ferromagnetic ground state for both Cr and Mn doped material. The transition temperature, below which ferromagnetic order is achieved, is currently too low for usable device applications. However, these materials provide a promising test bed for new physics and prototype devices.
9
Ho, Kai-Chung. "Monte carlo studies of metal-insulator transition in granular system /." View Abstract or Full-Text, 2002. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202002%20HO.
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Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2002.<br>Includes bibliographical references (leaves 47-48). Also available in electronic version. Access restricted to campus users.
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Lam, Jennifer. "The nature of the metal-insulator transition in SiGe quantum wells." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq20977.pdf.
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Carnio, Edoardo. "The metal-insulator transition in doped semiconductors : an ab initio approach." Thesis, University of Warwick, 2018. http://wrap.warwick.ac.uk/106449/.
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In this thesis we study the Anderson metal-insulator transition starting from an atomistically correct ab initio description of a doped semiconductor. In particular, we use density functional theory to simulate model systems of sulphur-doped silicon (Si:S) with few impurities in a large cell. From the resulting Kohn-Sham Hamiltonian, we build an effective tight-binding Hamiltonian for larger systems with an arbitrary number of dopants. Our effective model assumes the same potential around single and paired impurities, for up to ten nearest neighbours and disregarding configurations of three and more close impurities. We generate up to a thousand disorder realisations for systems of 16 3 to 22 3 atoms and a large range of impurity concentrations. From the diagonalisation of these realisations we study the formation of an impurity band in the band gap of the host semiconductor. With increasing impurity concentration, this band undergoes an Anderson metal-insulator transition, namely (i) it approaches and merges with the conduction band and (ii) its states delocalise starting from the band centre. From the multifractal fluctuations of the wave functions near criticality, we characterise the Anderson transition in terms of its critical concentration nc and exponent. We identify two regimes: for energies in a “hybridization region”, where the conduction band seems to influence the impurity band, we observe an increase from v ≈ 0:5 to v ≈ 1, compatibly with the experimental values; deeper in the band, instead, the estimates of v fluctuate between 1 and 1:5, compatibly with v ≈ 1:59 (v ≈1:3) found in the Anderson model without (with) electron-electron interactions. Our results suggest a possible resolution of the long-standing exponent puzzle due to the interplay between conduction and impurity states.
12
Lewalle, Alexandre. "Metallic behaviour and the metal-insulator transition in two-dimensional systems." Thesis, University of Cambridge, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.619842.
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Pounder, Neill Malcolm. "The electrical transport properties of niobium-silicon amorphous alloys." Thesis, University of Leeds, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305616.
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Lam, Jennifer Eleanor. "The nature of the metal-insulator transition in silicon germanide quantum wells." Thesis, University of Ottawa (Canada), 1997. http://hdl.handle.net/10393/4399.
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A study of the temperature dependence of the resistivity of gated SiGe quantum well structures has revealed a metal-insulator transition as a function of carrier density at zero magnetic field. Although early scaling theories (Abrahams et al., 1979) have argued against the existence of a metal-insulator transition at zero temperature in infinite 2D and 1D systems, more recent theoretical results using a random set of two-dimensional point potentials have shown that such a transition is allowed in two dimensions (Az'bel, 1992). Mounting experimental evidence for such a transition in 2D systems with short range scattering has accumulated in both semiconducting and superconducting structures (Kravchenko et al., 1995, and others). Pseudomorphic, CVD-grown p-type Si/Si$\sb{0.87}$Ge$\sb{0.13}$/Si quantum wells of various widths (65-200 A) have been studied. The samples were gated using a Ti-Au Schottky gate to allow for carrier density variation. Measurement of the transport to quantum lifetime ratio indicates that the transport is dominated by short range scattering. In the temperature range from 400 mK - 4.2 K, the temperature dependence shows a transition from a metallic phase in the high density regime to an insulating phase in the low density regime with a transition boundary close to 2.2 $\times$ 10$\sp $ cm$\sp{-2}$. The scaling properties of the observed metal-insulator transition will be discussed, and compared to previous scaling results from silicon MOSFETs. Below 400 mK, the onset of another transition is accompanied by a sharp drop in resistivity with temperature followed by a monotonic decrease in resistivity below 115 mK. The phase diagram was explored using temperature and density dependences of the current-voltage characteristics.
15
Bilewska, Katarzyna. "Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films." Doctoral thesis, Katowice : Uniwersytet Śląski, 2019. http://hdl.handle.net/20.500.12128/12529.
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Przejście metal-izolator jest jednym z najciekawszych zjawisk, którymi zajmuje się fizyka ciała stałego. Nie tylko ze względu na możliwe zastosowania tego zjawiska, ale przede wszystkim ze względu na samą jego fizykę, ogromna ilość prac jest poświęcona badaniu go. Geneza i kluczowe czynniki jego występowania takie jak odpychanie kulombowskie, rozszczepienie spin-orbita czy inne, różnią się znacznie w zależności od rozważanego materiału. Pośród dużej grupy materiałów nikielaty ziem rzadkich, obiekt badań tej pracy, reprezentują tylko jeden rodzaj przejścia.
W tych materiałach przejście metal-izolator czeka jeszcze na pełne wyjaśnienie. Obecny opis uwzględnia nierównowagowe rozłożenie ładunku (charge disproportionation) na jonach niklu, co z kolei wpływa na przekrywanie się pasm niklu i tlenu a w związku z tym na przewodnictwo elektryczne w materiale. Zaobserwowano spójną zmienność właściwości elektrycznych między członkami rodziny nikielatów w odniesieniu do pierwiastka ziem rzadkich. Zmiany te związane są jednocześnie z poziomem zniekształceń strukturalnych. Jeśli zniekształcenie strukturalne jest siłą napędową przejścia, można również wykazać jego wpływ w odniesieniu do wymiarów materiału. Teza dyskutowana w niniejszej rozprawie zakłada, że nanoskopowe wymiary nikielatu samarowego wpływają na jego strukturę krystaliczną, a co za tym idzie, na strukturę elektronową i przejście metal-izolator. Ta korelacja jest możliwym punktem wyjścia do opracowania teorii dotyczącej przejścia metal-izolator w cienkich warstwach.
Postawiona teza jest weryfikowana poprzez badanie szeregu materiałów tlenków samarowo-niklowych w postaci cienkich warstw napylanych magnetronowo, o zmiennej grubości i zmiennych warunkach syntezy. Otrzymano prawie amorficzne, polikrystaliczne oraz silnie steksturowane warstwy o grubości od 20 do 500 nm. Do scharakteryzowania materiałów i zbadania przejścia metal-izolator zastosowano zarówno techniki pomiarowe powierzchniowo czułe, jak i metody objętościowe. Badania utrudniało jednak zanieczyszczenie powierzchni tlenkiem chromu wynikające z niezbędnej końcowej obróbki termicznej.
Pomimo tych trudności udało się zweryfikować występowanie przejścia metal-izolator. Zjawisko to jest skorelowane z uzyskanymi widmami Ramana i zmianami temperatury pozycji modów ramanowskich. Zmiany zaobserwowane w powłokach polikrystalicznych były w ogólności mniejsze i mniej wyraźne niż w przypadku warstw z preferowaną orientacją. Ponadto, chociaż warstwy amorficzne nie wykazywały prawidłowej struktury krystalicznej i modów Ramana odpowiednich dla rombowej struktury niekielatu, zauważono zmianę struktury elektronowej w szerszym zakresie temperatur. W rzeczywistości wszystkie dane uzyskane za pomocą spektroskopii fotoelektronów rentgenowskich sugerują, że w każdej badanej próbce zachodzą zmiany struktury elektronowej pasma niklu wywołane temperaturą. Porównanie różnic energii wiązania składowych Ni 2p3/2 i jej zmian w zależności od temperatury wskazują, że największe zmiany zachodzą w najcieńszych warstwach. Warstwa, która miała duże niedopasowanie strukturalne w stosunku do podłoża, duże przesunięcie temperatury Ramana i wysoką temperaturę MI, również ma wysoki rozdział energii XPS i wielkość jego zmiany temperatury.
Wyniki pokazują, że dysproporcja ładunku niklu nie jest charakterystyczna dla stanu izolatora w cienkich warstwach SmNiO3. Pojawia się także w stanie metalicznym, chociaż jest mniejsza. Ponadto występuje nawet w powłokach, które nie zawierają odpowiedniej struktury krystalicznej. Przejście występuje w wyższej temperaturze w przypadku cieńszych warstw, które również mają wyższy poziom niedopasowania sieci. Argumentuje się, że w przypadku tak cienkiej warstwy odkształcenie jest wystarczająco duże, aby wymagać wyższej energii cieplnej, aby wywołać zmiany długości wiązań nikiel-tlen i zachodzenia pasm na siebie. Preferowana orientacja krystaliczna również wpływa na przejście, jednak niewielka liczba próbek utrudniała uchwycenie szczegółów tego zachowania.
Przedstawiony w pracy efekt grubości i struktury krystalicznej jest odmianą klasycznego efektu jonów metali ziem rzadkich. Przejście metal-izolator można modyfikować, zmieniając wymiary materiałów i orientację kryształów. Efekt anizotropii i rozmiaru jest ważny z uwagi na przyszłe dostosowanie przejścia MI do konkretnych potrzeb.
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Gonzalez, Rosillo Juan Carlos. "Volume resistive switching in metallic perovskite oxides driven by the metal-Insulator transition." Doctoral thesis, Universitat Autònoma de Barcelona, 2017. http://hdl.handle.net/10803/405305.
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Los óxidos de perovskita fuertemente correlacionados son una clase de materials con fascinantes propiedades físicas intrínsecas debido a la interacción de efectos de carga, spin, órbita y cristalinos. Efectos exóticos, como superconductividad, ferromagnetismo, ferroelectricidad o transiciones metal-aislante se producen gracias a la competición de los diferentes grados de libertad del sistema. El uso de estos efectos en una nueva generación de dispositivos es una fuente de inspiración continua para la comunidad científica.
Los dispositivos de Memoria Resistiva de Acceso aleatorio (RRAM) son uno de los candidatos más prometedores para ganar la carrera hacia la memoria universal del futuro, debido a sus excelentes propiedades en términos de escalabilidad, fatiga frente a ciclado, retención y velocidad de operación. Están basadas en el efecto de Conmutación Resistiva (RS), dónde dos (o más) estados de resistencia, reversibles y no volátiles son inducidos mediante la aplicación de un campo eléctrico intenso. Este fenómeno ha sido observado en una gran variedad de óxidos, donde es ampliamente aceptado que el movimiento de oxígeno juega un papel fundamental para explicar su origen. Sin embargo, el mecanismo físico preciso que gobierna el efecto depende del material, y en algunos de ellos, dicho mecanismo aún no es comprendido en su totalidad. Esta falta de compresión es hoy en vía es uno de los cuellos de botella que está retrasando el uso generalizado de esta tecnología.
En esta tesis, presentamos un novedoso mecanismo de RS basado en la Transición Metal-Aislante (MIT) perovskitas metálicas con correlación electrónica fuerte. Hemos estudiado el comportamiento RS de tres diferentes familias de perovskitas metálicas: La1-xSrxMnO3, YBa2Cu3O7-d y RENiO3 y demostramos que estos tres sistemas con conducción mixta eletrónica-iónica pueden experimentar una MIT, como consecuencia de la aplicación del campo eléctrico intenso, y que puede transformar su volumen bulk. Esta conmutación resistiva de carácter volúmico tiene una naturaleza diferente the los usuales tipos filamentar e interfacial, y abre nuevas oportunidades para el diseño de nuevos dispositivos robustos.
Hemos caracterizado conciencudamente el efecto de RS a la nanoescala mediante Microscopía de Fuerzas Atómicas en modo Conducción (C-AFM). Espectroscopía de Fuerza Túnel (STS) y medidas de transporte dependientes de la temperatura han sido realizadas en los diferentes estados resistivos para obtener detalles de su estructura electrónica. Hemos reproducido con éxito el comportamiento memristivo nanoscópico en una escala micrómetrica mediante el uso de sondas de W-Au en una estación de puntas. Usando esta aproximación, hemos llevado a cabo medidas en diferentes atmósferas, las cuales sugieren el intercambio de oxígeno con la atmósfera. Además, presentamos una prueba de concepto de una configuración de tres terminales, donde la conmutación resistiva es inducida en la puerta del dispositivo. En el caso particular del superconductor YBa2Cu3O7-d, hemos estudiado la influencia en las propiedades superconductoras de zonas de alta resistencia embebidas en la matriz del material. Esta aproximación sienta las bases hacia el diseño de dispositivos con zonas de anclaje de vórtices reconfigurables.
La interpretación de los resultados se hará en términos de una transición volúmica de tipo Mott, que estimamos ser de validez general para perovskitas metálicas de óxidos complejos.<br>Strongly correlated perovskite oxides are a class of materials with fascinating intrinsic physical functionalities due to the interplay of charge, spin, orbital ordering and lattice effects. The exotic phenomena arising from these competing degrees of freedom include superconductivity, ferromagnetism, ferroelectricity and metal-insulator transitions, among others. The use of these exotic phenomena in a new generation of devices with new and enhanced functionalities is continuing inspiring the research community.
In this sense, Resistive-Random Access Memories (RRAM) are one of the most promising candidates to win the race towards the universal memory of the future, which could overcome the limitations of actual technologies (Flash and Dynamic-RAM), due to their excellent properties in terms of scalability, endurance, retention and switching speeds. They are based on the Resistive Switching effect (RS), where the application of an electric field produces a reversible, non-volatile change in the resistance between two or more resistive states. This phenomenon has been observed in a large variety of oxide materials, where the motion of oxygen is widely accepted to play a key role in their outstanding properties. However, the exact mechanism governing this effect is material-dependent and for some of them it is still far to be understood. This lack of understanding is actually one of the main bottlenecks preventing the widespread use of this technology.
In this thesis, we present a novel Resistive Switching mechanism based on the Metal-Insulator Transition (MIT) in metallic perovskite oxides with strong electron electron interaction. We analyse the RS behaviour of three different families of metallic perovskites: La1-xSrxMnO3, YBa2Cu3O7-δ and RENiO3 and demonstrate that the MIT of these mixed electronic-ionic conductors can be tuned upon the application of an electric field, being able to transform the entire bulk volume. This volume RS is different in nature from interfacial or filamentary type and opens new possibilities of robust device design. Thorough nanoscale electrical characterization of the RS effect in these systems has been performed by means of Conductive-Atomic Force Microscopy (C-AFM). Scanning Tunnelling Spectroscopy (STS) and temperature-dependent transport measurements were performed in the different resistive states to get insight into their electronic features. The nanoscale memristive behaviour of these systems is successfully reproduced at a micrometric scale with W-Au tips in probe station experiments. Using this approach, atmosphere dependent measurements were undertaken, where oxygen exchange with the ambience is strongly evidenced. In addition,
we present a proof-of-principle result from a 3-Terminal configuration where the RS effect is applied at the gate of the device. In the particular case of superconducting YBa2Cu3O7-δ films, we have studied the influence of high resistance areas, which are embedded in the material, on the superconducting transport properties enabling vortex pinning modification and paving the way towards novel reconfigurable vortex pinning sites.
We interpret the RS results of these strongly correlated systems in terms of a Mott volume transition, that we believe to be of general validity for metallic perovskite complex oxides. We have verified that strongly correlated metallic perovskite oxides are a unique class of materials very promising for RS applications due to its intrinsic MIT properties that boosts a robust volumetric resistive switching effect. This thesis settles down the framework to understand the RS effect in these strongly correlated pervoskites, which could eventually lead to a new generation of devices exploiting the intrinsic MIT of these systems.
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Blümer, Nils [Verfasser]. "Mott-Hubbard Metal-Insulator Transition and Optical Conductivity in High Dimensions / Nils Blümer." Aachen : Shaker, 2003. http://d-nb.info/1172609020/34.
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Proskuryakov, Yuri. "Interactions, localisation and the metal to insulator transition in two-dimensional semiconductor systems." Thesis, University of Exeter, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.288367.
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Qiu, Lei. "Exploring 2D Metal-Insulator Transition in p-GaAs Quantum Well with High rs." Case Western Reserve University School of Graduate Studies / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1386337954.
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Che, Lah Nurul Akmal. "Conductivity studies of the size-induced metal-insulator transition (SIMIT) in silver nanoparticles." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:a2337bd5-0502-4bc0-a565-971e0fa8f6fc.
Full textAbstract:
In his tenth decade, Professor Sir Nevill Mott, in a letter to Professor Peter P. Edwards, reminiscenced about the fundamental difference between metals and non-metals; thus he wrote of the situation at the absolute zero of temperature (T = 0 K) '... there a metal conducts, and a non-metal doesn't'. This simple but amazingly powerful commentary stands as a powerful abiding description of the challenge of the fascinating question as to what makes a metal conduct. Many years of experimental studies of metals and nonmetals, targeted at the experimental verification in divided-metal systems ranging from transition mesoscopic metal compounds to microscopic colloidal metals, the occurrence of the electronic transition between these two canonical states of matter have always an enduring fascination where the possibility of a Size-Induced Metal-Insulator Transition (SIMIT) within a small particle of a bulk metal is accessed. In contrast to the situation of a discontinuous Metal-to-Insulator Transition (MIT) in macroscopic systems containing huge numbers of interacting particles and electrons (i.e. a doped semiconductor), the onset of the SIMIT in a single, isolated microscopic particle of a metallic element of the Periodic Table is expected to be a continuous metal-insulator transition. Here, we report an experimental study on the microwave electrical conductivity across the possible SIMIT in particles of the most conductive of all metals in Periodic Table - silver (Ag). Particular emphasis has been placed on the development and application of a non-intrusive microwave measurement technique, taken across a broad range microwave frequencies. The aim is to look for, and interrogate the nature of the SIMIT in such ultra-fine Ag particles. We have attempted to develop a coherent description of the different dielectric responses that yield electrical conductivity over a wide range of mesoscopic and microscopic Ag particles, from which the SIMIT is identified and rationalised. We find that, at 294 K, the mesoscopic Ag particles approximately 5 nm exhibit an 'effective' microwave conductivities which is reduced by some two orders-of-magnitude below that of bulk metallic Ag.
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Nájera, Ocampo Oscar. "Study of the dimer Hubbard Model within Dynamical Mean Field Theory and its application to VO₂." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS462/document.
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J'étudie en détail la solution d'un modèle simplifié d'électrons fortement corrélés, à savoir le modèle de Hubbard dimérisé. Ce modèle est la réalisation la plus simple d'un problème de cluster DMFT. Je fournis une description détaillée des solutions dans une région de coexistence où l'on trouve deux états (méta) stables des équations DMFT, l'un métallique et l'autre isolant. De plus, je décris en détail comment ces états disparaissent à leurs lignes critiques respectives. Je clarifie le rôle clé joué par la corrélation intra-dimère, qui agit ici en complément des corrélations de Coulomb.Je passe en revue la question importante du passage continue entre unisolant Mott et un isolant Peierls où je caractérise une variété de régimes physiques. Dans un subtil changement de la structure électronique, lesbandes de Hubbard évoluent des bandes purement incohérentes (Mott) à desbandes purement cohérentes (Peierls) à travers un état inattendu au caractère mixte. Je trouve une température d'appariement singulet T* en-dessous de laquelle les électrons localisés à chaque site atomique peuvent se lier dans un singulet et minimiser leur entropie. Ceci constitue un nouveau paradigme d'un isolant de Mott paramagnétique.Enfin, je discute la pertinence de mes résultats pour l'interprétation de différentes études expérimentales sur VO₂. Je présente plusieurs arguments qui me permettent d'avancer la conclusion que la phase métallique, à vie longue (métastable) induite dans les expériences pompe-sonde, et l'état métallique métastable M₁, thermiquement activé dans des nano-domaines, sont identiques. De plus, ils peuvent tous être qualitativement décrits, dans le cadre de notre modèle, par un métal corrélé dimérisé<br>We study in detail the solution of a basic strongly correlated model,namely, the dimer Hubbard model. This model is the simplest realization ofa cluster DMFT problem.We provide a detailed description of the solutions in the ``coexistentregion'' where two (meta)stable states of the DMFT equations are found, onea metal and the other an insulator. Moreover, we describe in detail howthese states break down at their respective critical lines. We clarify thekey role played by the intra-dimer correlation, which here acts in additionto the onsite Coulomb correlations.We review the important issue of the Mott-Peierls insulator crossoverwhere we characterize a variety of physical regimes. In a subtle change inthe electronic structure the Hubbard bands evolve from purely incoherent(Mott) to purely coherent (Peierls) through a state with unexpected mixedcharacter. We find a singlet pairing temperature T* below which thelocalized electrons at each atomic site can bind into a singlet and quenchtheir entropy, this uncovers a new paradigm of a para-magnetic Mottinsulator.Finally, we discuss the relevance of our results for the interpretation ofvarious experimental studies in VO₂. We present a variety of argumentsthat allow us to advance the conclusion that the long-lived (meta-stable)metallic phase, induced in pump-probe experiments, and the thermallyactivated M₁ meta-stable metallic state in nano-domains are the same.In fact, they may all be qualitatively described by the dimerizedcorrelated metal state of our model
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Capone, Massimo. "The Mott Transition: Role of Frustration and Orbital Degeneracy." Doctoral thesis, SISSA, 2000. http://hdl.handle.net/20.500.11767/4230.
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Zhu, Zhi Huai. "The metal-insulator transition in Mn-substituted Sr₃Ru₂O₇ by a photoemission study." Thesis, University of British Columbia, 2009. http://hdl.handle.net/2429/13906.
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We have studied the metal-insulator transition (MIT) in Mn-substituted Sr₃Ru₂O₇ by x-ray photoemission (XPS) and angle-resolved photoemission spectroscopy (ARPES). In XPS, both the surface- and bulk-sensitive spectra
show a two-peak structure, corresponding to the well screened and the unscreened excitations. The evolution of the well screened peak with Mn is that the higher the concentration of Mn impurities, the lower the intensity
of the peak, indicating that the screening channels are determined by the metallic property of a system. In ARPES, a strong doping dependence is also observed for the Fermi surface, which loses the 2D sheet associated
with the dxy band and reduces to the 1D dxz/yz Fermi surfaces, as the system changes from metal to a Mott insulator. As for the band dispersion, we see that the dxy band shifts in energy toward the chemical potential,
becoming degenerate with the dxz/yz bands, to the point of being indistinguishable. A nesting vector (0.33, 0.33, 0) has been determined from the Fermi surface of the 10% Mn doped sample, as in response to-or alternatively
inducing-a charge/orbital ordering across the Mott transition. The opening of an energy gap is observed below the MIT both as a function of temperature and Mn substitution. To obtain a microscopic understanding of
the MIT, we have performed a local-density approximation calculations for the electronic structures of Sr₃Ru₂O₇ and found that spin-orbital coupling induces an unexpected magnetic anisotropy. This might play a key role in the emergence of the MIT and magnetic superstructure in Mn-substituted Sr₃Ru₂O₇, as well as the nematic behavior in the parent compound.
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Hossain, M. A. "Metal-insulator transition, orbital symmetries and gaps in correlated oxides : an impurity control approach." Thesis, University of British Columbia, 2009. http://hdl.handle.net/2429/16753.
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The primary objective of this research is to develop and study newer ways to control electronic properties of correlated oxide systems using impurities. This goal has been achieved by introducing dilute localized 3d Mn impurities in place of a delocalized 4d Ru sites in a 2-dimensional Ru-O matrix and doping an electronically reconstructed polar surface of YBCO via surface impurities.
The first part of the work concentrates on X-ray Absorption Spectroscopy (XAS) and
Resonant Soft X-ray Scattering (RSXS) studies on lightly Mn doped Sr₃Ru₂O₇ . Our goal
is to understand the electronic structure of the material both at room and low temperature
and ultimately to understand the mechanism behind the low temperature metal-insulator
transition in this compound. With XAS, we zoom into the local electronic structure of
the impurities themselves and discovered unusual valence and crystal field level inversion
in the Mn impurities. With the help of density functional theory and cluster multiplet
calculations, we developed a model to describe the hierarchy of the crystal eld levels of
the Mn impurities. Long range magnetic properties of the compound has been probed
with Resonant Soft X-ray Scattering (RSXS) on the Mn and Ru L-edges. We have found
and analyzed the (¼,¼,0) structurally forbidden diffraction peak and connected it to an antiferromagnetic instability in the parent compound. Doping dependent scattering studies revealed the magnetic structure of the low temperature insulating phase and ultimately the mechanism behind the metal-insulator transition itself.
Angle Resolved Photoemission Spectroscopy (ARPES) has been used to investigate the low energy electronic structure of underdoped high-Tc superconductor YBCO. It has been revealed that due to the presence of polar surfaces there is electronic reconstruction on the surface of a cleaved YBCO sample. A novel technique has been devised that allows one to control the surface doping level in-situ by evaporating Potassium (K) on the YBCO surface. We showed that K tunes the surface doping level by donating electrons and thereby makes it possible to continuously tune the surface doping level throughout the phase diagram.
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Leighton, Christopher. "Persistent photoconductivity and the metal-insulator transition in Cd(_1-x)Mn(_x)Te:In." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/5008/.
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The persistent photoconductivity (PPC) effect in the diluted magnetic semiconductor Cd(_1-x)Mn(_x)Te:In has been studied in detail. Electrical transport measurements have been made on a large number of samples to build up an understanding of the phototransport properties of this material. In particular, the compositional dependence of the phototransport parameters has been measured up to x ≈ 0.2. Several samples exhibit an elevated temperature PPC effect which has been interpreted in terms of the formation of multiple DX centres. These samples can have a quenching temperature of up to 190 K, suggestmg that Cd(_1-x)Mn(_x)Te:In could be an interesting material in terms of applications of room temperature persistent photoconductors. The low field magnetoresistance has been measured and analysed quantitatively in order to attempt to identify the origin of the magnetoresistive effects in insulating and metallic samples. The positive magnetoresistance has been found to be linked with the magnetization of the sample. An anomalous negative magnetoresistance has been observed tinder certain experimental conditions. This negative magnetoresistance has been interpreted in terms of the formation of bound magnetic polarons and their contribution to spin-disorder scattering. The main body of this thesis is concerned with the study of the Metal-Insulator Transition (MIT). The PPC effect allows us to study the MIT in a continuous fashion by fine timing the carrier density by illumuiation. In this way we have made the first zero magnetic field study of the MIT in a magnetic semiconductor. The critical behaviour has been found to be consistent with the scaling theory of electron localization, which predicts a critical form σ = σ(_0)(n/n(_c) – 1)(^v). The critical conductivity exponent, v was determined to be close to one, while the critical carrier density, n(_e), was found to be ~ 2 x 10(^17) cm(^-3), for x = 0.08. The temperature dependence of the conductivity has been quantitatively analysed m both the metallic and insulating phases. On the insulating side of the transition, variable range hopping (VRH) conduction has been observed at low temperatures (down to 300 mK). The temperature dependence is consistent with VRH conduction with electron-electron interaction effects taken into account. In the metallic phase the temperature dependence of the conductivity (up to ~ 1 K) is consistent with a model where the zero temperature value of the conductivity is corrected by electron-electroninteraction effects, and the effects of weak localization. The magnitudes of these corrections are found to be in reasonable agreement with theoretical predictions. The electrical transport has also been studied in the weakly localized regime in Cd(_1-x)Mn(_x)Te:In and Cd(_1-x)Mn(_x)Te:In, Al. A rapid decrease in the conductivity occurs at low temperatures ( < 1.5 K). This is interpreted in terms of the effect of the s-d exchange interaction, which leads to the formation of bound magnetic polarons. It is suggested that this drop in conductivity can only be observed in the paramagnetic phase, and that spinglassordering has a significant effect on the temperature dependence of the conductivity at low temperatures.
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Rodríguez, Domínguez Laura. "Implications of phase coexistence in VO(2) thin-films across the metal-insulator transition." Doctoral thesis, Universitat de Barcelona, 2021. http://hdl.handle.net/10803/673256.
Full textAbstract:
A Metal-insulator transition (MIT) is the ability of some materials to change between metal and insulator electric behaviours as a function of some external stimuli such as temperature, stress, voltage, magnetic field or light. The Fermi level position with respect to the band structure determines one character or the other, and in some materials this band structure is very sensitive to electron-electron correlations. This is the case of some transition metal oxides, which despite having partially filled bands allowing, in theory, metallic conduction, electron- electron interactions split the half-filled band in a lower energy band that is full first and a higher energy sub-band still empty, resembling an insulator.
An archetypical example is vanadium dioxide (VO2), a system with a 3d electronic configuration which leads to a first-order MIT happening near room temperature (~68 ºC) with a change in conductivity of several orders of magnitude, accompanied by a structural phase transition (SPT) that occurs simultaneously. This results in a high-temperature state given by a metallic rutile (tetragonal) phase that turns into a semiconductor with monoclinic M1 structure at the low-temperature state. Thus, the electronic and structural elements of the transition in VO2 are tightly entangled, whereby mechanical stress induces lattice deformations in the crystal that distort the surroundings of V atoms affecting the orbital properties within the unit cell, the lattice’s electrostatic potential field and vibrational modes.
This thesis explores in detail the consequences of in-plane tensile strain in epitaxial VO2 thin films grown on rutile (001)-oriented TiO2 substrates by the pulsed laser deposition (PLD) technique, how the strain relaxes as film thickness increases, and how the strain relaxation itself gives raise to new phases and phase coexistence phenomena of interest. What makes this work novel with respect to other strain studies is the focus on the relation between local properties and global behaviour. As the thesis will show, space-averaged measurements of transport properties or X-ray diffraction (XRD) can miss out on a lot of interesting and important physics that happens on a nanoscopic level, including nano-tweeds or phase coexistence in the form of metal-insulator (M-I) phase boundaries. This work has used various microscopy techniques effective at different scales, from optical microscopy (micron scale) to scanning-probe microscopy (nanoscale) to transmission electron microscopy (atomic scale), yielding a very complete picture of the MIT in strained VO2 films across the different scales.
After a brief introductory chapter contextualizing the thesis, chapter one describes the
growth and average structural characteristics of the films, including their evolution due to aging. It was grown a thickness set from ~2 nm up to ~150 nm and surprisingly, the films do not relax with thickness neither conventionally nor progressively, resulting in crack formation, M-I phase separation toward the cracks and strong-strain gradients within the film.
The microstructure far from crack-edges is presented in chapter two. It shows that the
strained VO2 regions are not stabilized in the standard high-temperature rutile phase but in a metrically tetragonal structure that triples the rutile periodicity with monoclinic (2/m) symmetry. Moreover, the orientational variants of this new phase (x3M) coexist forming tweed patterns of few nanometers in size.
Chapter three explores the functional consequences of the coexistence between strain- relaxed insulating phase near the films’ cracks and metallic phase away from them. The peculiar pattern induced by crack formation leads to the concept of “self-pixelation”, whereby each island of VO2 bounded by insulating cracks behaves de facto as a “pixel” whose MIT can be individually triggered independently of the rest of the film.
Finally, the last chapter contains the general conclusions and outlook for future work.<br>Una transició metall-aïllant (MIT) es la habilitat d’alguns materials per canviar el seu comportament elèctric de metall a aïllant en funció d’estímuls externs com la temperatura, estrès, voltatge, camp magnètic o la llum. Aquest és el cas d’alguns òxids de metalls de transició, els quals malgrat tenir bandes d’energia parcialment ocupades permetent, en teoria, conducció metàl·lica, les interaccions electró-electró divideixen aquesta banda parcialment ocupada en una d’energia més baixa que s’omple primer i una altra d’energia major que queda buida, semblant a un aïllant.
Un arquetip d’això és el diòxid de vanadi (VO2), un sistema amb configuració electrònica 3d1 i una MIT de primer ordre, que succeeix ~68 ºC amb un canvi de conductivitat de varis ordres de magnitud, produint-se també una transició de fase estructural (SPT) simultàniament. Aquesta dualitat resulta en un estat d’alta temperatura metàl·lic amb estructura rutil (tetragonal) que es transforma en un semiconductor amb estructura monoclínica M1 en el seu estat de baixa temperatura. Per tant, els elements electrònics i estructurals de la transició al VO2 estan estretament entrelligats.
L’estrès mecànic indueix deformacions a la xarxa del cristall que distorsiona els voltants dels àtoms de V afectant les propietats orbitals dins de la cella unitat, camp de potencial electrostàtic i modes vibracionals.
Aquesta tesi explora en detall les conseqüències de la tensió en el pla, en capes primes epitaxials de VO2 crescudes sobre substrats rutil de TiO2 orientats en (001), per la tècnica de dipòsit de làser polsat (PLD). El que fa que sigui diferent respecte la resta d’estudis amb estrès mecànic és la focalització en la relació que hi ha entre propietats locals i comportament global del material. Com es mostrarà a la tesi, les mesures espacials mitjana poden perdre’s processos físics importants a nivell nanoscòpic, com nano-tweeds o coexistència de fases amb fronteres metall-aïllant (M-I), i per això les mostres s’han caracteritzat amb varies microscòpies a diferents escales (micro, nano i escala atòmica) aconseguint una visió completa de la MIT en capes tensionades de VO2.
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Liu, Mengkun. "Ultrafast far-infrared studies of Vanadates - multiple routes for an insulator to metal transition." Thesis, Boston University, 2012. https://hdl.handle.net/2144/12484.
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Thesis (Ph.D.)--Boston University
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.<br>The metal insulator transition in vanadates has been studied for decades and yet new discoveries still spring up revealing new physics, especially among two of the most studied members: Vanadium sesquioxide (V203) and Vanadium dioxide (V02). Although subtleties abound, both of the materials have first order insulator to metal phase transitions that are considered to be related to strong electron-electron (e-e) correlation. Further, ultrafast spectroscopy of strongly correlated materials has generated great interest in the field given the potential to dynamically distinguish the difference between electronic (spin) response versus lattice responses due to the associated characteristic energy and time scales.
In this thesis, I mainly focus on utilizing ultrafast optical and THz spectroscopy to study phase transition dynamics in high quality V203 and V02 thin films epitaxially grown on different substrates. The main findings of the thesis are:
(1) Despite the fact that the insulator to metal transition (IMT) in V203 is electron-correlation driven, lattice distortion plays an important role. Coherent oscillations in the far-infrared conductivity are observed resulting from coherent acoustic phonon modulation of the bandwidth
W. The same order of lattice distortion induces less of an effect on the electron transport in V02 in comparison to V203. This is directly related to the difference in latent heat of the phase transitions in V02 and V203.
(2) It is possible for the IMT to occur with very little structural change in epitaxial strained V02 films, like in the case of Cr doped or strained V203. However, in V02, this necessitates a large strain which is only possible by clamping to a substrate with larger c axis parameter through epitaxial growth. This is demonstrated for V02 films on Ti02 substrates.
(3) Initiating an ultrafast photo-induced insulator-to-metal transition (IMT) is not only possible with above bandgap excitation, but also possible with high-field far-infrared excitation. With the help of the field enhancement in metamaterial split ring resonator gaps, we obtain picosecond THz electric field transients of several MVIem which is sufficient to drive the insulator to metal transition in V02.
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Neupane, Krishna Prasad. "Studies of the Insulator-Metal Transition in La1-xCaxMnO3 and Thin Film Growth of Nd0.2Sr0.8MnO3." Scholarly Repository, 2009. http://scholarlyrepository.miami.edu/oa_dissertations/231.
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Two experimental projects involving perovskite manganese oxide compounds are presented. The first involved dielectric and transport studies of the insulator-metal transition as a function of charge-carrier doping in La1-xCaxMnO3 (0 < x < 0.15) bulk samples. The results provide new insight into the role of competing magnetic, lattice and Coulomb energies in determining the insulator-metal transition near x=0.22. The second project involved the growth, structural characterization, and resistive anisotropy of a-axis oriented Nd0.2Sr0.8MnO3 thin films with thicknesses t in the range 10 nm< t < 150 nm. Thicker films develop regular crack arrays which are the origin of a highly anisotropic in-plane electrical resistance. These cracks form parallel to the crystallographic c-axis on films with tensile strain deposited on NdGaO3 (100) and La0.3Sr0.7Al0.65Ta0.35O3 (110) substrates. Films grown under compressive strain on LaAlO3 (110) substrates have no cracks.
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Spitzig, Alyson. "The importance of Joule heating on the voltage-triggered insulator-to-metal transition in VO₂." Thesis, University of British Columbia, 2017. http://hdl.handle.net/2429/62808.
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The large change in resistivity in the material VO₂ has attracted considerable attention since it was first discovered in 1959. Recently, the ability to trigger the insulator-to-metal transition (IMT) with a strong electric field has been observed, but there has been debate about whether the transition is due to field-effects. We apply a voltage bias across a VO₂ thin film via a conductive atomic force microscope (CAFM) tip and measure the resultant current. We observe the IMT as a jump in the measured current in the IV curves. We fit the IV curves to the Poole-Frenkel (PF) conduction mechanism in the insulating state, immediately preceding the IMT. The PF conduction mechanism describes the thermal excitation of electrons into the conduction band in insulators, facilitated by strong electric fields. The PF mechanism is temperature dependent, and we use the temperature dependence to calculate the local temperature of the film before the transition. We directly compare the local electric field and local temperature of the film immediately preceding the IMT. We determine that the transition is not solely due to the applied electric field, but rather that the tip has locally warmed the film close to its IMT temperature through Joule heating.<br>Science, Faculty of<br>Physics and Astronomy, Department of<br>Graduate
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Weerasinghe, Hasitha C. "Electrical characterization of metal-to-insulator transition in iron silicide thin films on sillicone substrates." [Tampa, Fla] : University of South Florida, 2006. http://purl.fcla.edu/usf/dc/et/SFE0001677.
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Watson, Deborah Lee. "Quantum interference effects in the magnetoresistance of semiconductor structures near the metal to insulator transition." Thesis, University of Exeter, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286547.
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Wei, Haoming, Marcus Jenderka, Michael Bonholzer, Marius Grundmann, and Michael Lorenz. "Modeling the conductivity around the dimensionality-controlled metal-insulator transition in LaNiO3/LaAlO3 (100) superlattices." American Institute of Physics, 2015. https://ul.qucosa.de/id/qucosa%3A23552.
Full textAbstract:
A dimensionality controlled metal insulator transition in epitaxial [LaNiO3 (d nm)/LaAlO3(2nm)]10
(100) superlattices (thereafter [d/2]10 SLs) is demonstrated for decreasing LaNiO3 single layer
thickness from 4nm down to 1.2 nm. The [4/2]10 SL shows metallic behavior with positive resistivity
temperature coefficient, while the [2/2]10 SL shows a metal-insulator transition with crossover from
3D to two-dimensional single-layer dimensionality. Strong localization appears for the [1.2/2]10 SL
with the resistivity being dominated by two-dimensional variable range hopping with a localization
length of about 0.035 nm.
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Wei, Haoming, Marius Grundmann, and Michael Lorenz. "Confinement-driven metal-insulator transition and polarity-controlled conductivity of epitaxial LaNiO3/LaAlO3 (111) superlattices." American Institute of Physics, 2016. https://ul.qucosa.de/id/qucosa%3A23553.
Full textAbstract:
Recently, topological conductivity has been predicted theoretically in LaNiO3(111)-based
superlattices. Here we report high-quality epitaxial LaNiO3/LaAlO3 superlattices on (111)-oriented
SrTiO3 and LaAlO3 single crystals. For both substrates a metal-insulator transition with decreasing
number of LaNiO3 monolayers is found. While the electrical transport is dominated by twodimensional
variable range hopping for superlattices grown on polar mismatched SrTiO3(111), it
switches to a thermally activated single gap behavior on polar matched LaAlO3(111). The gap
energy of the polar double-layer LaNiO3 superlattices can be tuned via the thickness of the insulating
LaAlO3 layers.
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Meetei, Oinam Nganba. "Metal-Insulator Transition and Novel Magnetism Driven by Coulomb Interactions, Spin-Orbit Coupling and Disorder." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1405698402.
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O'Neal, Jared. "A Numerical Study of a Disorder-driven 2D Mott Insulator-to-Metal Quantum Phase Transition." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492701913534985.
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Vidas, Luciana. "The insulator-metal phase transition in VO2 measured at nanometer length scales and femtosecond time scales." Doctoral thesis, Universitat Politècnica de Catalunya, 2019. http://hdl.handle.net/10803/666959.
Full textAbstract:
The physics of transition-metal oxides presents a challenge to our current understanding of condensed matter physics. The main difficulty arises from a competition between electron-electron and electron-phonon interactions to dictate the properties of these complex materials. This issue is particularly apparent in vanadium dioxide, which undergoes an electronic and structural phase transition close to room temperature. Despite more than 50 years of research, the origin of the transformation is still actively debated, with contradictory interpretations often reported. The main goal of this thesis is to re-evaluate the phase transition in VO2 with a combination of new experimental techniques, ranging from the midinfrared to hard x-rays, that can probe the transformation at nanometer length scales and femtosecond time-scales. This allows to disentangle the roles of phase separation, laser-induced heat, and electron and phonon dynamics to the insulator-metal transition. The results from these experiments provide a unified and new picture of the nature of this process, both in and out of equilibrium, in which the electron-phonon interactions are the main driving mechanism. Furthermore, the new techniques and analysis presented here for VO2 can be applied to the study of other controversial complex materials that exhibit remarkable properties, and answer thereby some of the key outstanding questions in condensed matter physics.<br>La física de los óxidos de metales de transición constituye un gran desafío a nuestra comprensión actual de la materia condensada. El mayor obstáculo surge de la competición entre las interacciones electrón-electrón y electrónfonón para dictar las propiedades de tales materiales complejos. Este problema es particularmente evidente en el dióxido de vanadio, el cual experimenta una transición de fase tanto electrónica como estructural a una temperatura ligeramente superior a la ambiente. A pesar de más de 50 años de investigaciones, el origen de la transformación sigue siendo motivo de debate, con multitud de interpretaciones a menudo contradictorias. El objetivo principal de esta tesis es reevaluar la transición aislante-metal de VO2 empleando una combinación de técnicas experimentales nuevas, desde la región del infrarrojo medio a los rayos X duros, que permiten el estudio de la transición de fase a escalas nanométricas y en tiempos de femtosegundos. Esto facilita el esclarecimiento de los roles que desempeñan aspectos como la separación de fases, el calor inducido por láser y las dinámicas de electrones y fonones en la transición de fase de VO2. Los resultados de estos experimentos ofrecen una visión unificada sobre la naturaleza de este fenómeno, tanto en equilibrio como fuera de él, en la que la interacción de los electrones con fonones son el principal mecanismo responsable de impulsar la transición. Asimismo, los análisis y técnicas nuevos presentados en esta tesis para el estudio de VO2 pueden ser empleados para la investigación de otros materiales complejos que también exhiben propiedades extraordinarias y cuyo entendimiento presenta serias controversias. De esta manera, se daría respuesta a algunas de las preguntas clave pendientes de la física de la materia condensada.
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Maliepaard, Michael Cornelis. "The metal-insulator transition in GaAs and Inâ†0â†.â†5â†3Gaâ†0â†.â†4â†7As." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303254.
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D'ELIA, ALESSANDRO. "VO2 a prototypical Phase Change Material: spectroscopic study of the orbital contribution across the Metal Insulator Transition." Doctoral thesis, Università degli Studi di Trieste, 2020. http://hdl.handle.net/11368/2967985.
Full textAbstract:
Il VO2 è un affascinante sistema con configurazione 3d1 che esibisce una transizione metallo isolane (MIT) a 67°C accompagnata da una transizione strutturale, passando da una fase monoclina isolante a bassa temperatura ad una metallica tetragonale ad alta temperatura. Dalla sua scoperta, la MIT è stata ampiamente studiata sia per il suo potenziale applicativo che per investigarne la natura.
Diversi modelli teorici sono stati proposti in letteratura per spiegare la natura della MIT come una transizione strutturale di Peierls o una transizione Mott-Hubbard innescata dalla repulsione Coulombiana tra gli elettroni. Tuttavia una chiara descrizione teorica non è ancora stata trovata dal momento che le proprietà del VO2 dipendono dalla complessa interazione di diversi gradi di libertà: orbitale, strutturale ed elettronico.
Di conseguenza, al fine di sfruttare le caratteristiche della MIT in applicazioni tecnologiche, uno studio dettagliato dell'interazione tra i vari gradi di libertà è di fondamentale importanza.
Con l'obbiettivo di sciogliere l'intreccio tra ordine reticolare, ordine degli orbitali e interazione elettronica, in questa tesi sono stati studiati quattro campioni di VO2 con proprietà strutturali differenti. Tre film sottili cristallini alla cui struttura reticolare è applicato un diverso grado di strain ed un film nanostrutturato disordinato. Questi campioni sono stati studiati tramite avanzate tecniche spettroscopiche di raggi x quali X-ray Absorption Near Edge Structure (XANES), Resonant Photoemission (ResPES) e Constant Initial State (CIS).
La combinazione di queste tecniche ha permesso di determinare l'influenza dello strain sul contributo multi-orbitale alla MIT. in aggiunta, interessanti proprietà quali il controllo della popolazione del livelo di Fermi possono essere modulate controllando lo strain applicato. nel campione disordinato, in cui il la transizione di Peierls è inibita dalla mancanza di ordine, è stato possibile osservare una transizione puramente elettronica, cioè la prova sperimentale che la transizione strutturale non è strettamente necessaria per innescare la transizione metallo-isolante.<br>VO2 is a fascinating 3d1 system undergoing a temperature triggered (67 °C) Metal Insulator Transition (MIT) coupled with a structural phase transition, from a low temperature monoclinic insulator to a high temperature tetragonal metal. Since its discovery, the MIT has been widely studied with a twofold interest: its applicative potential and its nature.
Different theoretical models have been proposed to explain the occurrence of the insulating phase of VO2 like a structurally driven Peierls transition or a Mott-Hubbard transition triggered by electron mutual Coulomb repulsion. However, a clear theoretical picture is missing since VO2 properties are determined by a complex interplay among lattice, orbital and electronic degrees of freedom. Therefore, in order to exploit the MIT features for technological application, a detailed study of the influence and interplay between the different degrees of freedom is of paramount importance.
With the aim of disentangling the lattice-orbital-electronic intrigue, in this thesis, four samples with different structural properties have been studied. Three thin strained films and one nanostructured disordered VO2 film have been investigated using advanced spectroscopic techniques like X-ray absorption Near Edge Structure (XANES), Resonant Photoemission (ResPES) and Constant Initial State (CIS). The combination of these techniques allowed to determine the strain influence over the multi-orbital contribution to the MIT. In addition, interesting features like Fermi Level population can be modulated tuning the strain. In the disordered sample, in which the Peierls mechanism is quenched, it has been possible observe the occurrence of a purely electronic transition, i.e. structural transition is not necessary to trigger the MIT.
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Durandurdu, Murat. "Polyamorphism in Semiconductors." Ohio University / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1040060243.
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Zhou, Wei. "Oblique Angle Deposition Effects on Magnetron-Sputtered Metal Films." Miami University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=miami1501067883261477.
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Heidkamp, Marcus. "Spin coherence and -dephasing of donor and free conduction band electrons across the metal-insulator transition in Si:GaAs." kostenfrei, 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=977727149.
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Read, Daniel Edward. "Electrical and magnetic properties of n-Cd(_1-x)Mn(_x) Te close to the metal-insulator transition." Thesis, Durham University, 2001. http://etheses.dur.ac.uk/3783/.
Full textAbstract:
Electrical transport and magnetic measurements have been made on n-Cd(_1-x)Mn(_x) Te (0.047 < X < 0.197) for samples doped with hi or In,Al. Results are presented for measurements made as a function of temperature (40 mK < T < 300 K), applied magnetic field (0 T < B < 6 T) and photogenerated carrier density. Low field magnetic susceptibility measurements have identified a transition from a paramagnetic phase to a spin glass phase at low temperatures. The measured temperature dependence of the magnetic susceptibility is consistent with a cluster glass model. Measurements of the spin glass freezing temperature have been carried out on four samples having different Mn fractions The results obtained are in agreement with the limited number of previous measurements on nominally undoped samples. At low temperatures photo-induced changes in magnetism have been measured in both the paramagnetic and the spin glass phase. The change in magnetisation on illumination is due to an increased number of bound magnetic polarons formed around quasi localised s-spins. High temperature susceptibility measurements have been used to examine the parameters characterising the paramagnetic phase, and their variation with Mn fraction. Electrical transport measurements at very low temperatures (T < 800 mK) have shown a strong temperature dependent electron localisation. This has resulted in the confirmation of insulating behaviour in a sample (x = 0.047) having n. At low temperatures and in zero field an activated form of the conductivity is observed. In applied magnetic fields (B > 50 mT) Efros-Shklovskii variable range hopping is observed in the insulating phase. These results are attributed to the formation of a hard gap in the density of states, having a magnetic origin. At higher fields an insulator-metal phase transition occurs. In the metallic phase the conductivity can be described by a quantum correction to the zero temperature conductivity due to the effect of electron-electron interactions. Results obtained before and after illumination are consistent with scaling theory of electron localisation, having a critical exponent close to unity, indicative of the importance of electron-electron interactions. A reduction in the value of the critical field is seen after increasing the carrier density (B(_c) = 2.0 and 1.3 T for n = 3.3 xl0(^17)cm(^-3) and 3.8 x10(^17) cm(^-3) respectively).At low temperatures an anisotropy in the resistivity has been measured for samples in the spin glass phase. Magnetoresistance measurements have shown results consistent with previous measurements, in addition to a large, low field component that is attributed to the effect of magnetic field on electrons in the variable range hopping regime.
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Wilkinson, Aidan. "Transport phenomena in two-phase systems." Thesis, Loughborough University, 2017. https://dspace.lboro.ac.uk/2134/25574.
Full textAbstract:
The physics of two-phase systems is explored here, particularly magneto-transport and low temperature d.c. conductivity in thin films. The extraordinary magnetoresistance (EMR) effect was analysed in the context of previous experimental and theoretical considerations. The magnetoresistance (MR) may be enhanced by up to two orders of magnitude by changing the geometry. This was investigated using finite element analysis. Thin film samples consisting of a layered structure of Germanium-Tin-Germanium (Ge-Sn-Ge) were created in collaboration with Shandong University in China. Ge layers were kept at a constant thickness across all samples, with variable Sn thickness. Regions of Sn form island-like shapes ten times larger than the average film thickness, as is seen in scanning electron microscope (SEM) images. Raman spectroscopy was conducted on these samples, from which it is concluded that the Ge layers are amorphous in nature. It was seen that there is a relationship between the electrical resistance and the film thickness which is indicative of a metal-insulator transition (MIT). The temperature dependence of resistivity was subsequently investigated. The temperature coefficient of resistivity (TCR) of the samples is seen to become negative as the thickness of the Sn layer is reduced below a certain critical thickness. Depending on their thickness, samples were designated as metallic or insulator, and various models associated with metals and insulators fitted to the data. While it is impossible to be absolutely certain of the validity of each of the models, some are a better fit than others. The same temperature dependence of resistivity was measured with an applied magnetic field. This is compared with the previous EMR investigation, however the MR of the samples is only of the order of a few percent which corresponds to ordinary MR, seen in most metals. The magnetic field measurements suppress a resistivity down-turn at very low temperatures (T < 10K) which suggests the presence of superconductivity. Analysis of dr=dT shows that the onset of superconductivity is lower for samples with a lower Sn thickness. Additionally, the deposition rate of the Sn layer affects the resistivity significantly; a higher deposition rate causes a decrease in resistivity. It is supposed that this is due to a change in the microstructure of the film. Finally, piezo-resistivity was considered by applying mechanical compression to the samples. The added pressure causes a drop in resistivity.
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Butrouna, Kamal H. "A Systematic Transport and Thermodynamic Study of Heavy Transition Metal Oxides with Hexagonal Structure." UKnowledge, 2014. http://uknowledge.uky.edu/physastron_etds/24.
Full textAbstract:
There is no apparent, dominant interaction in heavy transition metal oxides (TMO), especially in 5d-TMO, where all relevant interactions are of comparable energy scales, and therefore strongly compete. In particular, the spin-orbit interaction (SOI) strongly competes with the electron-lattice and on-site Coulomb interaction (U). Therefore, any tool that allows one to tune the relative strengths of SOI and U is expected to offer an opportunity for the discovery and study of novel materials. BaIrO3 is a magnetic insulator driven by SOI whereas the isostructural BaRuO3 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of SOI and U via a systematic chemical substitution of the Ru4+(4d4) ions for Ir4+(5d5) ions in BaIrO3, i.e., in high quality single crystals of BaIr1-xRuxO3(0.0 < x < 1.0) . Our investigation of structural, magnetic, transport and thermal properties reveals that Ru substitution directly rebalances the competing energies so profoundly that it generates a rich phase diagram for BaIr1-xRuxO3 featuring two major effects: (1) Light Ru doping (0 < x < 0.15) prompts a simultaneous and precipitous drop in both the magnetic ordering temperature TC and the electrical resistivity, which exhibits metal-insulator transition at around TC. (2) Heavier Ru doping (0.41 < x < 0.82) induces a robust metallic and spin frustration state. For comparison and contrast, we also substituted Rh4+(4d5) ions for Ir4+(5d5) ions in BaIrO3, i.e. BaIr1-xRhxO3(0.0 < x < 0.10), where Rh only reduces the SOI, but without altering the band filling. Hence, this system remains tuned at the Mott instability and is very susceptible to disorder scattering which gives rise to Anderson localization.
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Herwadkar, Aditi A. "Electronic structure and magnetism in some transition metal nitrides Mn-doped ScN, dilute magnetic semiconductor and CrN, Mott insulator /." online version, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=case1164816868.
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Herwadkar, Aditi Dr. "Electronic structure and magnetism in some transition metal nitrides: MN-doped ScN, dilute magnetic semiconductor and CrN, Mott insulator." Case Western Reserve University School of Graduate Studies / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=case1164816868.
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Schütz, Philipp [Verfasser], Ralph [Gutachter] Claessen, Matthias [Gutachter] Bode, and Andrea [Gutachter] Caviglia. "Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO\(_3\) / Philipp Schütz ; Gutachter: Ralph Claessen, Matthias Bode, Andrea Caviglia." Würzburg : Universität Würzburg, 2020. http://d-nb.info/1219430102/34.
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Peres, Marcelos Lima. "Localização de Anderson e transição metal-isolante em filmes de Pb1-xEuxTe do tipo p." Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-29082008-081824/.
Full textAbstract:
Neste trabalho, realizamos o estudo da transição metal-isolante e da localização de Anderson na liga de Pb1-xEuxTe do tipo p para x variando de 0 até 0.1. As propriedades de transporte nessa liga (mobilidade, concentração de portadores e resistividade elétrica) foram obtidas utilizando o método de caracterização elétrica por efeito Hall entre as temperaturas de 300 K e 10 K. Nessa região de temperatura, foi possível observar uma transição metal-isolante para x > 0.05. Verificamos que a transição é do tipo Anderson e ocorre devido à desordem presente na liga. Para baixas temperaturas (T < 10 K) e em amostras com x > 0.01, verificamos a presença de magnetorresistência positiva e negativa aplicando campos magnéticos de até 11T. Nas amostras metálicas, a presença de magnetorresistência negativa é causada pelo efeito conhecido como localização de Anderson (efeito de interferência quântica construtiva entre as funções de onda) e a presença de magnetorresistência positiva é causada, principalmente, pelo acoplamento spin-órbita, e é chamada de antilocalização. Nas amostras isolantes, a magnetorresistência negativa é originada pelo efeito Zeeman enquanto que a magnetoresistência positiva é causada pela redução do comprimento de localização. Assim, os valores positivos e negativos da magnetoresistência têm origens diferentes dependendo do regime de condução (metálicoou isolante). Por esse motivo, o estudo dos resultados experimentais apresentados nesse trabalho foi dividido em duas partes: uma parte que trata as amostras metálicas (região de desordem fraca) e outra parte para as amostras isolantes (região de desordem forte). A partir dessa divisão, e utilizando os modelos teóricos disponíveis na literatura, foi possível fazer uma análise das medidas experimentais de magnetotransporte. Como resultado, identificamos os principais mecanismos de interação (espalhamento inelástico, efeito Zeeman, acoplamento spin-órbita, etc.) que interferem no transporte e nos efeitos de localização e antilocalização.<br>In this work, we investigated Anderson localization and the metal-insulator transition in p-type films of Pb1-xEuxTe for x varying from 0 up to 0.1. The transport properties of this alloy (mobility, carrier concentration and electrical resistivity) were obtained using the Hall method of electrical caracterization for temperatures ranging from 300 K down to 10 K. In this temperature range, it was possible to observe a metal-insulator transition for x > 0:05. The transition is of the Anderson type and is due to the disorder present in the alloy. For low temperatures (T < 10 K) and for samples with x > 0.01, we observed positive and negative magnetoresistance for magnetic fields up to 11 T. For metallic samples, the negative magnetoresistance originates from Andersons localization (constructive quantum interference effect between the wave functions) while positive magnetoresistence is caused, mainly, by the spin-orbit scattering, and it is called antilocalization. For insulating samples, negative magnetoresistance is originated from the Zeeman effect while positive magnetoresistance is caused by the localization length reduction. Therefore, positive and negative magnetoresistance values have different origins depending on the conduction regime (metallic or insulating). For this reason, our experimental investigation, presented in this work, was separated into two parts: the first one treats the metallic samples (weak-disorder regime) and the other treats the insulating samples (strong-disorder regime). From this division, and using available theoretical models, it was possible to analyze the magnetotransport experimental measurements. As a result, we identify the main interaction mechanisms (inelastic scattering, Zeeman effect, spin-orbit coupling, etc.) that interfere on the transport and localization and antilocalization effects.
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Tshepe, Tshakane. "Metal-insulator transition in boron-ion implanted type IIa diamond." Thesis, 2000. https://hdl.handle.net/10539/26301.
Full textAbstract:
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg,
in fulfillment of the requirements for the degree of Doctor of Philosophy.<br>High purity natural type Il a diamond specimens were used in this study. Conducting
layers in the surfaces of these diamonds were generated using low-ion dose multiple
implantation-annealing steps. The implantation energies and the ion-doses were spread
evenly to intermix the point-defects, thereby increasing the probability of interstitialvacancy
recombinations and promoting dopant-interstitial-vacancy combination resulting
in activated dopant sites in the implanted layers. The process used to prepare our samples
is known as cold-implantation-rapid-annealing (CIRA). Carbon-ion and boron-ion
implantation was used to prepare the diamond specimens, and de-conductivity measurements
in the temperature range of 1.5-300 K were made following each CIRA sequence.
An electrical conductivity crossover from the Mott variable range hopping (VRH)
to the Efros-Shklovskii VRH conduction was observed when the temperature of insulating
samples was lowered. The conductivity crossover temperature Tcross decreases with
increasing concentration of the boron-ion dose in the implanted layers, indicating the narrowing
of the Coulomb gap in the single-particle density of states near the Fermi energy. (Abbreviation abstract)<br>Andrew Chakane 2019
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Luo, Ji-Chang, and 羅際昌. "Metal-insulator transition in few-layer MoS2." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/13014086139344380438.
Full textAbstract:
碩士<br>國立交通大學<br>電子物理系所<br>103<br>Comparing to three-dimensional bulks, two-dimensional materials exhibit many novel physical properties and show excellently their applications in nano- and opto-electronics. In addition to graphene, other two-dimensional materials, having layered structure of semiconductor material such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), draw much attention because of the presence of indirect bandgap. The MoS2 can be used for making field-effect transistors (FETs) to give a high on/off ratio of above 106.
In this work, the electron transport in MoS2 is studied by two terminal devices with a back gating electrode. We use mechanical exfoliation, electron beam lithography and thermal evaporation to make few-layer MoS2 FET devices. From the device characterizations, we estimate the mobility, carrier concentration and localization length at different temperature range from 80 to 200 K and for MoS2 having different thickness. At a temperature higher than 200 K, the MoS2 shows an insulating to metallic phase transition. The transition could be attributed to the thermally excited carriers. On the other hand, the insulating phase appears at temperatures lower than 200 K and the insulating phase can be converted to the metallic phase by applying a back-gate voltage as well. The back gating voltage is used to increase the carrier concentration. The metallic phase was observed and the transition occurred at a conductivity very close to the ideal value of e2/h. At last, we show that the metallic phase can also be induced by applying a high electric field at zero gating voltage.