No description available.

Graduation date: 1998

In this work, optical pump – near-infrared probe and near-infrared pump – mid-infrared probe spectroscopy are used for the investigation of pressure-induced insulator-tometal transitions in transition metal oxide compounds. The materials under study are a-Fe₂O₃, also known as hematite, and VO₂. Both materials undergo pressureinduced metallization. However, the physical mechanisms of this phase transition are very different for these systems and have not been fully understood up to now. Using ultrafast pump-probe spectroscopy we obtain an insight into the evolution of the band structure and electron dynamics across the insulator-to-metal transition. In the case of VO₂, our near-infrared pump – mid-infrared probe experiments reveal a non-vanishing pumping threshold for photo-induced metallization even at our highest pressures around 20 GPa. This demonstrates the existence of localized charge carriers and the corresponding persistence of a band gap. Besides the threshold behaviour for photo-induced metallization, the carrier relaxation time scale, and the linear reflectivity and transmissivity have been studied under pressure increase. An anomaly in the threshold behaviour as well as the linear reflectivity and transmissivity at a critical pressure around 7 GPa indicates band gap filling under pressure. This is further supported by results obtained under decompression, where the changes of the linear reflectivity turned out to be almost fully reversible. The observations on VO₂ are highly reproducible and can be explained in terms of a pressure-induced bandwidth-driven insulator-to-metal transition. Fe₂O₃ has been studied via optical pump – near-infrared probe spectroscopy up to pressures of 60 GPa. In the pressure range up to 40 GPa, the changes of the response can be explained by photo-induced absorption and bleaching. The pressure-dependent study of the relaxation dynamics allows to identify cooling of the electron system as origin of the picosecond relaxation process.

<p>  </p> <p>In this study, VO2 films were grown on conducting oxide SrRuO3 layers. Apart from applications in magnetism, SrRuO3 is a widely studied template material to create multi-functional oxide heterostructures. Here, SrRuO3 buffered SrTiO3 (111) and Si/SiO2 were selected as platforms for VO2 growth. The properties of VO2 thin films grown on SrRuO3 buffer layers, as well as thermally and electric-field induced metal-insulator transition were systematically studied. Numerous growth experiments were conducted to identify the optimal growth conditions. Utilizing the current shunting associated with the conductive underlayer, electric-field induced metal-insulator transition was investigated in both the in-plane and out-of-plane configurations. A distributed resistance network with general applicability to understanding metal-insulator transitions is proposed to predict the electrical behavior of VO2 grown on conducting layers.</p>

Two-dimensional (2D) carrier systems built from semiconductor heterostructures have been at the center of a wide variety of experimental and theoretical research over the past decades. The quality improvement of GaAs/AlGaAs systems has allowed the observation of several peculiar ground states stabilized by the subtle interplay between carrier-carrier interaction, disorder and magnetic field. More recently, 2D systems in semiconductor heterostructures have also been used as a prime substrate for further confinement of the carriers to mesoscopic systems of major interest for the emerging fields of quantum computing and spintronics. This thesis addresses both magnetotransport measurements in hole open quantum dots (QDs) and thermopower studies of 2D holes in (311)A GaAs heterostructures. In the first part of this thesis, we describe the fabrication process for hole GaAs open QDs and investigate their magnetotransport properties at very low temperature T. Below 500 mK, the magnetoconductance of the open QDs exhibits clear signatures of coherent transport, namely magnetoconductance fluctuations and weak anti-localization. From these effects, we extract a T dependence for the dephasing time, together with an upper limit for the spin-orbit scattering time using the random matrix theory. Both the dephasing time and the spin-orbit scattering time are found to be much smaller than for electrons in similar systems. In the second part of this work, we report low-T thermopower measurements in the parallel magnetic field-induced metal-insulator transition (MIT) of 2D GaAs hole heterojunctions with different interface-dependent mobilities. When the magnetic field is increased, the diffusion thermopower decreases across the MIT. The reduction of the diffusion thermopower is more pronounced for the lower mobility sample where it reverses its sign. This behaviour indicates that the system does not undergo any ground state modification through the MIT but rather that the parallel magnetic field induces a dramatic change of the dominant hole scattering mechanisms. Finally, the last part of this thesis is devoted to the thermopower study of the insulating phase (IP) observed in 2D GaAs bilayer hole systems around the total Landau level filling factor n = 1. Our measurements show that the diffusion thermopower diverges with decreasing T in the IP. This divergence of the diffusion thermopower at low T indicates the opening of an energy gap in the system's ground state and suggests the formation of a pinned bilayer hole Wigner crystal around n = 1.

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. / 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 Anderson’s 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.

Thesis advisor: Ziqiang Wang / Gutzwiller wave function is an important theoretical technique for treating local electron-electron correlations nonperturbatively in condensed matter and materials physics. It is concerned with calculating variationally the ground state wave function by projecting out multi-occupation configurations that are energetically costly. The projection can be carried out analytically in the Gutzwiller approximation that offers an approximate way of calculating expectation values in the Gutzwiller projected wave function. This approach has proven to be very successful in strongly correlated systems such as the high temperature cuprate superconductors, the sodium cobaltates, and the heavy fermion compounds. In recent years, it has become increasingly evident that strongly correlated systems have a strong propensity towards forming inhomogeneous electronic states with spatially periodic superstrutural modulations. A good example is the commonly observed stripes and checkerboard states in high-$T_\mathrm c$ superconductors under a variety of conditions where superconductivity is weakened. There exists currently a real challenge and demand for new theoretical ideas and approaches that treats strongly correlated inhomogeneous electronic states, which is the subject matter of this thesis. This thesis contains four parts. In the first part of the thesis, the Gutzwiller approach is formulated in the grand canonical ensemble where, for the first time, a spatially (and spin) unrestricted Gutzwiller approximation (SUGA) is developed for studying inhomogeneous (both ordered and disordered) quantum electronic states in strongly correlated electron systems. The second part of the thesis applies the SUGA to the $t$-$J$ model for doped Mott insulators which led to the discovery of checkerboard-like inhomogeneous electronic states competing with $d$-wave superconductivity, consistent with experimental observations made on several families of high-$T_{\mathrm c}$ superconductors. In the third part of the thesis, new concepts and techniques are developed to study the Mott transition in inhomogeneous electronic superstructures. The latter is termed ``SuperMottness'' which is shown to be a general framework that unifies the two paradigms in the physics of strong electronic correlation: Mott transition and Wigner crystallization. A cluster Gutzwiller approximation (CGA) approach is developed that treats the local ($U$) and extended Coulomb interactions ($V$) on equal footing. It is shown with explicit calculations that the Mott-Wigner metal-insulator transition can take place far away from half-filling. The mechanism by which a superlattice potential enhances the correlation effects and the tendency towards local moment formation is investigated and the results reveal a deeper connection among the strongly correlated inhomogeneous electronic states, the Wigner-Mott physics, and the multiorbital Mott physics that can all be united under the notion of SuperMottness. It is proposed that doping into a superMott insulator can lead to coexistence of local moment and itinerant carriers. The last part of the thesis studies the possible Kondo effect that couples the local moment and the itinerant carriers. In connection to the sodium rich phases of the cobaltates, a new Kondo lattice model is proposed where the itinerant carriers form a Stoner ferromagnet. The competition between the Kondo screening and the Stoner ferromagnetism is investigated when the conduction band is both at and away from half-filling. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Esta tese apresenta um estudo sistemático da síntese e das propriedades físicas de amostras policristalinas de Nd IND. 1-X R IND. X NiO IND. 3; R = Sm, Eu, 0 < OU = X < OU = 1. Estes materiais apresentam uma transição de fase metal-isolante MI em temperaturas 200 < OU = T IND. MI < OU = 400 K. Amostras foram produzidas a partir do método de precursores sol-gel, sinterizadas a 1000 GRAUSC e sob pressões de O IND. 2 de até 80 bar. O estudo da influência da substituição de R POT. 3+ na matriz de NdNiO IND. 3 foi realizado a partir das caracterizações através de medidas de difração de raios-X DRX, difração de neutrons como função da temperatura DRN, transporte elétrico ro(T), transporte térmico capa(T), coeficiente Seebeck S(T), calorimetria diferencial e susceptibilidade magnética qui(T). Os resultados de DRX revelaram que as amostras são monofásicas e cristalizam-se na estrutura perivskita distorcida ortorrombicamente, grupo espacial Pbnm. As medidas de DRN realizadas nas amostras de Nd IND. 1-XEu IND. X NiO IND. 3 mostraram a evolução dos parâmetros de rede e do volume da cela unitária V como função da temperatura. Estas caracterizações revelaram que, em T DA ORDEM DE T IND. MI, ocorre uma expansão em V, assim como um aumento do ângulo de ligação Ni-O e uma diminuição do ângulo de \"superexchange\" teta. Medidas de ro(T) revelam a ocorrência da transição MI em um amplo intervalo de temperatura 200 < OU = T IND. MI 400 K. Através destas medidas verificou-se também a presença de histerese térmica ocorre decresce continuamente com o aumento de x, até anular-se em x > 0,5 e em x > 0,35 para R=Sm e Eu, respectivamente. Este resultado foi confirmado através das medidas de capa(T) e S(T). Além disso, verificou-se a importância da contribuição da rede na capa(T). As medidas de S(T) indicam que os portadores de carga são elétrons e que a densidade de ) estados no nível de Fermi N(E IND. F) foi estimada ser da ordem de 10 POT. 23 (eVcm POT. 3) POT. -1. Os valores de T IND. MI e a presença ou não de histerese térmica foram verificados através das medidas de DSC. Um estudo foi feito para verificar qual a maneira mais precisa de subtrair a contribuição dos íons terras-raras nas medidas de susceptibilidade magnética qui(T) dos compostos Nd IND. 1-X R IND. X NiO IND. 3. Após estas correções foi possível verificar o ordenamento magnético da sub-rede do Ni POT. 3+ na região de temperatura T IND. n DA ORDEM DE 200 K para as amostras com x < 0,4 e x < 0,25 para R=Sm e Eu, respectivamente. Ajustes lineares feitos em qui(T) acima de temperaturas T > 200 K revelaram valores de momentos magnéticos efetivos müeff variando de 1,7 a 1,8 mü IND. B o que está em concordância com o valor esperado de müeff DA ORDEM DE 1,76 mü IND. B do íon livre de Ni POT. 3+. Adicionalmente, uma separação precisa do termo independente da susceptibilidade magnética foi efetuada e a susceptibilidade de Pauli dos materiais foi encontrada. Foi possível então obter uma estimativa da densidade de estados no nível de Fermi N(E IND. F), que gerou valores similares aos obtidos via medidas do coeficiente Seebeck. Entretanto, o comportamento de qui(T) corrigido abaixo de T IND. n revelou características de um antiferromagnetismo não convencional devido a um aumento monotônico de qui(T) com o decréscimo da temperatura e a presença de irreversibilidade nas curvas resfriadas a campo magnético zero ZFC e do refinamento de estrutura estimou-se que a valência no Ni nas amostras de Nd IND. 1-X R IND. X NiO IND. 3 DA ORDEM DE 3. Estimativas grosseiras da largura de bando W do O 2ro e da energia de transferência de carga delta para a série de compostos Nd IND. 1-X R IND. X NiO IND. 3 revelaram valores compatíveis com aqueles ) encontrados na literatura. De maneira geral, as caracterizações das propriedades estruturais, de transporte e magnéticas sugerem que os compostos Nd IND. 1-X R IND. X NiO IND. 3 podem ser classificados como sistemas onde correlações eletrônicas e flutuações dessas correlações ocorrem. Foram discutidas algumas limitações acerca da aplicabilidade dos modelos vigentes para a explicação da transição metal-isolante nos niquelatos aqui estudados. / This work reports a systematic study on the synthesis and general physical properties of polycrystalline samples of Nd IND. 1-X R IND. X NiO IND. 3; R = Sm, Eu, 0 < OU = X < OU = 1. These compounds exhibit a metal-insulator MI phase transition in a broad range of temperature 200 < OU = T IND. MI < OU = 400 K. The samples were prepared through sol-gel precursors and sintered at extreme conditions: high temperatures 1000 GRAUSC and under oxygen pressures up to 80 bar. These samples were characterized by several techniques including X-ray powder diffraction XRD, neutron diffraction as a function of temperature NRD, electrical resistivity ro(T), thermal conductivity capa(T), Seebeck coefficient S(T), differential scanning calorimetry DSC, and magnetic susceptibility qui(T). The results of XRD revealed that all samples are single phase and crystallize in an orthorhombic structure, space group Pbnm. The NRD data, combined with the Rietveld analysis, indicated small changes in the lattice parameters a, b, and c and in the volume V of the unit cell T DA ORDEM DE T IND. MI. Such a small change in these parameters is accompained by either a little decrease of the superexchange angle teta and a small expansion of the Ni-O bond-length. The ro(T) data exhibit interesting features such as: (1) a metallic-like behavior of ro(T) at high temperatures; (2) a huge increase of the magnitude of ro(T) at T DA ORDEM DE T IND. MI; and (3) a thermal hysteresis occurring just below T DA ORDEM DE T IND. MI in a temperature interval as large as 100K. Such a thermal hysteresis is characteristic of a first order MI transition and was found to vanish with increasing substitution of x. This strongly suggests that increasing x modify the character of this transition to second order. Thermal properties were carried out and confirmed the change of this MI transition with increasing x. In addition, an analysis of the capa(T) data indicate that phonons are the major thermal carriers in these nickelates. Also, the Seebeck coefficient S(T) data revealed features of a conventional metal at higher temperatures with electrons as carriers. An accurate analysis of the S(T) data based on simple band structure arguments indicate a density of states at the Fermi level of 10 POT. 23 (eVcm POT. 3) POT. -1 and energy gaps in the insulating regime close to 20 meV. The character of the first order transition in lightly substituted samples at T DA ORDEM DE T IND. MI was also inferred from the DSC data. The S(T) data confirmed the occurrence of the metal-insulator transition and the already observed change from first to second order character with increasing x. The magnetic susceptibility ípsilon(T) data have been precisely corrected by a systematic subtraction of the R POT.3+-ion contribution of the measured qui(T). Linear adjusts of the corrected curves above 150K where found to fit the Curie-Weiss law with effective magnetic moment of mü IND. EFF ~ 1.76 mü IND. B, which is close to the free-ion value of mü IND. EFF ~ 1.76 mü IND. B (Ni POT. 3+). These results indicate that the Ni POT. 3+ array displays an antiferromagnetic ordering below a well-defined temperature T IND. N, which is close to T IND.MI for Nd IND. 1-X R IND. X NiO IND. 3 and lightly substituted samples. However, the evolution of the susceptibility of the Ni POT. 3+ array displays features which are fingerprints of unconventional antiferromagnetic state. These features, observed mostly below T IND. N, include a field independent irreversibility of qui(T) and a systematic increase of qui(T) with decreasing temperature, resembling that of a paramagnet. An analysis of the electronic contribution to ípsilon(T) resulted in a density of states at the Fermi level close to the one estimated from the S(T) data. These results are discussed within the context of recent experimental results and theories employed to explain the origin of the metal-insulator transition in these nickelates.

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. / 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 Anderson’s 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.

In this dissertation, we study properties of quantum interaction phenomena in two-dimensional (2D) and one-dimensional (1D) electronic systems in p-GaAs micro- and nano-scale devices. We present low-temperature magneto-transport data from three forms of low-dimensional systems 1) 2D hole systems: in order to study interaction contributions to the metallic behavior of 2D systems 2) Bilayer hole systems: in order to study the many body, bilayer quantum Hall state at nu = 1 3) 1D hole systems: for the study of the anomalous conductance plateau G = 0.7 ???? 2e2/h The work is divided into five experimental studies aimed at either directly exploring the properties of the above three interaction phenomena or the development of novel device structures that exploit the strong particle-particle interactions found in p-GaAs for the study of many body phenomena. Firstly, we demonstrate a novel semiconductor-insulator-semiconductor field effect transistor (SISFET), designed specifically to induced 2D hole systems at a ????normal???? AlGaAs-on-GaAs heterojunction. The novel SISFETs feature in our studies of the metallic behavior in 2D systems in which we examine temperature corrections to ????xx(T) and ????xy(T) in short- and long-range disorder potentials. Next, we shift focus to bilayer hole systems and the many body quantum Hall states that form a nu = 1 in the presence of strong interlayer interactions. We explore the evolution of this quantum Hall state as the relative densities in the layers is imbalanced while the total density is kept constant. Finally, we demonstrate a novel p-type quantum point contact device that produce the most stable and robust current quantization in a p-type 1D systems to date, allowing us to observed for the first time the 0.7 structure in a p-type device.