<p>The role of the surroundings, or <italic> environment </italic>, is essential in understanding funda- mental quantum-mechanical concepts, such as quantum measurement and quantum entanglement. It is thought that a dissipative environment may be responsible for certain types of quantum (i.e. zero-temperature) phase transitions. We observe such a quantum phase transition in a very basic system: a resonant level coupled to a dissipative environment. Specifically, the resonant level is formed by a quantized state in a carbon nanotube, and the dissipative environment is realized in resistive leads; and we study the shape of the resonant peak by measuring the nanotube electronic conductance.</p><p>In sequential tunneling regime, we find the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weakly than the conventional T<super>-1</super>. Moreover, the observed scaling signals a close connec- tion between fluctuations that influence tunneling phenomenon and macroscopic models of the electromagnetic environment.</p><p>In the resonant tunneling regime (temperature smaller than the intrinsic level width), we characterize the resonant conductance peak, with the expectation that the width and height of the resonant peak, both dependent on the tunneling rate, will be suppressed. The observed behavior crucially depends on the ratio of the coupling between the resonant level and the two contacts. In asymmetric barriers the peak width approaches saturation, while the peak height starts to decrease.</p><p>Overall, the peak height shows a non-monotonic temperature dependence. In sym- metric barriers case, the peak width shrinks and we find a regime where the unitary conductance limit is reached in the incoherent resonant tunneling. We interpret this behavior as a manifestation of a quantum phase transition.</p><p>Finally, our setup emulates tunneling in a Luttinger liquid (LL), an interacting one-dimensional electron system, that is distinct from the conventional Fermi liquids formed by electrons in two and three dimensions. Some of the most spectacular properties of LL are revealed in the process of electron tunneling: as a function of the applied bias or temperature the tunneling current demonstrates a non-trivial power-law suppression. Our setup allows us to address many prediction of resonant tunneling in a LL, which have not been experimentally tested yet.</p> / Dissertation

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-11T12:56:20Z No. of bitstreams: 2 Tese - Thiago Mureebe Carrijo - 2018.pdf: 5034985 bytes, checksum: 627c2d90a4e7fa2b7ebc112a3b54e16f (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-11T15:48:17Z (GMT) No. of bitstreams: 2 Tese - Thiago Mureebe Carrijo - 2018.pdf: 5034985 bytes, checksum: 627c2d90a4e7fa2b7ebc112a3b54e16f (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-06-11T15:48:17Z (GMT). No. of bitstreams: 2 Tese - Thiago Mureebe Carrijo - 2018.pdf: 5034985 bytes, checksum: 627c2d90a4e7fa2b7ebc112a3b54e16f (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-05-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, we have studied how local quantum uncertainty (LQU) and linear entropy behave in critical systems. Two models of spin 1/2 systems were investigated: XY and XY with triple interaction (XYT), both unidimensional. It was concluded that these measures and their derivatives locate the phase transitions of these systems and a justification was given for the difference of behavior between the LQU and the linear entropy in the region near the transition in the XY model. A measure of multipartite quantum correlations was also created according to a set of axiomatic criteria, which was tested in the family of Werner-GHZ quantum states, producing results in accordance with the expected result, according to the literature. / Neste trabalho, foi estudado como a incerteza quântica local (LQU) e a entropia linear se comportam em sistemas críticos. Foram investigados dois modelos de sistemas de spin 1/2: XY e XY com interação tripla (XYT): ambos unidimensionais. Concluiu-se que essas medidas e suas derivadas marcam as transições de fase desses sistemas e foi dada uma justificativa para a diferença de comportamento entre a LQU e a entropia linear na região próxima da transição no modelo XY. Também foi criada uma medida de correlações quânticas multipartidas segundo um conjunto de critérios axiomáticos, a qual foi testada na família de estados quânticos Werner-GHZ, produzindo resultados em conformidade com esperado segundo o que há na literatura.

Orientador: Kyoko Furuya / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-12T22:08:27Z (GMT). No. of bitstreams: 1 Viscondi_ThiagodeFreitas_M.pdf: 15446195 bytes, checksum: d4e6bd6c75ebc928ffe531009f94d745 (MD5) Previous issue date: 2009 / Resumo: Examinamos vários aspectos do modelo de um condensado de Bose-Einstein aprisionado em um potencial de três poços simetricamente dispostos, incluindo os efeitos geralmente negligenciados de interação entre partículas em modos locais distintos, que conhecemos como colisões cruzadas. Por intermédio de uma extensão do formalismo de pseudo-spins de Schwinger, aproveitamos a estrutura algébrica natural do sistema, de forma a construir o análogo clássico do modelo utilizando os estados coerentes próprios das representações totalmente simétricas do grupo SU(3). Empregando esta aproximação semiclássica, estudamos os diferentes regimes dinâmicos populacionais presentes no sistema, que podem ser divididos em três grandes conjuntos, os quais denominamos como dinâmicas de condensados gêmeos, poço vazio e vórtice. Estes regimes estão relacionados ao comportamento dos pontos de equilíbrio do modelo, que apresentam bifurcações e alterações de estabilidade, ferramentas essenciais à compreensão dos fenômenos não lineares de tunelamento do condensado. A dinâmica de condensados gêmeos representa um subregime integrável do sistema, onde observamos a supressão do tunelamento bosônico, conhecida como auto-aprisionamento macroscópico. Os estados de vórtice são responsáveis por configurações de rotação do condensado na armadilha, ao passo que os estados de poço vazio exibem desocupação persistente em um dos modos locais. Todos os resultados análogos clássicos são comparados a cálculos quânticos exatos, no intuito de observar as origens da quebra de correspondência clássico-quântica, que quantificamos com uma medida de emaranhamento multipartite, conhecida como pureza generalizada. Também consideramos a transição de fase quântica presente no modelo para interações bosônicas atrativas, a qual associamos a uma mudança da dinâmica populacional do sistema, observada como uma fragmentação das representações do estado fundamental sobre o espaço de fase / Abstract: We examined several aspects of a Bose-Einstein condensate trapped in a symmetrically arranged triple-well potential, including the effects of the generally neglected interaction between particles in different local modes, known as cross-collisions. By means of an extension of the Schwinger¿s pseudospins formalism, we take advantage of the system¿s algebraic structure in order to obtain the classical analogue of the model, by using the coherent states of the fully symmetric representations of the SU(3) group. Employing this semiclassical approximation, we studied the different dynamical regimes of the system, which can be divided into three large groups, which we call as twin-condensate, single depleted well and vortex dynamics. These dynamical regimes are related to the behavior of the fixed points of the model, which exhibit bifurcations and changes of stability, essential tools to the understanding of the nonlinear tunneling phenomena. The twin-condensate dynamics is an integrable subregime of the system, where we observe the suppression of bosonic tunneling, known as macroscopic self-trapping. The vortex states are responsible for the rotational configurations of the condensate in the trap, while the single well depleted states exhibit one persistent vacant local mode. All the classical analogue results are compared to exact quantum calculations, in order to observe the origins of the broken quantum-classical correspondence, which we quantified with a measure of multipartite entanglement, known as generalized purity. We also consider the quantum phase transition for attractive bosonic interactions, which we connect to a change in population dynamics of the system, observed as the phase space fragmentation of the ground state representations / Mestrado / Física / Mestre em Física

Submitted by Geyciane Santos (geyciane_thamires@hotmail.com) on 2015-08-06T15:26:04Z No. of bitstreams: 1 Dissertação - Anne Beatriz Rocha Abreu.pdf: 1366448 bytes, checksum: 47c7b96a4f310e46ae8f497afc56d8cb (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:52:03Z (GMT) No. of bitstreams: 1 Dissertação - Anne Beatriz Rocha Abreu.pdf: 1366448 bytes, checksum: 47c7b96a4f310e46ae8f497afc56d8cb (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:54:27Z (GMT) No. of bitstreams: 1 Dissertação - Anne Beatriz Rocha Abreu.pdf: 1366448 bytes, checksum: 47c7b96a4f310e46ae8f497afc56d8cb (MD5) / Made available in DSpace on 2015-08-07T13:54:27Z (GMT). No. of bitstreams: 1 Dissertação - Anne Beatriz Rocha Abreu.pdf: 1366448 bytes, checksum: 47c7b96a4f310e46ae8f497afc56d8cb (MD5) Previous issue date: 2013-10-02 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we study the antiferromagnetic anisotropic Heisenberg spin-1 model with interactions between nearest neighbors (J1 along the rows and J01 along the columns) and between the next nearest neighbors (J2 along the diagonals) on a square lattice. We apply initially the method of linear spin wave theory (LSWT) to study the behavior of the quantum phase transition (T=0) and compare the results with qualitative values obtained for the model J1􀀀J01 􀀀J2, where we explore the two cases of spin-1/2 and spin-1. We analyse the phase diagram in the plane ( J01=J1) versus ( J2=J1). In the case of spin-1/2 the results are not satisfactory in the region of small value of , where disordered region is present for any value of in contradiction with other results available in the literature that present the disordered phase for > 1, whereas for < 1 we have absence of disordered phase with a phase transition of rst order direct between the phases antiferromagnetic (AF) e collinear antiferromagnetic (CAF). The AF state (Néelstate) is characterized by spins oriented antiparallel over all the square lattice. In the CAF state, the spins are oriented parallel in columns and alternated in opposite senses between a horizontal chains. The literature provides only one result in the case of spin-1, which was obtained years ago by Bishop et al. using the cluster coupled method (CCM), in which they demonstrate that no longer exists disordered intermediate phase, not even to = 1, featuring only rst order transitions ( < 1) and second order ( > 1) with presence of a tricritical point. On the other hand, our results for spin waves shows a phase diagram qualitatively similar to those obtained by other methods in the case of spin-1/2. Facing these controversy results found in spin-1, using CCM and LSWT, we will apply the technique of Sachdev operators (bond operators). Our results indicate that we have intermediate desordered phase for any value of > 0. / Neste trabalho estudamos o Modelo de Heisenberg Antiferromagnéico Anisotrópico de spin-1 com interações entre primeiros (J1 ao longo das linhas e J01 ao longo das colunas) e segundos vizinhos (J2 ao longo das diagonais) numa rede quadrada. Aplicamos inicial- mente o método da teoria de ondas de spin linear (LSWT) para estudar o comportamento da transição de fase quântica (T=0) e comparamos os resultados qualitativos com os valores obtidos para o modelo J1 􀀀 J01 􀀀 J2, onde exploramos os dois casos de spin-1/2 e spin-1. Analisamos o diagrama de fase no plano ( J01 =J1) versus ( J2=J1). No caso de spin-1/2 os resultados não são satisfatórios na região de pequeno valor de , onde a região desordenada está presente para qualquer valor de em contradição com outros resultados disponíveis na literatura que apresenta a fase desordenada para > 1, enquanto que para < 1 temos ausência desta fase desordenada com uma transição de fase direta de primeira ordem entre as fases antiferromagnética (AF) e colinear antifer-romagnética (CAF). O estado AF (estado de Néel) é caracterizado por spins orientados antiparalelamente sobre toda a rede quadrada. No caso do estado CAF, os spins estão orientados paralelamente em colunas e alternados em sentidos opostos entre cadeias na horizontal. No caso de spin-1 apenas disponibilizamos de um resultado na literatura, que foi obtido anos atrás por Bishop e colaboradores usando o método do cluster acoplado (CCM), no qual demonstram não existir a fase desordenada intermediária, nem mesmo para = 1, apresentando apenas transições de primeira ordem ( < 1) e segunda ordem ( > 1) com presença de um ponto tricrítico. Por outro lado, nossos resultados de ondas de spin mostram um diagrama de fase qualitativamente similar aos encontrados por outros métodos no caso de spin-1/2. Diante desta controvérsia dos resultados encontrados no spin-1, usando CCM e ondas de spin linear, iremos aplicar a técnica dos operadores de Sachdev (operadores de enlace). Nossos resultados indicam que temos a fase desordenada intermediária para qualquer valor de > 0.

In this thesis I will outline various quantum phase transitions in 2D models of magnets that are amenable to simulation with quantum Monte Carlo techniques. The key player in this work is the theory of deconfined criticality, which generically allows for zero temperature quantum phase transitions between phases that break distinct global symmetries. I will describe models with different symmetries including SU(N), SO(N), and "easy-plane" SU(N) and I will demonstrate how the presence or absence of continuous transitions in these models fits together with the theory of deconfined criticality.

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<p>In Part I, the relationship between the topology of the Fermi surface and the entanglement entropy S is examined. Spinless fermionic systems on one and two dimensional lattices at fixed chemical potential are considered. The lattice is partitioned into sub-system of length L and environment, and the entanglement of the subsystem with the environment is calculated via the correlation matrix. S is plotted as a function of the next-nearest or next-next nearest neighbor hopping parameter, t. In 1 dimension, the entanglement entropy jumps at lifshitz transitions where the number of Fermi points changes. In 2 dimensions, a neck-collapsing transition is accompanied by a cusp in S, while the formation of electron or hole-like pockets coincides with a kink in the S as a function of the hopping parameter. The entanglement entropy as a function of subsystem length L is also examined. The leading order coefficient of the LlnL term in 2 dimensions was seen to agree well with the Widom conjecture. Of interest is the difference this coefficient and the coefficient of the term linear in L near the neck-collapsing point. The leading order term changes like |t-t_c|^1/2 whereas the first sub-leading term varies like |t-t_c|^1/3, where t_c is the critical value of the hopping parameter at the transition.</p> <p>In Part II, we study the statistics of fractionalized excitations in a bosonic model which describes strongly interacting excitons in a N-band insulator. The elementary excitations of this system are strings, in a large N limit. A string is made of a series of bosons whose flavors are correlated such that the end points of a string carries a fractionalized flavor quantum number. When the tension of a string vanishes, the end points are deconfined. We determine the statistics of the fractionalized particles described by the end points of strings. We show that either bosons or Fermions can arise depending on the microscopic coupling constants. In the presence of the cubic interaction in the Hamiltonian as the only higher order interaction term, it was shown that bosons are emergent. In the presence of the quartic interaction with a positive coupling constant, it was revealed that the elementary excitations of the system possess Fermion statistics.</p> / Master of Science (MSc)

<p>The following dissertation presents neutron scattering studies on three specific magnetic insulating oxide materials whose lattice is based on triangular structural motifs. Each of the three materials studied, LuCoGaO₄, Co₃V₂O₈ and Tb₂Ti₂O₇, displays an interesting disordered ground state that is reached by different mechanisms: site disorder, geometric frustration, and quantum fluctuations induced by a transverse magnetic field. The main focus of this work is the characterization of the resulting magnetic ground states and magnetic excitations within these systems.</p> <p>Chapters 3, 4 and 5 contain original work in the form of six research articles that have either been published or have been prepared for publication in peer-reviewed journals.</p> <p>Chapter 3 describes studies of the quasi two-dimensional triangular layered antiferromagnet LuCoGaO₄. This material is found to exhibit a spin glass ground state as a result of geometrical frustration and site disorder inherent in this system. Below the freezing temperature, this system exhibits static, two-dimensional correlations consistent with frozen short-range correlated regions in the plane of the bilayers that extend over roughly five unit cells. The dynamic correlations reveal typical spin glass behavior upon cooling. Furthermore, a resonant gapped spin-wave-like excitation is observed, that can be related to the anisotropy in the system. Such an excitation is relatively uncommon in spin glasses and has been studied for the first time in such detail.</p> <p>Chapter 4 is concerned with the study of the kagome staircase system Co₃V₂O₈. While prone to geometrical frustration due to its underlying kagome structural motif, this material is characterized by predominantly ferromagnetic interactions that lead to an unfrustrated, ferromagnetic ground state. In this chapter, departures from this conventional ground state by different disordering mechanisms are investigated. The first part focuses on the effects of site disorder by introducing quenched nonmagnetic impurities into the system. The growth of single crystals of (Co_1-xMg_x)₃V₂O₈ is reported. These crystals reveal that the ferromagnetic ground state is very sensitive to doping, and show that a low doping concentration of 19% leads to a suppression of the ferromagnetic ground state below 1.5 K. This could be understood as percolation problem on the quasi two-dimensional kagome lattice including site and bond percolation. The second part focuses on the influence of a transverse magnetic field on the ground state of Ising spins, introducing quantum fluctuations that lead to quantum phase transitions at ~6.25, 7 and 13 T. The observed quantum phase transitions are characterized by distinct changes in the magnetic structure and their associated spin excitation spectra.</p> <p>Chapter 5 presents studies on the pyrochlore antiferromagnet Tb₂Ti₂O₇, which is a proposed spin liquid candidate but whose actual ground state is still the topic of current debate. The ground state of Tb₂Ti₂O₇ was revisited by neutron scattering measurements, revealing a new phase in the low temperature low field phase diagram that can be described as a frozen antiferromagnetic spin ice that exhibits distinct elastic and inelastic scattering features.</p> / Doctor of Philosophy (PhD)