In this thesis we have studied phase transitions in a polymer solution and also in a two dimensional (2D) disordered system. In our study on the polymer system we present a model in an attempt to understand the observed crossover of a stiff polymer chain with side groups in a solution from its Gaussian (or ordinary random walk) behavior to a self avoiding walk (SAW) behavior. While our analysis based on usual persistent self avoiding walk (PSAW) and renormalization groupn this work does show a sudden onset of the excluded volume effect, much lik show a gradual increase in excluded volume effect, the relaxational method that we have developed ie that observed in the experiment [Murakami et al., Macromolecules 13, 345 (1980)]. We also have studied the transport properties of a 2D disordered system known as quantum percolation using stationary state and dynamical methods. Both methods involve the calculation of the transmission coefficient of a particle that enters through a lead at one side of the cluster and exits through another lead on the other side of the 2D cluster. Using the stationary state method we have shown that the transmission of a quantum particle crucially depends on the local phases of wave function. The states are observed to have high transmission if the local phases of the neighboring sites differ by 0 or ∼ 2π, where as those with a phase difference of ∼ π give low transmission. We have investigated the localization property of the 2D disordered system using both the stationary state and dynamical methods. Our study based on finite size scaling shows that, for a given energy, all states are exponentially localized for a sufficiently large disorder. As the disorder is decreased below a critical value (which depends on the energy of the particle), the particle state makes a transition to a power-law localized regime. A further decrease in disorder, to typically less than 15% (but dependent on energy), appears to result in a second phase transition from the power-law localization regime to an extended state regime.
In this thesis we have studied phase transitions in a polymer solution and also in a two dimensional (2D) disordered system. In our study on the polymer system we present a model in an attempt to understand the observed crossover of a stiff polymer chain with side groups in a solution from its Gaussian (or ordinary random walk) behavior to a self avoiding walk (SAW) behavior. While our analysis based on usual persistent self avoiding walk (PSAW) and renormalization groupn this work does show a sudden onset of the excluded volume effect, much lik show a gradual increase in excluded volume effect, the relaxational method that we have developed ie that observed in the experiment [Murakami et al., Macromolecules 13, 345 (1980)]. We also have studied the transport properties of a 2D disordered system known as quantum percolation using stationary state and dynamical methods. Both methods involve the calculation of the transmission coefficient of a particle that enters through a lead at one side of the cluster and exits through another lead on the other side of the 2D cluster. Using the stationary state method we have shown that the transmission of a quantum particle crucially depends on the local phases of wave function. The states are observed to have high transmission if the local phases of the neighboring sites differ by 0 or ∼ 2π, where as those with a phase difference of ∼ π give low transmission. We have investigated the localization property of the 2D disordered system using both the stationary state and dynamical methods. Our study based on finite size scaling shows that, for a given energy, all states are exponentially localized for a sufficiently large disorder. As the disorder is decreased below a critical value (which depends on the energy of the particle), the particle state makes a transition to a power-law localized regime. A further decrease in disorder, to typically less than 15% (but dependent on energy), appears to result in a second phase transition from the power-law localization regime to an extended state regime.
A. The Course.- Molecules Coupled to Their Environment.- Theories of Molecular Chirality: A Short Review.- Condensed Cooper Pairs and Macroscopic Quantum Phenomena.- Non-Equilibrium Statistical Mechanics: Dynamics of Macroscopic Observables.- Quantum Brownian Motion.- Localization Critical Exponents.- Stochastic Models of Population and Phase Relaxation.- Classical and Quantum, Lattice and Continuum Percolation.- Tunneling and Relaxation in Low Temperature Systems.- Dynamics of Quantum Particles: Coupled Coherent and Incoherent Motion.- Chaotic Motion of Molecular Chains.- Complex Surface Geometry in Nano-Structure Solids: Fractal versus Bernal-Type Models.- Aggregation Phenomena.- Electrodeposition: Phenomenology and Theory.- Screening in Electrolytes and in Polymer Solutions: The Charge Structure Function.- In Search of Scaling Laws in Porous Silica Gels.- Stochastic Aspects in Reaction Kinetics.- B. The Seminars.- I Quantum Theory of Large Systems.- Coherence and Quantum Mechanics.- The Dynamical Generation of Macroscopic Coherent Light.- Macroscopically Inhomogeneous Bose-Einstein Condensation.- Generalized Squeezing of Boson States.- Equilibrium States of Long-Range Interacting Quantum Lattice Systems.- Environment and Symmetry Breaking in Quantum Field Theory.- Gauge Chemistry.- II Localized and Extended States.- Coherence Effects in Excitation Transfer: Application to Hexagonal Photosynthetic Unit.- Random Matrix Theory and Anderson Localisation.- Local Moments and the Localization of Electrons in Liquids.- How Universal is the Scaling Theory of Localization?.- Dimers, Repulsions, and the Absence of Localisation.- III Transport and Reactions.- Electronic Excitations in Polysilanes: Frenkel Excitons of a Disordered Chain.- The Study of Energy Change in Radical Systems.- Decay From an Initial Unstable State in a Chemical Reaction Model.- The Effects of Static Disorder on Polaron Transport.- Diffusion in a Disordered Line.- Bimolecular Diffuion-Limited Reaction Kinetics at Steady State.- Dynamic Percolation Theory for Diffusion of Interacting Particles: Tracer Diffusion in a Multi-Component Lattice Gas.- Crossover from Dispersive to Diffusive Energy Transport.- The Coupling Scheme for Relaxations in Complex Correlated Systems.- Simulation of Excitation Transport in Disordered Media.- Dynamical Exponents for 1-D Random-Random Directed Walks.- IV Polymers.- Interactions between Poly(styrene-co-styrene Sulfonic Acid) and Poly(methylmethacrylate-co-4-vinyl Pyridine) in Dimethyl Sulfoxide Solution by Photon Correlation Spectroscopy.- Interactions of Sigma Conjugated Polymers With Strong Optical Fields.- Randomly Branched Polymers.- Multi-Particle Relaxation in Electronically Excited Polymers: Distribution of Transition Rates from Fluorescence Data - A Numerical Approach.- Understanding Heat Conduction in Oriented Polymers.- Static Correlations of Polymer Chains in Networks.- Theory of Polymers on Fractal Lattices.- V Disordered and Low-Dimensional Systems.- Self-Consistent Interpretation of Percolation in a Microemulsion.- Experimental Evidence of Fractal Aggregates in Dense Microemulsions.- Fractal Dynamics of the Catalytic CO-Oxydation - Application of Fractal Cellular Automata Models.- Electrodeposition: Fractal and Multifractal Measures.- High Resolution Spectroscopy of Langmuir-Blodgett Films.- Molecular Dynamics Simulation of the Transport of Small Molecules Across a Polymer Membrane.- Reactions in Microemulsions: Fractal Modeling.- Enhanced Membrane Rigidity in Charged Lamellar Phases.- VI Words of thanks.- Lecturers.- Participants.
The study of non-equilibrium behaviors of quantum matter is one of the research directions at the frontiers of condensed matter physics and, in general, of theoretical and experimental physics. Understanding the mechanisms which prevent many quantum particles from reaching thermal equilibrium can ultimately shed light on the foundations of statistical physics and provide insight into the anomalous long-time collective behaviors observed in experiments. In this thesis, we present new theoretical developments in this direction. In the first part of the thesis we discuss the non-equilibrium dynamics of the two-dimensional quantum Ising model, the prototypical model of magnetism. In particular, we investigate the dynamics of interfaces separating different spin domains. In some regimes, we can provide an exact description of their unitary time evolution, but we also discuss the robustness of our predictions away from the exactly-solvable limit. Moreover, we address the same problem in the presence of disorder and provide a hydrodynamic interpretation of our findings. In the second part of the thesis, we investigate the localization properties of disordered quantum systems. In particular, we develop a renormalization-group picture of the Anderson localization transition in generic geometries. This provides a new framework to understand the phase diagram of such systems and their critical behavior beyond the usual finite-size scaling analysis. We also provide further insights on this topic both by studying resonances and their connection with observables and by employing unsupervised non-parametric learning techniques to probe disordered systems in an experimentally accessible form. The results presented in this thesis constitute a step forward in understanding the non-equilibrium phases and regimes of quantum matter and they pave the way to interesting future developments.
The interplay between quenched disorder and interaction effects opens the possibility in a closed quantum many-body system of a phase transition at finite energy density between an ergodic phase, which is governed by the laws of statistical physics, and a localized one, in which the degrees of freedom are frozen and ergodicity breaks down. The possible existence of a quantum phase transition at finite energy density is strongly questioning our understanding of the fundamental laws of nature and has generated an active field of research called many-body localization. This thesis consists of three parts and is dedicated to the understanding and characterization of the phenomenon of many-body localization, approaching it from complementary facets. In particular, borrowing methods and tools from different fields, we analyze timely problems. The first part of the thesis is devoted to detecting the many-body localization transition and to characterize both the ergodic and the localized phase it separates. Here we provide a characterization from two different perspectives: the first one is based on the study of local entanglement properties. In the second one, using tools from quantum-chaos theory, we attempt to answer the question of understanding time-irreversibility, and thus probing the breaking of ergodicity. We analyze experimentally viable observables. Moreover, we propose two different quantities to distinguish an Anderson insulating phase from a many-body localized one, which is one of the issues in experiments. The second part focuses on understanding the existence of a putative subdiffusive multifractal phase. Analyzing the quantum dynamics of the system in this region of the phase diagram, we point out the importance of finite-size effects, questioning the existence of this multifractal phase. We speculate with a possible scenario in which the diffusivity and thus ergodicity could be restored in the thermodynamic limit. Furthermore, we find that the propagation is highly non-Gaussian, which could have an important effect on understanding the critical point of the according transition. We tackle this problem also from a different angle. A possible toy-model to understand many-body localization entails the Anderson model on a random-regular graph. Also in the latter model the possible existence of an intermediate multifractal phase has been conjectured. There, studying the survival return probability of a particle with time, we give a new characterization of multifractal phases and give indication of the possible existence of this phase. Nevertheless, we also outline possible caveats. In the last part of this thesis we study the interplay between symmetry and correlated disorder in a non-interacting fermionic system. We show another possible mechanism for breaking localization. In particular, we focus on studying information and particle transport, emphasizing how the two types of propagation can be different.
In this thesis, we study the quantum phase transition triggered by an external random po- tential in ultra-cold low-dimensional weakly-interacting Bose gases at zero temperature. In one-dimensional systems, the quantum phases are characterized by the decay at long-range of the one-body density matrix. This decay exhibits an algebraic behaviour in the superfluid quasi-condensed phase while in the insulating Bose glass phase this it becomes exponen- tial, signaling the complete loss of coherence in the system. In two-dimensional systems, a characterization based on the long-range behaviour of the one-body density matrix appears to be highly demanding from a computational point of view. We therefore characterized the superfluid-insulator transition in two-dimensional systems by the low-energy behaviour of the cumulative density of states of the Bogoliubov excitations. While in the superfluid condensed phase this quantity grows as the square of the excitation energy in agreement with the existence of a finite velocity of sound, in the insulating Bose glass phase this power law is less than quadratic. This study is performed in the framework of an extended Bogoliubov approach properly adapted to treat low-dimensional Bose gases. Using a systematic numerical study, we draw the interaction-disorder phase diagrams of the superfluid-insulator transition in 1D and 2D. The phase boundary follows two different power- laws depending on the length scales characterizing the spatial correlations of the disorder and the strength of interactions. The power-law exponents were found to be in agreement with the ones predicted by scaling arguments, both in the white noise and the Thomas-Fermi regimes. In the two-dimensional system, the classical percolation threshold in the Thomas-Fermi limit was found to overestimate the critical disorder below which the onset of superfluidity should be observed for a given interaction strength.
Understanding the effects of disorder in quantum many-body systems is one of the central challenges of contemporary statistical physics. Its interplay with interactions and quantum fluctuations profoundly modifies the nature of quantum states and their dynamics, giving rise to phenomena such as localization, ergodicity breaking, and the emergence of complex collective behavior. This thesis presents new results on two classes of disordered quantum systems. The first part is devoted to the study of the many-body localization transition, a paradigmatic example of disorder-induced ergodicity breaking. The critical properties of the transition are investigated using a numerical renormalization-group approach, establishing a connection between its scaling behavior and that of the Anderson localization transition on tree-like graphs. The second part focuses on the zero-temperature quantum spin-glass phase of the two-dimensional bond-disordered Heisenberg model. Combining large-scale numerical simulations with a semiclassical expansion, evidence is provided for the existence of the spin-glass phase. The effective model governing the semiclassical fluctuations is further analyzed to characterize the low-energy excitation spectrum and its localization properties. Finally, the weak-disorder regime is investigated and its implications for the ferromagnetic phase are discussed. Overall, the results presented in this thesis contribute to the understanding of the effects of disorder in interacting quantum many-body systems, encompassing both localization phenomena and the collective behavior of disordered quantum magnets.
Disordered elastic systems are studied in the quantum, classical, and dynamic regime in the present thesis. The thesis consists of two main parts: In the first part we study the low temperature phase diagram of one-dimensional weakly disordered quantum systems like charge or spin density waves and Luttinger liquids by a full finite temperature renormalization group (RG) calculation. In the classical region, for vanishing quantum fluctuations those results are supplemented by an exact solution of the model in the case of strong disorder, described by the ground state and the correlation function. Furthermore, by a mapping of the problem onto a Burgers equation with noise, in the case of weak disorder, we can derive an expression for the correlation length. At zero temperature we reproduce the (quantum) phase transition between a pinned (localized) and an unpinned (delocalized) phase for weak and strong quantum fluctuations, respectively, as found previously by Fukuyama or Giamarchi and Schulz. At finite temperatures the localization transition is suppressed: the random potential is wiped out by thermal fluctuations on length scales larger than the thermal de Broglie wave length of the phason excitations. The existence of a zero temperature transition is reflected in a rich cross-over phase diagram determined by the correlation functions. In particular we find four different scaling regions: a classical disordered, a quantum disordered, a quantum critical, and a thermal region. The results can be transferred directly to the discussion of the influence of disorder in superfluids. Finally we extend the RG calculation to the treatment of a commensurate lattice potential, and for the case of density waves we discuss the influence of quantum phase-slips, which might lead to a new scenario for the unpinning (delocalization) transition at zero temperature. Additionally, we analyze the current noise in a classical one-dimensional charge density wave system in the weak pinning regime by solving the overdamped equation of motion numerically. At low temperatures and just above the zero temperature depinning threshold, the power spectrum of the current noise reveals the existence of 1/f or flicker noise. Our results are in agreement with experimental measurements in quasi-one-dimensional charge density wave systems. In part two, we examine the viscous motion of an interface driven by a periodically oscillating external field in a random medium. The velocity exhibits a smeared depinning transition showing a pronounced hysteresis, which is absent in the adiabatic case. Its width is determined by a new length scale, introduced by the frequency of the external drive and - in the low frequency regime - by the critical exponents of the zero frequency depinning transition. Scaling arguments and an approximate renormalization group treatment predict a power law behavior of the velocity as a function of frequency and field amplitude at the dc depinning threshold, which is confirmed numerically. Thermal fluctuations lead to an additional smearing of the transition. If the amplitude of the external driving force is smaller than the depinning field, the motion of the interface by avalanches has to be taken into account in order to describe the velocity hysteresis. To check the validity of our model with respect to experimental results, we calculate the complex susceptibilities using an adiabatic and non-adiabatic approach numerically for finite systems and compare it to measurements of a superferromagnetic granular multilayer system and find that the model of an ac driven interface in a disordered environment can describe the main features of the experimental results. Additionally, the influence of a strong surface potential on the critical depinning of an elastic system, driven in a random medium is considered. If the surface potential prevents depinning completely, the curvature of the displacement profile at zero temperature exhibits a pronounced rhombic hysteresis curve with width of two times the (bulk) depinning field. The hysteresis disappears at non-zero temperatures if the driving force is changed adiabatically. If the surface depinns by the applied force or thermal creep, the curvature is reduced with increasing velocity. The results apply, e.g., to driven magnetic domain walls, flux-line lattices, and charge-density waves. As addendum we examine low dimensional interacting, but clean Bose systems at low temperatures. The interaction leads to scattering events of the over-condensate particles for which we calculate the scattering times using Fermi's golden rule. With that we derive the thermal conductivity and the related weak localization corrections. This is done for short and long-range interactions.