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Criticality in Quantum Dissipative Systems

Einar B. Stiansen

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

This thesis consists of five scientific papers in the field of condensed matter physics. In all papers we employ large scale Monte Carlo simulations to investigate quantum critical behavior in systems coupled to an environment. Special attention is paid to possible anisotropies between spatial fluctuations and fluctuations in imaginary time. Implications of the results to the loop current theory of cuprates are discussed.

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This thesis consists of five scientific papers in the field of condensed matter physics. In all papers we employ large scale Monte Carlo simulations to investigate quantum critical behavior in systems coupled to an environment. Special attention is paid to possible anisotropies between spatial fluctuations and fluctuations in imaginary time. Implications of the results to the loop current theory of cuprates are discussed.
This thesis presents seven research papers on topics in condensed-matter theory. Five of the papers report on Monte Carlo studies of quantum phase transitions in various (d+1)-dimensional statistical mechanics models featuring Caldeira-Leggettlike dissipation. The principal motivation for these studies was to investigate a particular bond-dissipative (2+1)-dimensional XY model of circulating currents in cuprate high-temperature superconductors. It has been proposed that quantum critical fluctuations associated with a local quantum critical point described by this model can explain the marginal-Fermi-liquid behaviour of the normal state of these compounds. We present simulation results for this model for both compact and noncompact phase variables and show unambiguously that the quantum critical point in the compact case is not local. If the phases are taken to be noncompact variables, the model is also a model of resistively shunted Josephson junction arrays. The results in this case reveal a more complicated phase diagram, but we have not been able to establish critical behaviour consistent with the scenario of local quantum criticality.\n\nThe study of extended quantum dissipative models is also motivated by the general effect on condensed-matter systems of the coupling to environmental degrees of freedom. Their influence on quantum critical phenomena is characterized by the dynamical critical exponent z, a measure of spatiotemporal anisotropy, the value of which can be estimated by naive scaling arguments. We confirm by numerical means that such scaling estimates give correct results to a good approximations (with a few reservations), irrespective of system dimensionality, order parameter symmetry, or whether the variables are compact or noncompact. Corrections to the naive scaling estimates have to be invoked for strongly super-Ohmic dissipation for d = 1 due to relatively large values of the anomalous scaling dimension η.\n\nThe two last research papers are concerned with the superconducting pairing state of the recently discovered class of iron-based high-temperature superconductors. Here, we calculate possible signatures of the proposed s±-wave pairing state in conductance-spectroscopy and Josephson-effect experiments.
We use quantum Monte Carlo simulations to calculate the phase diagram and the correlation functions for the quantum phase transitions in the two-dimensional dissipative quantum XY model with and without fourfold anisotropy. Without anisotropy, the model describes the superconductor-to-insulator transition in two-dimensional dirty superconductors. With anisotropy, the model represents the loop-current order observed in the underdoped cuprates and its fluctuations, as well as the fluctuations near the ordering vector in simple models of two-dimensional itinerant ferromagnets and itinerant antiferromagnets. These calculations test an analytic solution of the model which reexpressed it in terms of topological excitations: the vortices with interactions only in space but none in time, and warps with leading interactions only in time but none in space, as well as subleading interactions which are both space and time dependent. For parameters where the proliferation of warps dominates the phase transition, the critical fluctuations as functions of the deviation of the dissipation parameter $\ensuremath{\alpha}$ on the disordered side from its critical value ${\ensuremath{\alpha}}_{c}$ are scale invariant in imaginary time $\ensuremath{\tau}$ as the correlation length in time ${\ensuremath{\xi}}_{\ensuremath{\tau}}={\ensuremath{\tau}}_{c}{e}^{{[{\ensuremath{\alpha}}_{c}/({\ensuremath{\alpha}}_{c}\ensuremath{-}\ensuremath{\alpha})]}^{1/2}}$ diverges, where ${\ensuremath{\tau}}_{c}$ is a short-time cutoff. On the other hand, the spatial correlations develop with a correlation length ${\ensuremath{\xi}}_{x}\ensuremath{\approx}{\ensuremath{\xi}}_{0}ln({\ensuremath{\xi}}_{\ensuremath{\tau}})$, with ${\ensuremath{\xi}}_{0}$ of the order of a lattice constant. The dynamic correlation exponent zzz is therefore $\ensuremath{\infty}$. The Monte Carlo calculations also directly show warps and vortices. Their densities and correlations across the various transitions in the model are calculated and related to those of the order-parameter correlations in the dissipative quantum XY model.
In recent years, quantum phase transitions have attracted the interest of both theorists and experimentalists in condensed matter physics. These transitions, which are accessed at zero temperature by variation of a non-thermal control parameter, can influence the behaviour of electronic systems over a wide range of the phase diagram. Quantum phase transitions occur as a result of competing ground state phases. The cuprate superconductors which can be tuned from a Mott insulating to a d -wave superconducting phase by carrier doping are a paradigmatic example. This review introduces important concepts of phase transitions and discusses the interplay of quantum and classical fluctuations near criticality. The main part of the article is devoted to bulk quantum phase transitions in condensed matter systems. Several classes of transitions will be briefly reviewed, pointing out, e.g., conceptual differences between ordering transitions in metallic and insulating systems. An interesting separate class of transitions is boundary phase transitions where only degrees of freedom of a subsystem become critical; this will be illustrated in a few examples. The article is aimed at bridging the gap between high-level theoretical presentations and research papers specialized in certain classes of materials. It will give an overview on a variety of different quantum transitions, critically discuss open theoretical questions, and frequently make contact with recent experiments in condensed matter physics.
This thesis present the recently theoretical and numerical results on 2D dissipative quantum XY model. The two-dimensional quantum XY model is applicable to the quantum critical properties of several experimental systems, such as superconductor to insulator transitions, ferromagnetic and antiferromagnetic transitions in metals, and loop current order transition in the cuprates. Renormalization group methods are applied to solve the reformulated ac- tion of the original model in terms of two type topological excitations: vortices and warps. The calculations explain the extraordinary properties of the model studied through quan- tum Monte Carlo simulations: the separability of the correlation function in space and time, the correlation length in space proportional to logarithm of the correlation length in time near the transition from disordered phase to ordered phase. The running dynamical critical exponent is introduced to address the anisotropy between time and space. The effects of anisotropy fields have been examined through renormalization group method. The transi- tion from disordered phase to ordered phase of this model has been studied by quantum Monte Carlo. The divergence of temporal correlation length in function of (Kc − K)/Kc is examined by numerical simulation. The logarithmic relation between temporal correlation length and spacial correlation length is further confirmed. Also, the same logarithmic rela- tion for different correlation function with different space separation is found and implicitly confirmed the separability of correlation function in space and time.
This thesis presents six research papers on topics in condensed-matter theory and one paper on a computational method.In two of the papers the focus is on the search for Deconfined Quantum Criticality in models describing quantum antiferromagnets on the square lattice. DQC is conjectured to be a quantum critical point between Neel and valence bond solid ordering in the models. If true, it would represent a schism with the Landau Ginzburg Wilson paradigm of phase transitions, inwhich such a critical point cannot be generic. Verification of the existence of DQC could have consequences not only for our understanding of phase transitions in general, but more specifically also for the understanding of the cuprate superconductors.Despite extensive Monte Carlo simulations, we are not able to reach a conclusion on the character of the phase transition for one of the proposed DQC models. In another, more fundamental model, no phase transition is found at all, severely undermining a lot of the theoretical framework assumed to be true in earlier work.Four of the papers investigate thermodynamical properties of superconductors with several (three or more) superconducting bands. This research is motivated by the multiband iron based superconductors, but the results should be quite general.Using large scale Monte Carlo simulations, we reveal that a new, time reversal symmetry broken non-superconducting phase should be viable for repulsive inter-band interactions. This is in contrast to mean field studies where no such phase is found, signifying the importance of fluctuations in these systems, even in three dimensions.In the last paper we present a multi-histogram reweighting method for nonequilibrium Markov chains with discrete energies.