60,407 research outputs found
Quantum phase transitions
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 behavior 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 are 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 on bridging the gap between
high-level theoretical presentations and research papers specialized in certain
classes of materials. It will give an overview over a variety of different
quantum transitions, critically discuss open theoretical questions, and
frequently make contact with recent experiments in condensed matter physics.Comment: 50 pages, 7 figs; (v2) final version as publishe
A Particle-based Multiscale Solver for Compressible Liquid-Vapor Flow
To describe complex flow systems accurately, it is in many cases important to
account for the properties of fluid flows on a microscopic scale. In this work,
we focus on the description of liquid-vapor flow with a sharp interface between
the phases. The local phase dynamics at the interface can be interpreted as a
Riemann problem for which we develop a multiscale solver in the spirit of the
heterogeneous multiscale method, using a particle-based microscale model to
augment the macroscopic two-phase flow system. The application of a microscale
model makes it possible to use the intrinsic properties of the fluid at the
microscale, instead of formulating (ad-hoc) constitutive relations
Emergence and spontaneous breaking of approximate O(4) symmetry at a weakly first-order deconfined phase transition
We investigate approximate emergent nonabelian symmetry in a class of weakly
first order `deconfined' phase transitions using Monte Carlo simulations and a
renormalization group analysis. We study a transition in a 3D classical loop
model that is analogous to a deconfined 2+1D quantum phase transition in a
magnet with reduced lattice symmetry. The transition is between the N\'eel
phase and a twofold degenerate valence bond solid (lattice-symmetry-breaking)
phase. The combined order parameter at the transition is effectively a
four-component superspin. It has been argued that in some weakly first order
`pseudocritical' deconfined phase transitions, the renormalization group flow
can take the system very close to the ordered fixed point of the symmetric
sigma model, where is the total number of `soft' order parameter
components, despite the fact that is not a microscopic symmetry. This
yields a first order transition with unconventional phenomenology. We argue
that this occurs in the present model, with . This means that there is a
regime of lengthscales in which the transition resembles a `spin-flop'
transition in the ordered sigma model. We give numerical evidence for
(i) the first order nature of the transition, (ii) the emergence of
symmetry to an accurate approximation, and (iii) the existence of a regime in
which the emergent is `spontaneously broken', with distinctive features
in the order parameter probability distribution. These results may be relevant
for other models studied in the literature, including 2+1D QED with two
flavours, the `easy-plane' deconfined critical point, and the N\'eel--VBS
transition on the rectangular lattice.Comment: 16 pages. v2: updated to journal versio
Gravity as an emergent phenomenon: a GFT perspective
While the idea of gravity as an emergent phenomenon is an intriguing one,
little is known about concrete implementations that could lead to viable
phenomenology, most of the obstructions being related to the intrinsic
difficulties of formulating genuinely pregeometric theories. In this paper we
present a preliminary discussion of the impact of critical behavior of certain
microscopic models for gravity, based on group field theories, on the dynamics
of the macroscopic regime. The continuum limit is examined in light of some
scaling assumption, and the relevant consequences for low energy effective
theories are discussed, the role of universality, the corrections to scaling,
the emergence of gravitational theories and the nature of their thermodynamical
behavior.Comment: 1+26 page
The superfluid insulator transition of ultra-cold bosons in disordered 1d traps
We derive an effective quantum Josephson array model for a weakly interacting
one-dimensional condensate that is fragmented into weakly coupled puddles by a
disorder potential. The distribution of coupling constants, obtained from first
principles, indicate that weakly interacting bosons in a disorder potential
undergo a superfluid insulator transition controlled by a strong randomness
fixed point [Phys. Rev. Lett. 93, 150402 (2004)]. We compute renormalization
group flows for concrete realizations of the disorder potential to facilitate
finite size scaling of experimental results and allow comparison to the
behavior dictated by the strong randomness fixed point. The phase diagram of
the system is obtained with corrections to mean-field results.Comment: 10 pages, 6 figures, expanded version including a calculation of a
global phase diagra
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