82 research outputs found
State Hierarchy Induced by Correlated Spin Domains in short range spin glasses
We generate equilibrium configurations for the three and four dimensional
Ising spin glass with Gaussian distributed couplings at temperatures well below
the transition temperature T_c. These states are analyzed by a recently
proposed method using clustering. The analysis reveals a hierarchical state
space structure. At each level of the hierarchy states are labeled by the
orientations of a set of correlated macroscopic spin domains. Our picture of
the low temperature phase of short range spin glasses is that of a State
Hierarchy Induced by Correlated Spin domains (SHICS). The complexity of the low
temperature phase is manifest in the fact that the composition of such a spin
domain (i.e. its constituent spins), as well as its identifying label, are
defined and determined by the ``location'' in the state hierarchy at which it
appears. Mapping out the phase space structure by means of the orientations
assumed by these domains enhances our ability to investigate the overlap
distribution, which we find to be non-trivial. Evidence is also presented that
these states may have a non-ultrametric structure.Comment: 30 pages, 17 figure
Phase Fluctuations and Pseudogap Phenomena
This article reviews the current status of precursor superconducting phase
fluctuations as a possible mechanism for pseudogap formation in
high-temperature superconductors. In particular we compare this approach which
relies on the two-dimensional nature of the superconductivity to the often used
-matrix approach. Starting from simple pairing Hamiltonians we present a
broad pedagogical introduction to the BCS-Bose crossover problem. The finite
temperature extension of these models naturally leads to a discussion of the
Berezinskii-Kosterlitz-Thouless superconducting transition and the related
phase diagram including the effects of quantum phase fluctuations and
impurities. We stress the differences between simple Bose-BCS crossover
theories and the current approach where one can have a large pseudogap region
even at high carrier density where the Fermi surface is well-defined. The
Green's function and its associated spectral function, which explicitly show
non-Fermi liquid behaviour, is constructed in the presence of vortices. Finally
different mechanisms including quasi-particle-vortex and vortex-vortex
interactions for the filling of the gap above are considered.Comment: 129 pages, Elsart, 28 EPS figures; Physics Reports, in press. Authors
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