14,153 research outputs found
Spin glass models with Kac interactions
In this paper I will review my work on disordered systems -spin glass model
with two body and body interactions- with long but finite interaction
range . I will describe the relation of these model with Mean Field Theory
in the Kac limit and some attempts to go beyond mean field.Comment: Proceedings of the Stat-phys23 conferenc
Impurity scattering and localization in -wave superconductors
Strong evidence is presented for the localization of low energy quasiparticle
states in disordered -wave superconductors. Within the framework of the
Bogoliubov-de Gennes (BdG) theory applied to the extended Hubbard model with a
finite concentration of non-magnetic impurities, we carry out a fully
self-consistent numerical diagonalization of the BdG equations on finite
clusters containing up to sites. Localized states are identified
by probing their sensitivity to the boundary conditions and by analyzing the
finite size dependence of inverse participation ratios.Comment: 4 pages REVTeX with 2 embedded .ps figures; submitted to PRB as Rapid
Communicatio
Quantization and Periodicity of the Axion Action in Topological Insulators
The Lagrangian describing the bulk electromagnetic response of a
three-dimensional strong topological insulator contains a topological `axion'
term of the form '\theta E dot B'. It is often stated (without proof) that the
corresponding action is quantized on periodic space-time and therefore
invariant under '\theta -> \theta +2\pi'. Here we provide a simple, physically
motivated proof of the axion action quantization on the periodic space-time,
assuming only that the vector potential is consistent with single-valuedness of
the electron wavefunctions in the underlying insulator.Comment: 4 pages, 1 figure, version2 (section on axion action quantization of
non-periodic systems added
First steps of a nucleation theory in disordered systems
We devise a field theoretical formalism for a microscopic theory of
nucleation processes and phase coexistence in finite dimensional glassy
systems. We study disordered -spin models with large but finite range of
interaction. We work in the framework of glassy effective potential theory
which in mean-field is a non-convex, two minima function of the overlap. We
will associate metastability and phase coexistence with the existence of space
inhomogeneous solution of suitable field equations and we will study the
simplest of such solutions.Comment: 31 pages, 4 figures. Content revised, typos correcte
A simple stochastic model for the dynamics of condensation
We consider the dynamics of a model introduced recently by Bialas, Burda and
Johnston. At equilibrium the model exhibits a transition between a fluid and a
condensed phase. For long evolution times the dynamics of condensation
possesses a scaling regime that we study by analytical and numerical means. We
determine the scaling form of the occupation number probabilities. The
behaviour of the two-time correlations of the energy demonstrates that aging
takes place in the condensed phase, while it does not in the fluid phase.Comment: 8 pages, plain tex, 2 figure
On chaos in mean field spin glasses
We study the correlations between two equilibrium states of SK spin glasses
at different temperatures or magnetic fields. The question, presiously
investigated by Kondor and Kondor and V\'egs\"o, is approached here
constraining two copies of the same system at different external parameters to
have a fixed overlap. We find that imposing an overlap different from the
minimal one implies an extensive cost in free energy. This confirms by a
different method the Kondor's finding that equilibrium states corresponding to
different values of the external parameters are completely uncorrelated. We
also consider the Generalized Random Energy Model of Derrida as an example of
system with strong correlations among states at different temperatures.Comment: 19 pages, Late
Penumbral thermal structure below the visible surface
. The thermal structure of the penumbra below its visible surface
(i.e., ) has important implications for our present understanding
of sunspots and their penumbrae: their brightness and energy transport, mode
conversion of magneto-acoustic waves, sunspot seismology, and so forth. .
We aim at determining the thermal stratification in the layers immediately
beneath the visible surface of the penumbra: ( km below the visible continuum-forming layer). . We analyzed
spectropolarimetric data (i.e., Stokes profiles) in three Fe \textsc{i} lines
located at 1565 nm observed with the GRIS instrument attached to the 1.5-meter
solar telescope GREGOR. The data are corrected for the smearing effects of
wide-angle scattered light and then subjected to an inversion code for the
radiative transfer equation in order to retrieve, among others, the temperature
as a function of optical depth . . We find that the
temperature gradient below the visible surface of the penumbra is smaller than
in the quiet Sun. This implies that in the region the penumbral
temperature diverges from that of the quiet Sun. The same result is obtained
when focusing only on the thermal structure below the surface of bright
penumbral filaments. We interpret these results as evidence of a thick
penumbra, whereby the magnetopause is not located near its visible surface. In
addition, we find that the temperature gradient in bright penumbral filaments
is lower than in granules. This can be explained in terms of the limited
expansion of a hot upflow inside a penumbral filament relative to a granular
upflow, as magnetic pressure and tension forces from the surrounding penumbral
magnetic field hinder an expansion like this.Comment: 5 pages; 2 figures; accepted for publication in Astronomy and
Astrophysics Letter
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