272 research outputs found
Distribution of the local density of states, reflection coefficient and Wigner delay time in absorbing ergodic systems at the point of chiral symmetry
Employing the chiral Unitary Ensemble of random matrices we calculate the
probability distribution of the local density of states for zero-dimensional
("quantum chaotic") two-sublattice systems at the point of chiral symmetry E=0
and in the presence of uniform absorption. The obtained result can be used to
find the distributions of the reflection coefficent and of the Wigner time
delay for such systems.Comment: 4 pages, 3 figure
Probing ultracold Fermi gases with light-induced gauge potentials
We theoretically investigate the response of a two component Fermi gas to
vector potentials which couple separately to the two spin components. Such
vector potentials may be implemented in ultracold atomic gases using optically
dressed states. Our study indicates that light-induced gauge potentials may be
used to probe the properies of the interacting ultracold Fermi gas, providing.
amongst other things, ways to measure the superfluid density and the strength
of pairing.Comment: 8 pages, 3 figure
Pairing Fluctuations Determine Low Energy Electronic Spectra in Cuprate Superconductors
We describe here a minimal theory of tight binding electrons moving on the
square planar Cu lattice of the hole-doped cuprates and mixed quantum
mechanically with pairs of them (Cooper pairs). Superconductivity occurring at
the transition temperature T_c is the long-range, d-wave symmetry phase
coherence of these Cooper pairs. Fluctuations necessarily associated with
incipient long-range superconducting order have a generic large distance
behaviour near T_c. We calculate the spectral density of electrons coupled to
such Cooper pair fluctuations and show that features observed in Angle Resolved
Photo Emission Spectroscopy (ARPES) experiments on different cuprates above T_c
as a function of doping and temperature emerge naturally in this description.
These include `Fermi arcs' with temperature-dependent length and an antinodal
pseudogap which fills up linearly as the temperature increases towards the
pseudogap temperature. Our results agree quantitatively with experiment. Below
T_c, the effects of nonzero superfluid density and thermal fluctuations are
calculated and compared successfully with some recent ARPES experiments,
especially the observed `bending' or deviation of the superconducting gap from
the canonical d-wave form.Comment: 14 pages, 8 figures (to appear in Phys. Rev. B
A nearly closed ballistic billiard with random boundary transmission
A variety of mesoscopic systems can be represented as a billiard with a
random coupling to the exterior at the boundary. Examples include quantum dots
with multiple leads, quantum corrals with different kinds of atoms forming the
boundary, and optical cavities with random surface refractive index. The
specific example we study is a circular (integrable) billiard with no internal
impurities weakly coupled to the exterior by a large number of leads with one
channel open in each lead. We construct a supersymmetric nonlinear
-model by averaging over the random coupling strengths between bound
states and channels. The resulting theory can be used to evaluate the
statistical properties of any physically measurable quantity in a billiard. As
an illustration, we present results for the local density of states.Comment: 5 pages, 1 figur
Dynamic response of mesoscopic metal rings and thermodynamics at constant particle number
We show by means of simple exact manipulations that the thermodynamic
persistent current in a mesoscopic metal ring threaded by a
magnetic flux at constant particle number agrees even beyond linear
response with the dynamic current that is defined via the
response to a time-dependent flux in the limit that the frequency of the flux
vanishes. However, it is impossible to express the disorder average of in terms of conventional Green's functions at flux-independent
chemical potential, because the part of the dynamic response function that
involves two retarded and two advanced Green's functions is not negligible.
Therefore the dynamics cannot be used to map a canonical average onto a more
tractable grand canonical one. We also calculate the zero frequency limit of
the dynamic current at constant chemical potential beyond linear response and
show that it is fundamentally different from any thermodynamic derivative.Comment: 19 pages, postscript (uuencoded, compressed
Localization and delocalization in dirty superconducting wires
We present Fokker-Planck equations that describe transport of heat and spin
in dirty unconventional superconducting quantum wires. Four symmetry classes
are distinguished, depending on the presence or absence of time-reversal and
spin rotation invariance. In the absence of spin-rotation symmetry, heat
transport is anomalous in that the mean conductance decays like
instead of exponentially fast for large enough length of the wire. The
Fokker-Planck equations in the presence of time-reversal symmetry are solved
exactly and the mean conductance for quasiparticle transport is calculated for
the crossover from the diffusive to the localized regime.Comment: 4 pages, RevTe
The supersymmetric technique for random-matrix ensembles with zero eigenvalues
The supersymmetric technique is applied to computing the average spectral
density near zero energy in the large-N limit of the random-matrix ensembles
with zero eigenvalues: B, DIII-odd, and the chiral ensembles (classes AIII,
BDI, and CII). The supersymmetric calculations reproduce the existing results
obtained by other methods. The effect of zero eigenvalues may be interpreted as
reducing the symmetry of the zero-energy supersymmetric action by breaking a
certain abelian symmetry.Comment: 22 pages, introduction modified, one reference adde
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