250 research outputs found
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
Effect of electron-lattice interaction on the phase separation in strongly correlated electron systems with two types of charge carriers
The effect of electron-lattice interaction is studied for a strongly
correlated electron system described by the two-band Hubbard model. A two-fold
effect of electron-lattice interaction is taken into account: in non-diagonal
terms, it changes the effective bandwidth, whereas in diagonal terms, it shifts
the positions of the bands and the chemical potential. It is shown that this
interaction significantly affects the doping range corresponding to the
electronic phase separation and can even lead to a jump-like transition between
states with different values of strains.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Ballistic dynamics of a convex smooth-wall billiard with finite escape rate along the boundary
We focus on the problem of an impurity-free billiard with a random
position-dependent boundary coupling to the environment. The response functions
of such an open system can be obtained non-perturbatively from a supersymmetric
generating functional. The derivation of this functional is based on averaging
over the escape rates and results in a non-linear ballistic -model,
characterized by system-specific parameters. Particular emphasis is placed on
the {}``whispering gallery modes'' as the origin of surface diffusion modes in
the limit of large dimensionless conductance.Comment: 12 pages, no figure
Feasibility of a Small, Rapid Optical-to-IR Response, Next Generation Gamma Ray Burst Mission
We present motivations for and study feasibility of a small, rapid optical to
IR response gamma ray burst (GRB) space observatory. By analyzing existing GRB
data, we give realistic detection rates for X-ray and optical/IR instruments of
modest size under actual flight conditions. Given new capabilities of fast
optical/IR response (about 1 s to target) and simultaneous multi-band imaging,
such an observatory can have a reasonable event rate, likely leading to new
science. Requiring a Swift-like orbit, duty cycle, and observing constraints, a
Swift-BAT scaled down to 190 square cm of detector area would still detect and
locate about 27 GRB per yr. for a trigger threshold of 6.5 sigma. About 23
percent of X-ray located GRB would be detected optically for a 10 cm diameter
instrument (about 6 per yr. for the 6.5 sigma X-ray trigger).Comment: Elaborated text version of a poster presented at 2012 Malaga/Marbella
symposiu
Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction
We propose a minimal model for the Josephson current through a quantum dot in
a Kondo regime. We start with the model that consists of an Anderson impurity
connected to two superconducting (SC) leads with the gaps
, where for the lead at left and right. We show that, when one of the SC gaps is
much larger than the others , the starting model can
be mapped exactly onto the single-channel model, which consists of the right
lead of and the Anderson impurity with an extra onsite SC gap of
. Here and are
defined with respect to the starting model, and is the level width
due to the coupling with the left lead. Based on this simplified model, we
study the ground-state properties for the asymmetric gap, , using the numerical renormalization group (NRG) method. The
results show that the phase difference of the SC gaps , which induces the Josephson current, disturbs the screening of the
local moment to destabilize the singlet ground state typical of the Kondo
system. It can also drive the quantum phase transition to a magnetic doublet
ground state, and at the critical point the Josephson current shows a
discontinuous change. The asymmetry of the two SC gaps causes a re-entrant
magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte
Microwave-induced pi-junction transition in a superconductor / quantum-dot / superconductor structure
Using the nonequilibrium Green function, we show that microwave irradiation
can reverse the supercurrent flowing through a superconductor / quantum-dot /
superconductor structure. In contrast with the conventional sideband effect in
normal-metal / quantum-dot / normal-metal junctions, the photon-assisted
structures appear near ,
where is the resonant energy level of the quantum dot and is
the frequency of microwave field. Each photon-assisted structure is composed of
a negative and a positive peak, with an abrupt jump from the negative peak to
the positive peak around . The
microwave-induced -junction transition is interpreted in the picture of
photon-assisted Andreev bound states, which are formed due to multiple
photon-assisted Andreev reflection between the two superconductors. Moreover,
the main resonance located at can also be reversed with proper
microwave strength and frequency.Comment: 10 pagres, 3 figure
Non-exponential Dissipation in a Lossy Elastodynamic Billiard, Comparison with Porter-Thomas and Random Matrix Predictions
We study the dissipation of diffuse ultrasonic energy in a reverberant body
coupled to a waveguide, an analog for a mesoscopic electron in a quantum dot. A
simple model predicts a Porter-Thomas like distribution of level widths and
corresponding nonexponential dissipation, a behavior largely confirmed by
measurements. For the case of fully open channels, however, measurements
deviate from this model to a statistically significant degree. A random matrix
supersymmetric calculation is found to accurately model the observed behaviors
at all coupling strengths.Comment: 4 pages, 8 figures, figures resized, misprints correcte
Simple Bosonization Solution of the 2-channel Kondo Model: I. Analytical Calculation of Finite-Size Crossover Spectrum
We present in detail a simple, exact solution of the anisotropic 2-channel
Kondo (2CK) model at its Toulouse point. We reduce the model to a quadratic
resonant-level model by generalizing the bosonization-refermionization approach
of Emery and Kivelson to finite system size, but improve their method in two
ways: firstly, we construct all boson fields and Klein factors explicitly in
terms of the model's original fermion operators , and secondly
we clarify explicitly how the Klein factors needed when refermionizing act on
the original Fock space. This enables us to explicitly follow the adiabatic
evolution of the 2CK model's free-fermion states to its exact eigenstates,
found by simply diagonalizing the resonant-level model for arbitrary magnetic
fields and spin-flip coupling strengths. In this way we obtain an {\em
analytic} description of the cross-over from the free to the non-Fermi-liquid
fixed point. At the latter, it is remarkably simple to recover the conformal
field theory results for the finite-size spectrum (implying a direct proof of
Affleck and Ludwig's fusion hypothesis). By analyzing the finite-size spectrum,
we directly obtain the operator content of the 2CK fixed point and the
dimension of various relevant and irrelevant perturbations. Our method can
easily be generalized to include various symmetry-breaking perturbations.
Furthermore it establishes instructive connections between different
renormalization group schemes such as poor man's scaling, Anderson-Yuval type
scaling, the numerical renormalization group and finite-size scaling.Comment: 35 pages Revtex, 7 figures, submitted to Phys. Rev.
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