841 research outputs found
Interference Phenomena in Electronic Transport Through Chaotic Cavities: An Information-Theoretic Approach
We develop a statistical theory describing quantum-mechanical scattering of a
particle by a cavity when the geometry is such that the classical dynamics is
chaotic. This picture is relevant to a variety of systems, ranging from atomic
nuclei to microwave cavities; the main application here is to electronic
transport through ballistic microstructures. The theory describes the regime in
which there are two distinct time scales, associated with a prompt and an
equilibrated response, and is cast in terms of the matrix of scattering
amplitudes S. The prompt response is related to the energy average of S which,
through ergodicity, is expressed as the average over an ensemble of systems. We
use an information-theoretic approach: the ensemble of S-matrices is determined
by (1) general physical features-- symmetry, causality, and ergodicity, (2) the
specific energy average of S, and (3) the notion of minimum information in the
ensemble. This ensemble, known as Poisson's kernel, is meant to describe those
situations in which any other information is irrelevant. Thus, one constructs
the one-energy statistical distribution of S using only information expressible
in terms of S itself without ever invoking the underlying Hamiltonian. This
formulation has a remarkable predictive power: from the distribution of S we
derive properties of the quantum conductance of cavities, including its
average, its fluctuations, and its full distribution in certain cases, both in
the absence and presence prompt response. We obtain good agreement with the
results of the numerical solution of the Schrodinger equation for cavities in
which either prompt response is absent or there are two widely separated time
scales. Good agreement with experimental data is obtained once temperature
smearing and dephasing effects are taken into account.Comment: 38 pages, 11 ps files included, uses IOP style files and epsf.st
Quantum Monte Carlo Study of Disordered Fermions
We study a strongly correlated fermionic model with attractive interactions
in the presence of disorder in two spatial dimensions. Our model has been
designed so that it can be solved using the recently discovered meron-cluster
approach. Although the model is unconventional it has the same symmetries of
the Hubbard model. Since the naive algorithm is inefficient, we develop a new
algorithm by combining the meron-cluster technique with the directed-loop
update. This combination allows us to compute the pair susceptibility and the
winding number susceptibility accurately. We find that the s-wave
superconductivity, present in the clean model, does not disappear until the
disorder reaches a temperature dependent critical strength. The critical
behavior as a function of disorder close to the phase transition belongs to the
Berezinsky-Kosterlitz-Thouless universality class as expected. The fermionic
degrees of freedom, although present, do not appear to play an important role
near the phase transition.Comment: published version, more data added to Fig 5 and clarifications in
text, 8 page
On the Inequivalence of Weak-Localization and Coherent Backscattering
We define a current-conserving approximation for the local conductivity
tensor of a disordered system which includes the effects of weak localization.
Using this approximation we show that the weak localization effect in
conductance is not obtained simply from the diagram corresponding to the
coherent back-scattering peak observed in optical experiments. Other diagrams
contribute to the effect at the same order and decrease its value. These
diagrams appear to have no semiclassical analogues, a fact which may have
implications for the semiclassical theory of chaotic systems. The effects of
discrete symmetries on weak localization in disordered conductors is evaluated
and and compared to results from chaotic scatterers.Comment: 24 pages revtex + 12 figures on request; hub.94.
The Enhanced Sensitivity of the Transmission Phase of a Quantum Dot to Kondo Correlations
The strong sensitivity of the transmission phase through a quantum dot
embedded into one arm of a two-wave Aharonov-Bohm interferometer to the Kondo
effect is explained. The enhancement takes place because of the buildup of the
exchange scattering on the dot due to Kondo correlations even much above
. The enhanced exchange competes with the potential scattering, which is
always weak. Both cases of the Anderson impurity model and a multilevel quantum
dot are considered. In the latter case in addition to the description of
peculiar phase behavior a mechanism leading to ferromagnetic Kondo coupling in
quantum dots is proposed.Comment: 4 pages, 2 figure
Decoherence by Correlated Noise and Quantum Error Correction
We study the decoherence of a quantum computer in an environment which is
inherently correlated in time and space. We first derive the nonunitary time
evolution of the computer and environment in the presence of a stabilizer error
correction code, providing a general way to quantify decoherence for a quantum
computer. The general theory is then applied to the spin-boson model. Our
results demonstrate that effects of long-range correlations can be
systematically reduced by small changes in the error correction codes.Comment: 4 pages, 1 figure, Phys. Rev. Lett. in pres
Interactions and Broken Time-Reversal Symmetry in Chaotic Quantum Dots
When treating interactions in quantum dots within a RPA-like approach,
time-reversal symmetry plays an important role as higher-order terms -- the
Cooper series -- need to be included when this symmetry is present. Here we
consider model quantum dots in a magnetic field weak enough to leave the
dynamics of the dot chaotic, but strong enough to break time-reversal symmetry.
The ground state spin and addition energy for dots containing 120 to 200
electrons are found using local spin density functional theory, and we compare
the corresponding distributions with those derived from an RPA-like treatment
of the interactions. The agreement between the two approaches is very good,
significantly better than for analogous calculations in the presence of
time-reversal symmetry. This demonstrates that the discrepancies between the
two approaches in the time-reversal symmetric case indeed originate from the
Cooper channel, indicating that these higher-order terms might not be properly
taken into account in the spin density functional calculations.Comment: 4 pages, 3 figure
Mesoscopic Transport Through Ballistic Cavities: A Random S-Matrix Theory Approach
We deduce the effects of quantum interference on the conductance of chaotic
cavities by using a statistical ansatz for the S matrix. Assuming that the
circular ensembles describe the S matrix of a chaotic cavity, we find that the
conductance fluctuation and weak-localization magnitudes are universal: they
are independent of the size and shape of the cavity if the number of incoming
modes, N, is large. The limit of small N is more relevant experimentally; here
we calculate the full distribution of the conductance and find striking
differences as N changes or a magnetic field is applied.Comment: 4 pages revtex 3.0 (2-column) plus 2 postscript figures (appended),
hub.pam.94.
Signatures of Classical Periodic Orbits on a Smooth Quantum System
Gutzwiller's trace formula and Bogomolny's formula are applied to a
non--specific, non--scalable Hamiltonian system, a two--dimensional anharmonic
oscillator. These semiclassical theories reproduce well the exact quantal
results over a large spatial and energy range.Comment: 12 pages, uuencoded postscript file (1526 kb
Escape from noisy intermittent repellers
Intermittent or marginally-stable repellers are commonly associated with a
power law decay in the survival fraction. We show here that the presence of
weak additive noise alters the spectrum of the Perron - Frobenius operator
significantly giving rise to exponential decays even in systems that are
otherwise regular. Implications for ballistic transport in marginally stable
miscrostructures are briefly discussed.Comment: 3 ps figures include
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