261 research outputs found

### Pumped current and voltage for an adiabatic quantum pump

We consider adiabatic pumping of electrons through a quantum dot. There are
two ways to operate the pump: to create a dc current ${\bar I}$ or to create a
dc voltage ${\bar V}$. We demonstrate that, for very slow pumping, ${\bar I}$
and ${\bar V}$ are not simply related via the dc conductance $G$ as $\bar I =
\bar V G$. For the case of a chaotic quantum dot, we consider the statistical
distribution of ${\bar V} G - {\bar I}$. Results are presented for the limiting
cases of a dot with single channel and with multichannel point contacts.Comment: 6 pages, 4 figure

### Finite size effects and localization properties of disordered quantum wires with chiral symmetry

Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex

### Distribution of the reflection eigenvalues of a weakly absorbing chaotic cavity

The scattering-matrix product SS+ of a weakly absorbing medium is related by
a unitary transformation to the time-delay matrix without absorption. It
follows from this relationship that the eigenvalues of SS+ for a weakly
absorbing chaotic cavity are distributed according to a generalized Laguerre
ensemble.Comment: 4 pages, 1 figure, to appear in Physica E (special issue on Dynamics
of Complex Systems

### Weak localization and conductance fluctuations of a chaotic quantum dot with tunable spin-orbit coupling

In a two-dimensional quantum dot in a GaAs heterostructure, the spin-orbit
scattering rate is substantially reduced below the rate in a bulk
two-dimensional electron gas [B.I. Halperin et al, Phys. Rev. Lett. 86, 2106
(2001)]. Such a reduction can be undone if the spin-orbit coupling parameters
acquire a spatial dependence, which can be achieved, e.g., by a metal gate
covering only a part of the quantum dot. We calculate the effect of such
spatially non-uniform spin-orbit scattering on the weak localization correction
and the universal conductance fluctuations of a chaotic quantum dot coupled to
electron reservoirs by ballistic point contacts, in the presence of a magnetic
field parallel to the plane of the quantum dot.Comment: 4 pages, RevTeX; 2 figures. Substantial revision

### Conductance Fluctuations of Open Quantum Dots under Microwave Radiation

We develop a time dependent random matrix theory describing the influence of
a time-dependent perturbation on mesoscopic conductance fluctuations in open
quantum dots. The effect of external field is taken into account to all orders
of perturbation theory, and our results are applicable to both weak and strong
fields. We obtain temperature and magnetic field dependences of conductance
fluctuations. The amplitude of conductance fluctuations is determined by
electron temperature in the leads rather than by the width of electron
distribution function in the dot. The asymmetry of conductance with respect to
inversion of applied magnetic field is the main feature allowing to distinguish
the effect of direct suppression of quantum interference from the simple
heating if the frequency of external radiation is larger than the temperature
of the leads $\hbar\omega \gg T$.Comment: 7 pages, 5 figure

### Delay times and reflection in chaotic cavities with absorption

Absorption yields an additional exponential decay in open quantum systems
which can be described by shifting the (scattering) energy E along the
imaginary axis, E+i\hbar/2\tau_{a}. Using the random matrix approach, we
calculate analytically the distribution of proper delay times (eigenvalues of
the time-delay matrix) in chaotic systems with broken time-reversal symmetry
that is valid for an arbitrary number of generally nonequivalent channels and
an arbitrary absorption rate 1/\tau_{a}. The relation between the average delay
time and the ``norm-leakage'' decay function is found. Fluctuations above the
average at large values of delay times are strongly suppressed by absorption.
The relation of the time-delay matrix to the reflection matrix S^{\dagger}S is
established at arbitrary absorption that gives us the distribution of
reflection eigenvalues. The particular case of single-channel scattering is
explicitly considered in detail.Comment: 5 pages, 3 figures; final version to appear in PRE (relation to
reflection extended, new material with Fig.3 added, experiment
cond-mat/0305090 discussed

### Decay Rate Distributions of Disordered Slabs and Application to Random Lasers

We compute the distribution of the decay rates (also referred to as residues)
of the eigenstates of a disordered slab from a numerical model. From the
results of the numerical simulations, we are able to find simple analytical
formulae that describe those results well. This is possible for samples both in
the diffusive and in the localised regime. As example of a possible
application, we investigate the lasing threshold of random lasers.Comment: 11 pages, 11 figure

### Parametric pumping at finite frequency

We report on a first principles theory for analyzing the parametric electron
pump at a finite frequency. The pump is controlled by two pumping parameters
with phase difference $\phi$. In the zero frequency limit, our theory predicts
the well known result that the pumped current is proportional to $\sin\phi$.
For the more general situation of a finite frequency, our theory predicts a
non-vanishing pumped current even when the two driving forces are in phase, in
agreement with the recent experimental results. We present the physical
mechanism behind the nonzero pumped current at $\phi=0$, which we found to be
due to photon-assisted processes

### Transport Properties and Density of States of Quantum Wires with Off-diagonal Disorder

We review recent work on the random hopping problem in a
quasi-one-dimensional geometry of N coupled chains (quantum wire with
off-diagonal disorder). Both density of states and conductance show a
remarkable dependence on the parity of N. The theory is compared to numerical
simulations.Comment: 8 pages, to appear in Physica E (special issue on Dynamics of Complex
Systems); 6 figure

### Voltage-probe and imaginary potential models for dephasing in a chaotic quantum dot

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

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