47 research outputs found

### Theory of the oscillatory photoconductivity of a 2D electron gas

We develop a theory of magnetooscillations in the photoconductivity of a
two-dimensional electron gas observed in recent experiments. The effect is
governed by a change of the electron distribution function induced by the
microwave radiation. We analyze a nonlinearity with respect to both the dc
field and the microwave power, as well as the temperature dependence determined
by the inelastic relaxation rate.Comment: 4 pages, 3 figure

### Quasiclassical fluctuations of the superconductor proximity gap in a chaotic system

We calculate the sample-to-sample fluctuations in the excitation gap of a
chaotic dynamical system coupled by a narrow lead to a superconductor. Quantum
fluctuations on the order of magnitude of the level spacing, predicted by
random-matrix theory, apply if $\tau_E\ll\hbar/E_T$ (with $\tau_E$ the
Ehrenfest time and $E_T$ the Thouless energy). For \tau_E\agt\hbar/ E_T the
fluctuations are much greater than the level spacing. We demonstrate the
quasiclassical nature of the gap fluctuations in the large-$\tau_E$ regime by
correlating them to an integral over the classical dwell-time distribution.Comment: 4 pages, 3 figure

### 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

### Scattering Theory of Dynamic Electrical Transport

We have developed a scattering matrix approach to coherent transport through
an adiabatically driven conductor based on photon-assisted processes. To
describe the energy exchange with the pumping fields we expand the Floquet
scattering matrix up to linear order in driving frequency.Comment: Proceedings QMath9, September 12th-16th, 2004, Giens, Franc

### 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

### Electron transport and energy relaxation in dilute magnetic alloys

We consider the effect of the RKKY interaction between magnetic impurities on
the electron relaxation rates in a normal metal. The interplay between the RKKY
interaction and the Kondo effect may result in a non-monotonic temperature
dependence of the electron momentum relaxation rate, which determines the Drude
conductivity. The electron phase relaxation rate, which determines the
magnitude of the weak localization correction to the resistivity, is also a
non-monotonic function of temperature. For this function, we find the
dependence of the position of its maximum on the concentration of magnetic
impurities. We also relate the electron energy relaxation rate to the
excitation spectrum of the system of magnetic impurities. The energy relaxation
determines the distribution function for the out-of-equilibrium electrons.
Measurement of the electron distribution function thus may provide information
about the excitations in the spin glass phase.Comment: 15 pages, 5 figure

### Conductance fluctuations in a quantum dot under almost periodic ac pumping

It is shown that the variance of the linear dc conductance fluctuations in an
open quantum dot under a high-frequency ac pumping depends significantly on the
spectral content of the ac field. For a sufficiently strong ac field
$\gamma\tau_{\phi}<< 1$, where $1/\tau_{\phi}$ is the dephasing rate induced by
ac noise and $\gamma$ is the electron escape rate, the dc conductance
fluctuations are much stronger for the harmonic pumping than in the case of the
noise ac field of the same intensity. The reduction factor $r$ in a static
magnetic field takes the universal value of 2 only for the white--noise
pumping. For the strictly harmonic pumping $A(t)=A_{0}\cos\omega t$ of
sufficiently large intensity the variance is almost insensitive to the static
magnetic field $r-1= 2\sqrt{\tau_{\phi}\gamma} << 1$. For the quasi-periodic ac
field of the form $A(t)=A_{0} [\cos(\omega_{1} t)+\cos(\omega_{2} t)]$ with
$\omega_{1,2} >> \gamma$ and $\gamma\tau_{\phi} << 1$ we predict the novel
effect of enchancement of conductance fluctuations at commensurate frequencies
$\omega_{2}/\omega_{1}=P/Q$.Comment: 4 pages RevTex, 4 eps figures; the final version to appear in
Phys.Rev.

### Ehrenfest time dependent suppression of weak localization

The Ehrenfest time dependence of the suppression of the weak localization
correction to the conductance of a {\em clean} chaotic cavity is calculated.
Unlike in earlier work, no impurity scattering is invoked to imitate
diffraction effects. The calculation extends the semiclassical theory of K.
Richter and M. Sieber [Phys. Rev. Lett. {\bf 89}, 206801 (2002)] to include the
effect of a finite Ehrenfest time.Comment: 3 Pages, 1 Figure, RevTe

### Quantum-to-classical crossover of mesoscopic conductance fluctuations

We calculate the system-size-over-wave-length ($M$) dependence of
sample-to-sample conductance fluctuations, using the open kicked rotator to
model chaotic scattering in a ballistic quantum dot coupled by two $N$-mode
point contacts to electron reservoirs. Both a fully quantum mechanical and a
semiclassical calculation are presented, and found to be in good agreement. The
mean squared conductance fluctuations reach the universal quantum limit of
random-matrix-theory for small systems. For large systems they increase
$\propto M^2$ at fixed mean dwell time $\tau_D \propto M/N$. The universal
quantum fluctuations dominate over the nonuniversal classical fluctuations if
$N < \sqrt{M}$. When expressed as a ratio of time scales, the
quantum-to-classical crossover is governed by the ratio of Ehrenfest time and
ergodic time.Comment: 5 pages, 5 figures: one figure added, references update

### Quantum correction to the Kubo formula in closed mesoscopic systems

We study the energy dissipation rate in a mesoscopic system described by the
parametrically-driven random-matrix Hamiltonian H[\phi(t)] for the case of
linear bias \phi=vt. Evolution of the field \phi(t) causes interlevel
transitions leading to energy pumping, and also smears the discrete spectrum of
the Hamiltonian. For sufficiently fast perturbation this smearing exceeds the
mean level spacing and the dissipation rate is given by the Kubo formula. We
calculate the quantum correction to the Kubo result that reveals the original
discreteness of the energy spectrum. The first correction to the system
viscosity scales proportional to v^{-2/3} in the orthogonal case and vanishes
in the unitary case.Comment: 4 pages, 3 eps figures, REVTeX