Coulomb blockade in superconducting quantum point contacts

Amplitude of the Coulomb blockade oscillations is calculated for a single-mode Josephson junction with arbitrary electron transparency $D$. It is shown that the Coulomb blockade is suppressed in ballistic junctions with $D\to 1$. The suppression is described quantitatively as the Landau-Zener transition in imaginary time.Comment: 5 pages, 3 figures include

Theory of the Franck-Condon blockade regime

Strong coupling of electronic and vibrational degrees of freedom entails a low-bias suppression of the current through single-molecule devices, termed Franck-Condon blockade. In the limit of slow vibrational relaxation, transport in the Franck-Condon-blockade regime proceeds via avalanches of large numbers of electrons, which are interrupted by long waiting times without electron transfer. The avalanches consist of smaller avalanches, leading to a self-similar hierarchy which terminates once the number of transferred electrons per avalanche becomes of the order of unity. Experimental signatures of self-similar avalanche transport are strongly enhanced current (shot) noise, as expressed by giant Fano factors, and a power-law noise spectrum. We develop a theory of the Franck-Condon-blockade regime with particular emphasis on effects of electron cotunneling through highly excited vibrational states. As opposed to the exponential suppression of sequential tunneling rates for low-lying vibrational states, cotunneling rates suffer only a power-law suppression. This leads to a regime where cotunneling dominates the current for any gate voltage. Including cotunneling within a rate-equation approach to transport, we find that both the Franck-Condon blockade and self-similar avalanche transport remain intact in this regime. We predict that cotunneling leads to absorption-induced vibrational sidebands in the Coulomb-blockaded regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.

Resistively-shunted superconducting quantum point contacts

We have studied the Josephson dynamics of resistively-shunted ballistic superconducting quantum point contacts at finite temperatures and arbitrary number of conducting modes. Compared to the classical Josephson dynamics of tunnel junctions, dynamics of quantum point contacts exhibits several new features associated with temporal fluctuations of the Josephson potential caused by fluctuations in the occupation of the current-carrying Andreev levels.Comment: 5 pages, RevTex, 3 postscript figures include

Coulomb blockade oscillations of conductance in the regime of strong tunneling

We study the transport through a quantum dot coupled to two leads by single-mode point contacts. The linear conductance is calculated analytically as a function of a gate voltage and temperature T in the case when transmission coefficients of the contacts are close to unity. As a function of the gate voltage, the conductance shows Coulomb blockade oscillations. At low temperatures, the off-resonance conductance vanishes as T^2, in agreement with the theory of inelastic co-tunneling. Near a resonance, the low-energy physics is governed by a multi-channel Kondo fixed point.Comment: Revtex, 8 pages, 2 figure

Adiabatic charge pumping in almost open dots

We consider adiabatic charge transport through an almost open quantum dot. We show that the charge transmitted in one cycle is quantized in the limit of vanishing temperature and one-electron mean level spacing in the dot. The explicit analytic expression for the pumped charge at finite temperature is obtained for spinless electrons. The pumped charge is produced by both non-dissipative and dissipative currents. The latter are responsible for the corrections to charge quantization which are expressed through the conductance of the system.Comment: 5 pages, 1 figur

Non-equilibrium current noise in mesoscopic disordered SNS junctions

Current noise in superconductor-normal metal-superconductor (SNS) junctions is calculated within the scattering theory of multiple Andreev reflections (MAR). It is shown that the noise exhibits subharmonic gap singularities at $eV=2\Delta/n$, $n=1,2,...$ both in single-mode junctions with arbitrary transparency $D$ and in multi-mode disordered junctions. The subharmonic structure is superimposed with monotonic increase of the effective transferred charge $q^*=S_I(0)/2I$ with decreasing bias voltage. Other features of the noise include a step-like increase of $q^*$ in junctions with small $D$, and a divergence $S_I(0) \propto V^{-1/2}$ at small voltages and excess noise $S_{ex} = 2eI_{ex}$, where $I_{ex}$ is the excess current, at large voltages, in junctions with diffusive transport.Comment: 5 page

Penetration of the magnetic field into the twinning plane in the type I and II superconductors

It is demonstrated that in the type I and II superconductors with low-transparent twinning planes (TP) the penetration of external parallel magnetic field into the region of the twinning plane can be energetically favorable. In the type I superconductors the twinning planes become similar to Josephson junctions and the magnetic field penetrates into the center of the TP in the form of soft Josephson-like vortices. This leads to increase in the critical magnetic field values. The corresponding phase diagram in the parameter plane "temperature - magnetic field" essentially differs from the one obtained without taking the finite value of the magnetic field near the TP into account. Comparison between obtained phase diagrams and experimental data for different type I superconductors can allow to estimate the value of the TP transparency, which is the only fitting parameter in our theory.Comment: 5 pages, 2 figure

Supercurrent noise in quantum point contacts

Spectral density of current fluctuations in a short ballistic superconducting quantum point contact is calculated for arbitrary bias voltages $V$. Contrary to a common opinion that the supercurrent flow in Josephson junctions is coherent process with no fluctuations, we find extremely large current noise that is {\em caused} by the supercurrent coherence. An unusual feature of the noise, besides its magnitude, is its voltage dependence: the noise decreases with increasing $V$, despite the fact that the dc current grows steadily with $V$. At finite voltages the noise can be qualitatively understood as the shot noise of the large charge quanta of magnitude $2\Delta /V$ equal to the charge transferred during one period of Josephson oscillations.Comment: 12 pages, revtex, 2 figures by fax/conventional mail upon reques

Coulomb "blockade" of Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate is dramatically affected by the Coulomb blockade and can be controlled by gate voltage. In the case of strong spin-orbit coupling the relaxation rate is maximal in the Coulomb blockade valleys whereas for the weak spin-orbit coupling the maximum of the nuclear spin relaxation rate is near the Coulomb blockade peaks.Comment: 4 pages, 3 figure

Variable range cotunneling and conductivity of a granular metal

The Efros-Shklovskii law for the conductivity of granular metals is interpreted as a result of a variable range cotunneling process. The cotunneling between distant resonant grains is predominantly elastic at low T << T_c, while it is inelastic (i.e., accompanied by creation of electron-hole pairs on a string of intermediate non-resonant grains) at T > T_c. The corresponding E-S temperature T_ES in the latter case is slightly (logarithmically) T-dependent. The magnetoresistance in the two cases is different: it may be relatively strong and negative at T much below T_c, while at T>T_c it is suppressed due to inelastic processes which destroy the interference.Comment: Submitted to JETP Letter