113 research outputs found
Real-time diagrammatic approach to transport through interacting quantum dots with normal and superconducting leads
No full textWe present a real-time diagrammatic theory for transport through interacting quantum dots tunnel coupled to normal and superconducting leads. Our formulation describes both the equilibrium and nonequilibrium superconducting proximity effects in a quantum dot. We study a three-terminal transistor geometry, consisting of a single-level quantum dot tunnel coupled to two phase-biased superconducting leads and one voltage-biased normal lead. We compute both the Josephson current between the two superconductors and the Andreev current in the normal lead, and analyze their switching on and off as well as transitions between 0 and pi states as a function of gate and bias voltages. For the limit of large superconducting gaps in the leads, we describe the formation of Andreev bound states within an exact resummation of all orders in the tunnel coupling to the superconducting leads, and we discuss their signature in the nonequilibrium Josephson and Andreev currents and the quantum-dot charge
Nonequilibrium Josephson and Andreev current through interacting quantum dots
No full textWe present a theory of transport through interacting quantum dots coupled to normal and superconducting leads in the limit of weak tunnel coupling. A Josephson current between two superconducting leads, carried by first-order tunnel processes, can be established by the nonequilibrium proximity effect. Both the Andreev and the Josephson currents are suppressed for bias voltages below a threshold set by the Coulomb charging energy. A pi-transition of the supercurrent can be driven by tuning gate or bias voltages
Superconducting proximity effect in interacting quantum dots revealed by shot noise
No full textWe study the full counting statistics of charge transport through a quantum dot tunnel coupled to one normal and one superconducting lead with a large superconducting gap. As a function of the level detuning, there is a crossover from a regime with strong superconducting correlations in the quantum dot to a regime in which the proximity effect on the quantum dot is suppressed. We analyze the current fluctuations of this crossover in the shot-noise regime. In particular, we predict that the full counting statistics changes from Poissonian with charge 2e, typical for Cooper pairs, to Poissonian with charge e, When the superconducting proximity effect is present. Thus, the onset of the superconducting proximity effect is revealed by the reduction of the Fano factor from 2 to 1. (C) 2010 Elsevier Ltd. All rights reserved
Rashba spin precession in quantum-Hall edge channels,
No full textQuasi-one-dimensional edge channels are formed at the boundary of a two-dimensional electron system subject to a strong perpendicular magnetic field. We consider the effect of Rashba spin-orbit coupling, induced by structural inversion asymmetry, on their electronic and transport properties. Both our analytical and numerical results show that spin-split quantum-Hall edge channels exhibit properties analogous to that of Rashba-split quantum wires. Suppressed backscattering and a long spin lifetime make these edge channels an ideal system for observing voltage-controlled spin precession. Based on the latter, we propose a magnetless spin-dependent electron interferometer
Universal Rashba spin precession of two-dimensional electrons and holes
No full textWe study spin precession due to Rashba spin splitting of electrons and holes in semiconductor quantum wells. Based on a simple analytical expression that we derive for the current modulation in a broad class of experimental situations of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we conclude that the Datta-Das spin transistor i) is feasible with holes and ii) its functionality is not affected by integration over injection angles. The current modulation shows a universal oscillation period, irrespective of the different forms of the Rashba Hamiltonian for electrons and holes. The analytic formulas approximate extremely well exact numerical calculations of a more elaborate Kohn-Luttinger model
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