arXiv subjects

R. Raimondi

Publications and source records attributed to R. Raimondi.

46 records · Page 3Linked to original sources

arXiv1999

Andreev Tunneling in Strongly Interacting Quantum Dots

We review recent work on resonant Andreev tunneling through a strongly interacting quantum dot connected to a normal and to a superconducting lead. We derive a general expression for the current flowing in the structure and discuss the linear and non-linear transport in the nonperturbative regime. New effects associated to the Kondo resonance combined with the two-particle tunneling arise. The Kondo anomaly in the $I-V$ characteristics depends on the relative size of the gap energy and the Kondo temperature.

cond-mat.mes-hall↗

arXiv1999

Non-linear effects and dephasing in disordered electron systems

The calculation of the dephasing time in electron systems is presented. By means of the Keldysh formalism we discuss in a unifying way both weak localization and interaction effects in disordered systems. This allows us to show how dephasing arises both in the particle-particle channel (weak localization) and in the particle-hole channel (interaction effect). First we discuss dephasing by an external field. Besides reviewing previous work on how an external oscillating field suppresses the weak localization correction, we derive a new expression for the effect of a field on the interaction correction. We find that the latter may be suppressed by a static electric field, in contrast to weak localization. We then consider dephasing due to inelastic scattering. The ambiguities involved in the definition of the dephasing time are clarified by directly comparing the diagrammatic approach with the path-integral approach. We show that different dephasing times appear in the particle-particle and particle-hole channels. Finally we comment on recent experiments.

cond-mat.mes-hall↗

arXiv1998

Properties of superconductor - Luttinger liquid hybrid systems

In this paper we review some recent results concerning the physics of superconductor - Luttinger liquid proximity systems. We discuss both equilibrium (the pair amplitude, Josephson current, and the local density of states) and nonequilibrium (the subgap current) properties.

cond-mat.mes-hall↗

arXiv1998

Sub-gap conductance in ferromagnetic-superconducting mesoscopic structures

We study the sub-gap conductance of a ferromagnetic mesoscopic region attached to a ferromagnetic and a superconducting electrode by means of tunnel junctions. In the absence of the exchange field, the ratio $r= γ/ ε_T$ of the two tunnel junction resistances determines the behaviour of the sub-gap conductance which possesses a zero-bias peak for $r\gg 1$ and for $r\ll 1$ a peak at finite voltage. We show that the inclusion of the exchange field leads to a peak splitting for $r\ll 1$, while it shifts the zero-bias anomaly to finite voltages for $r\gg 1$.

cond-mat.mes-hall↗

arXiv1998

Andreev tunnelling in quantum dots: A slave-boson approach

We study a strongly interacting quantum dot connected to a normal and to a superconducting lead. By means of the slave-boson technique we investigate the low temperature regime and discuss electrical transport through the dot. We find that the zero bias anomaly in the current-voltage characteristics which is associated to the occurance of the Kondo resonance in the quantum dot, is enhanced in the presence of superconductivity, due to resonant Andreev scattering.

cond-mat.mes-hall↗

arXiv1997

Phase coherent transport in hybrid superconducting nanostructures

This article is an overview of recent experimental and theoretical work on transport in phase-coherent hybrid nanostructures, with particular emphasis on dc electrical conduction. A summary of multiple scattering theory and the quasi-classical methods is presented and comparisons between the two are made. Several paradigms of phase-coherent transport are discussed, including zero-bias anomalies, reentrant and long range proximity effects, Andreev interferometers and superconductivity-induced conductance suppression.

cond-mat.supr-con↗

arXiv1996

Boundary conditions for quasiclassical equations in the theory of superconductivity

In this paper we derive effective boundary conditions connecting the quasiclassical Green's function through tunnel barriers in superconducting - normal hybrid (S-N or S-S') structures in the dirty limit. Our work extends previous treatments confined to the small transparency limit. This is achieved by an expansion in the small parameter $r^{-1}=T/2(1-T)$ where T is the transparency of the barrier. We calculate the next term in the $r^{-1}$ expansion for both the normal and the superconducting case. In both cases this involves the solution of an integral equation, which we obtain numerically. While in the normal case our treatment only leads to a quantitative change in the barrier resistance $R_b$, in the superconductor case, qualitative different boundary conditions are derived. To illustrate the physical consequences of the modified boundary conditions, we calculate the Josephson current and show that the next term in the $r^{-1}$ expansion gives rise to the second harmonic in the current-phase relation.

cond-mat.supr-con↗

arXiv1996

Phase coherent transport in hybrid superconducting structures: the case of d-wave superconductors

We examine the effect of d-wave symmetry on zero bias anomalies in normal-superconducting tunnel junctions and phase-periodic conductances in Andreev interferometers. In the presence of d-wave pairing, zero-bias anomalies are suppressed compared with the s-wave case. For Andreev interferometers with aligned islands, the phase-periodic conductance is insensistive to the nature of the pairing, whereas for non-aligned islands, the nature of the zero-phase extremum is reversed.

cond-mat↗

arXiv1995

Diffusive and ballistic motion in superconducting hybrid structures

We examine transport properties of superconducting hybrid mesoscopic structures, in both the diffusive and ballistic regimes. For diffusive structures, analytic results from quasi-classical theory are compared with predictions from numerical, multiple-scattering calculations performed on small structures. For all structures, the two methods yield comparable results and in some cases, quantitative agreement is obtained. These results not only demonstrate that quasi-classical theory can yield the ensemble averaged conductance $ $ of small structures of dimensions of order 10- 20 Fermi wavelengths, but also establish that numerical scattering calculations on such small structures can yield results for $ $ which are characteristic of much larger systems. Having compared the two approaches, we extend the multiple-scattering analysis to the ballistic limit, where the sample dimensions become smaller than the elastic mean free path and demonstrate that the properties of certain Andreev interferometers are unchanged in the clean limit.

cond-mat↗

arXiv1995

Giant Conductance Oscillations In Mesoscopic Andreev Interferometers

We analyze the electrical conductance $G(ϕ)$ of a two-dimensional, phase coherent structure in contact with two superconductors, which is known to be an oscillatory function of the phase difference $ϕ$ between the superconductors. It is predicted that for a metallic sample, the amplitude of oscillation is enhanced by placing a normal barrier at the interface and that, by tuning the strength of the barrier, can be orders of magnitude greater than values observed in recent experiments. Giant oscillations can also be obtained without a barrier, provided a crucial sum rule is broken. This can be achieved by disorder induced normal scattering. In the absence of zero phase inter-channel scattering, the conductance possesses a zero phase minimum.

cond-mat↗