Superconducting proximity effect and zero-bias anomaly in transport through quantum dots weakly attached to ferromagnetic leads

Abstract

The Andreev transport through a quantum dot coupled to two external ferromagnetic leads and one superconducting lead is studied theoretically by means of the real-time diagrammatic technique in the sequential and cotunneling regimes. We show that the tunnel magnetoresistance (TMR) of the Andreev current displays a nontrivial dependence on the bias voltage and the level detuning, and can be described by analytical formulas in the zero temperature limit. The cotunneling processes lead to a strong modification of the TMR, which is most visible in the Coulomb blockade regime. We find a zero-bias anomaly of the Andreev differential conductance in the parallel configuration, which is associated with a nonequilibrium spin accumulation in the dot triggered by Andreev processes.

Quantum dot as spin current generator and energy harvester

“…It has been found inter alia that the thermoelectric figure of merit may be increased by Coulomb [50] and interference effects as well as direct hopping between the dots [51][52][53][54][55]. Impressive numerous experiments on the double quantum dots have to be noted [56][57][58][59] including those reporting the observation of the Kondo effect [60][61][62].…”

Andreev Transport through a Magnetic Molecule Weakly Coupled to Ferromagnetic Leads

“…Since each Cooper pair involves two electrons of opposite spin, the minority spin subband is a bottleneck for transport in the parallel configuration and one generally finds I P S I AP S . This is especially the case for larger bias voltages when all the Andreev states are active in transport and the current does not depend on J, such that TMR p 2 {p1 ¡ p 2 q [7]. However, when singleelectron states are mainly responsible for the Andreev current, the situation is more complex and one finds a strong dependence of the TMR on the type of exchange interaction, see Fig.…”

“…We do not impose any restrictions on the intrinsic exchange coupling J and consider the case of both ferromagnetic (FM, J ¡ 0) and antiferromagnetic (AFM, J 0) coupling. For comparison, we also present the results for J 0, which corresponds to a single quantum dot case [7]. This gives us the possibility to explicitly see how the presence of a large-spin molecule affects the transport characteristics.…”

“…In the following we are interested in the behavior of the Andreev current in both the parallel (I P S ) and antiparallel (I AP S ) configuration of ferromagnetic leads, together with tunnel magnetoresistance, which we define as [7], TMR pI AP S ¡ I P S q{I P S . For the magnetic molecule we assume that it is described by spin S 2 and magnetic anisotropy D{U 0.05.…”

Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads

“…Quantifying the relative magnitude of both processes is however not straightforward. It can be done in quantum dot based CPS's with ferromagnetic contacts, in which by varying the magnetic configuration of ferromagnetic leads between the parallel and antiparallel alignment, one can infer the knowledge about the amount of CAR processes relative to DAR ones [37,40,43,45]. In an ideal case of half-metallic leads, for bias voltages smaller than the dots' charging energy, the Andreev transport is only due to CAR processes, and the current is maximized in the antiparallel configuration of leads' magnetic moments, while in the case of parallel configuration the Andreev current vanishes.…”