Nonlinear electric and thermoelectric Andreev transport through a hybrid quantum dot coupled to ferromagnetic and superconducting leads

TL;DR

This paper investigates nonlinear Andreev transport in a quantum dot system coupled to ferromagnetic and superconducting leads, revealing how spin polarization influences charge and spin currents under voltage and temperature biases.

Contribution

It introduces a detailed analysis of spin-dependent nonlinear responses in a hybrid quantum dot system, highlighting the effects of lead polarization and tunneling rate dominance on subgap currents.

Findings

Subgap current depends on ferromagnetic lead polarization.

Spin-dependent potential exhibits nonmonotonic behavior with dot level detuning.

Current rectification and thermocurrent maximization are achievable.

Abstract

We discuss the nonlinear Andreev current of an interacting quantum dot coupled to spin-polarized and superconducting reservoirs when voltage and temperature biases are applied across the nanostructure. Due to the particle-hole symmetry introduced by the superconducting (S) lead, the subgap spin current vanishes identically. Nevertheless, the Andreev charge current depends on the degree of polarization in the ferromagnetic (F) contact since the shift of electrostatic internal potential of the conductor depends on spin orientation of the charge carrier. This spin-dependent potential shift characterizes nonlinear responses in our device. We show how the subgap current versus the bias voltage or temperature difference depends on the lead polarization in two cases, namely (i) S-dominant case, when the dot-superconductor tunneling rate ($\Gamma_R$) is much higher than the ferromagnet-dot tunnel coupling ($\Gamma_L$), and (ii) F-dominant case, when $\Gamma_L\gg \Gamma_R$. For the ferromagnetic dominant case the spin-dependent potential shows a nonmonotonic behavior as the dot level is detuned. Thus the subgap current can also exhibit interesting behaviors such as current rectification and the maximization of thermocurrents with smaller thermal biases when the lead polarization and the quantum dot level are adjusted.