Spin-Dependent Ringing and Beats in a Quantum Dot System

TL;DR

This paper investigates spin-dependent quantum oscillations and beats in a quantum dot system with Zeeman splitting, analyzing how these phenomena depend on magnetic configurations, temperature, and bias, revealing complex spin transport behaviors.

Contribution

It introduces a detailed analysis of spin-dependent ringing and beats in quantum dots using nonequilibrium Green functions, highlighting the effects of Zeeman energy and magnetic alignments.

Findings

Spin-dependent oscillations and beats are controllable via Zeeman energy.

Temperature increases suppress ringing and beat phenomena.

Ferromagnetic leads sustain finite spin currents even at zero Zeeman energy.

Abstract

We report spin-dependent quantum coherent oscillations (ringing) and beats of the total and spin currents flowing through a quantum dot with Zeeman split levels. The spin dependent transport is calculated via nonequilibrium Green function in the transient after a bias voltage is turned on at t=0. The dot is coupled to two electrodes that can be ferromagnetic or nonmagnetic. In the ferromagnetic case both parallel and antiparallel alignments are considered. The coherent oscillation and beat frequencies are controlled via the Zeeman energy E_Z. In particular, for E_Z=0 no beats are observed and the spin current is zero for nonmagnetic leads. In the ferromagnetic case a finite spin current is found for E_Z=0. The effects of temperature are also analyzed. We observe that with increasing temperature the ringing response and the beats tend to disappear. Additionally, the spin current goes to zero for nonmagnetic leads, remaining finite in the ferromagnetic case. The tunnel magnetoresistance (TMR) also reveals quantum coherent oscillations and beats, and it attains negative values for small enough temperatures and short times.