Cotunneling through two-level quantum dots weakly coupled to ferromagnetic leads

TL;DR

This paper theoretically investigates spin-polarized cotunneling in two-level quantum dots with ferromagnetic leads, revealing how conductance and TMR depend on the dot's ground state, aiding in identifying singlet or triplet states.

Contribution

It introduces a theoretical analysis of cotunneling effects in two-level quantum dots with ferromagnetic leads, highlighting how ground state configurations influence transport properties.

Findings

Differential conductance shows a broad minimum at low bias for singlet ground state.

Maximum in conductance at zero bias occurs for triplet ground state with antiparallel leads.

TMR behavior varies significantly depending on the dot's ground state.

Abstract

The spin-polarized transport through two-level quantum dots weakly coupled to ferromagnetic leads is considered theoretically in the Coulomb blockade regime. It is assumed that the dot is doubly occupied, so that the current flows due to cotunneling through singlet and triplet states of the dot. It is shown that transport characteristics strongly depend on the ground state of quantum dot. If the ground state is a singlet, differential conductance ($G$) displays a broad minimum at low bias voltage, while tunnel magnetoresistance (TMR) is given by the Julliere value. If triplet is the ground state of the system, there is a maximum in differential conductance at zero bias when the leads are magnetized in antiparallel. The maximum is accompanied by a minimum in TMR. The different behavior of $G$ and TMR may thus help to determine the ground state of the dot and the energy difference between the singlet and triplet states.