Strong spin Seebeck effect in Kondo T-shaped double quantum dots

TL;DR

This paper theoretically explores the spin Seebeck effect in a T-shaped double quantum dot system exhibiting the two-stage Kondo effect, revealing enhanced spin thermopower sensitive to lead polarization.

Contribution

It demonstrates the influence of the two-stage Kondo effect and lead spin polarization on the spin thermoelectric properties of double quantum dots, using numerical renormalization group methods.

Findings

Spin thermopower is enhanced at the second Kondo screening stage.

Spin Seebeck effect can be strong with minimal lead polarization (~1%).

Thermal conductance follows a modified Wiedemann-Franz law.

Abstract

We theoretically investigate the thermoelectric and spin thermoelectric properties of a T-shaped double quantum dot strongly coupled to two ferromagnetic leads, focusing on transport regime where the system exhibits the two-stage Kondo effect. We study the dependence of the (spin) Seebeck coefficient, the corresponding power factor and the figure of merit on temperature, leads' spin polarization and dot level position. We show that the thermal conductance fulfills a modified Wiedemann-Franz law. We also demonstrate that the spin thermopower is enhanced at temperatures corresponding to the second stage of Kondo screening. Very interestingly, the spin-thermoelectric response of the system is found to be highly sensitive to the spin polarization of the leads. In some cases spin polarization of the order of 1% is sufficient for a strong spin Seebeck effect to occur. This is explained as a consequence of the interplay between the two-stage Kondo effect and the exchange field induced in the double quantum dot. All calculations are performed with the aid of numerical renormalization group technique.