Spin Seebeck Effect in Normal-Metal--Chiral-Insulator Heterostructure

TL;DR

This paper develops a theoretical framework for phonon-mediated spin Seebeck effect in NM-CI heterostructures, revealing nonlinear phenomena like negative differential SSE and spin-current rectification for spintronic applications.

Contribution

It introduces a novel theoretical approach to analyze spin transport via chiral phonons, highlighting nonlinear effects and potential for thermally controlled spin diodes.

Findings

Identification of negative differential SSE due to thermal bias and electron density competition.

Discovery of spin-current rectification enabling thermally controlled spin diodes.

Analysis of interfacial spectral density's role in spin transport behavior.

Abstract

Phonons can carry angular momentum and exhibit chirality through the circular polarization of atomic motion. This enables a phonon-mediated spin Seebeck effect (SSE) via the conversion of phonon angular momentum into electron spin angular momentum. In this Letter, we develop a theoretical framework for calculating the spin current in a normal-metal (NM)-chiral-insulator (CI) heterostructure within the nonequilibrium Green's function formalism. We discuss the influence of (i) the thermal bias across the NM-CI interface, (ii) the chemical potential of the NM, and (iii) the insertion of an additional interfacial layer, on the spin transport properties. We identify two remarkable nonlinear spin transport phenomena: negative differential SSE and spin-current rectification. The negative differential SSE arises from the competition between the thermal bias and the thermally excited electron density. The spin-current rectification suggests the possibility of realizing a thermally controlled spin diode. We also find that the spin transport behavior is closely associated with an effective interfacial spectral density. This work provides a novel route toward thermally controlled spintronic devices using chiral phonons.