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

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–chiral-insulator (NM–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 modification of the interfacial on-site potential on the spin transport properties. We identify two characteristic 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 density of thermally excited electrons. 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 suggests a novel route toward thermally controlled spintronic devices using chiral phonons.