Thermally-generated spin current in the topological insulator Bi$_2$Se$_3$

TL;DR

This study demonstrates that the topological insulator Bi$_2$Se$_3$ efficiently converts thermal gradients into transverse spin currents, with a spin Nernst ratio surpassing that of heavy metals like Pt and W.

Contribution

The paper provides the first quantitative measurement of thermally-generated spin currents in Bi$_2$Se$_3$, establishing its superior spin Nernst ratio compared to other materials.

Findings

Bi$_2$Se$_3$ generates substantial thermally-driven spin currents.

The lower bound for spin current to thermal gradient ratio is (4.9 ± 0.9) × 10^6 A/m^2/K.

The spin Nernst ratio for Bi$_2$Se$_3$ is -0.61 ± 0.11, the largest among measured materials.

Abstract

We complete measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents in the topological insulator Bi$_2$Se$_3$ by quantifying the efficiency with which thermal gradients can generate transverse spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnesistance for bilayers of Bi$_2$Se$_3$/CoFeB. We find that Bi$_2$Se$_3$ does generate substantial thermally-driven spin currents. A lower bound for the ratio of spin current to thermal gradient is $J_s/\nabla_x T$ = (4.9 $\pm$ 0.9) $\times$ 10$^{6}$ ($\hbar/2e$) A m$^{-2}$ / K $\mu$m$^{-1}$, and a lower bound for the magnitude of the spin Nernst ratio is $-$0.61 $\pm$ 0.11. The spin Nernst ratio for Bi$_2$Se$_3$ is the largest among all materials measured to date, 2-3 times larger compared to previous measurements for the heavy metals Pt and W.