Thermally generated spin current in the topological insulator Bi₂Se₃

We present measurements of thermally generated transverse spin currents in the topological insulator Bi₂Se₃, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi₂Se₃/CoFeB. We find that Bi₂Se₃ does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is_r^J_x^s_T = (4.9 ± 0.9) × 10⁶ (₂^ħ_e )_K^A_μm^m2₁, and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi₂Se₃ is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi₂Se₃ can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy.