Towards an Entropic Inductor in Topological Materials

3D topological insulators are characterized by an insulating bulk and extended surface states exhibiting a helical spin texture. In this work, we investigate the hyperfine interaction between the spin-charge coupled transport of electrons and the nuclear spins in these surface states.  We employ nonequilibrium Green's function analysis to show that a current will polarize nuclear spins on the surface of a 3D topological insulator. We also show that electrical work can be extracted from a bath of polarized nuclear spins as they drive an electronic current [1]. This current, induced due to thermal relaxation of polarized nuclear spins has an inductive nature. The inductive effect is most prominent in topological insulators which have a large number of spinful nuclei per coherent segment, of which the volume is given by the mean free path length, Fermi wavelength and penetration depth of the surface state. The polarized nuclear spins also act on electrons as an effective in-plane magnetic field, known as the Overhauser field. This field causes an offset in the in-plane magnetoresistance allowing the nuclear polarization to be measured [2], an important first step in experimentally realizing an entropic inductor.

[1] A.M. Bozkurt, S. Kölling, A. Brinkman, and I. Adagideli, SciPost Phys. Core 8, 023 (2025) 

[2]  S. Kölling, I. Adagideli and A. Brinkman; Phys. Rev. B 112, 045110 (2025)