Spin-Seebeck effect on the surface of topological insulators
May 14, 2014 - In the recent Physical Review B article, Prof. Nikolic and physics graduate students Po-Hao Chang and Farzad Mahfouzi, together with Prof. Naoto Nagaosa from the University of Tokyo and RIKEN Advanced Science Institute in Japan, have predicted that applying a temperature gradient across newly discovered topological insulator (TI) materials will induce spin current I3Sx and associated voltage signal VISHE in the three-terminal junction shown in the Figure.
The spin-Seebeck effect (SSE) is one of the key phenomena in recently invigorated field of spin caloritronics. In the SSE, spin current or spin accumulation is induced by a temperature gradient applied across a ferromagnetic material. The effect was observed in a surprisingly wide range of materials, including ferromagnetic insulators where conventional charge Seebeck effect does not exist due to the absence of conduction electrons.
Since theories of SSE have been focused on the interplay of magnons and phonons out of equilibrium, the very recent experimental observation of "giant SSE" (up to a thousand times larger than observed in measurements on magnetic materials) in nonmagnetic semiconductor InSb placed into a large external magnetic field came as a surprise.
The prediction made by Nikolic group and collaborators suggests that one can re-create the same phenomenology as in the "giant SSE," but without using any external magnetic field, by exploiting strong relativistic effects (i.e., spin-orbit coupling) on the surface of TI materials. They possess a usual band gap in the bulk, while hosting metallic surfaces whose low-energy quasiparticles behave as massless Dirac fermions with spins locked to their momenta. One of the key effects that TIs bring into spintronics is nonequilibrium spin polarization in the direction transverse to injected unpolarized charge current. If layers of TI can be oriented in a way proposed in the article and illustrated in the Figure on the right, which has not yet been considered experimentally, the nonequilibrium spin polarization will appear in the direction of electron motion. This will make possible SSE and several other phenomena of interest to spintronics.
ABOUT NIKOLIC GROUP
ABOUT PHYSICAL REVIEW B
Physical Review B is the largest and most comprehensive international journal specializing in condensed matter and materials physics, publishing important papers on a wide range of topics. It has an impact factor of 3.767 (2012) and is ranked number one in total citations in condensed matter physics.