Giant spin Seebeck effect in a non-magnetic material

A giant spin Seebeck effect—three orders of magnitude greater than previously detected—has been observed in a non-magnetic material, InSb; the proposed mechanism relies only on phonon drag and spin–orbit interactions in a spin-polarized system, not on magnetic exchange. Thermal spin power as a new type of heat cycle Heat cycles provide almost all of the energy that a modern civilization consumes. The thermoelectric cycle, a latecomer compared with steam and gases, generates electrical power through the Seebeck effect, whereby an electric voltage is generated when a conductor is placed in a temperature gradient. The 2008 discovery of the spin Seebeck effect ( go.nature.com/dlvhz2 ) — whereby a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material — led to a new line of research in spintronics. In this issue of Nature , Jaworski et al . describe something similar but three orders of magnitude more powerful, a 'giant spin Seebeck effect' in a material (indium antimonide, InSb) that is non-magnetic but that has strong spin–orbit coupling and phonon–electron drag. They propose a mechanism for this phenomenon that relies on spin polarization only, not on magnetic exchange. The results, say the authors, show that the spin Seebeck effect can be of a magnitude that may make spin-based thermal-energy converters a reality, and possibly competitive with existing technologies. The spin Seebeck effect is observed when a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material, where it gets converted into a measurable voltage. It has been previously observed with a magnitude of microvolts per kelvin in magnetically ordered materials, ferromagnetic metals 1 , semiconductors 2 and insulators 3 . Here we describe a signal in a non-magnetic semiconductor (InSb) that has the hallmarks of being produced by the spin Seebeck effect, but is three orders of magnitude larger (millivolts per kelvin). We refer to the phenomenon that produces it as the giant spin Seebeck effect. Quantizing magnetic fields spin-polarize conduction electrons in semiconductors by means of Zeeman splitting, which spin–orbit coupling amplifies by a factor of ∼25 in InSb. We propose that the giant spin Seebeck effect is mediated by phonon–electron drag, which changes the electrons’ momentum and directly modifies the spin