Roberto Myers
Professor, Materials Science and Engineering
and Electrical and Computer Engineering
Ohio State University

Using Asymmetric Crystals for New Functionality: Moving Spins with Heat Using
Spin-Orbit Coupling and Dopant-less Diodes Using Polarization Charge

Abstract
The lack of inversion symmetry in certain crystals leads to interesting properties. Two such properties, the spin-orbit effect and polarization charge, can be exploited for new thermoelectric and optoelectronic devices. In the first part of the talk, I will introduce the heat/spin conversion phenomenon, the spin-Seebeck effect. Phonons interact with the magnetic moments in a material driving them away from equilibrium inducing a diffusive spin current, which can be converted in a neighboring material into an electric voltage. Via the spin-orbit effect, the heat/spin interactions can occur even in non-magnetic, but spin-polarized materials. For example in InSb, the spin-Seebeck effect exhibits a magnitude of up to 8 mV/K, which is as large as the highest known charge based thermoelectric effect opening new possibilities for thermal energy conversion devices based on spin. In the second part of the talk, the anisotropic property of polarization charge in (Ga,Al)N compounds is introduced. If the composition of (Ga,Al)N is graded along the dipole direction, a slab of bound polarization charge is uniformly distributed in the crystal, which is compensated by free charges. By grading the composition in a back and forth manner within a nanowire (GaNàAlNàGaN) a polarization-induced pn-junction is formed. Unlike other pn-diodes, the polarization-induced diodes can be formed without intentional impurity doping, do not freeze out at low temperature, yet still exhibit rectification and ultraviolet light emission at the bandgap. This work was done in collaboration with C. M. Jaworski, J. P. Heremans, S. Carnevale, T. F. Kent, J. Yang, P. J. Phillips, and M. J. Mills.

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