Nonequilibrium spin currents and spin polarization in noncollinear antiferromagnetic insulators
Alexey Kovalev, Department of Physics, University of Nebraska, Lincoln
An ability to control spin is important for probing many spin related phenomena in the field of spintronics. Spin-orbit torque is an important example in which spin flows across magnetic interface and helps to control magnetization dynamics. As spin can be carried by electrons, spin-triplet pairs, Bogoliubov quasiparticles, magnons, spin superfluids, spinons, etc., studies of spin currents can have implications across many disciplines. In this talk, I first review the most common ways to generate spin flows and then concentrate on how spin can be controlled via magnons in insulating materials. We develop a linear response theory based on the Luttinger approach of the gravitational scalar potential and apply our theory to a wide range of antiferromagnetic insulators, ranging from collinear honeycomb antiferromagnets to breathing pyrochlore noncollinear antiferromagnets. From our analysis, we suggest looking for the magnon-mediated spin Nernst effect in collinear antiferromagnets  that are invariant under (i) a global time reversal symmetry or under (ii) a combined operation of time reversal and inversion symmetries. In both cases, the thermal Hall effect is zero while the spin Nernst effect could be present due to the Dzyaloshinskii–Moriya interaction. An experiment consistent with such predictions has recently been reported for MnPS3 antiferromagnet . Our theory also applies to noncollinear antiferromagnets, such as kagome KFe_3(OH)_6(SO_4)_2, where we predict both the spin Nernst response  and generation of nonequilibrium spin polarization  by temperature gradients, the latter effect constitutes the magnonic analogue of the Edelstein effect of electrons. As dissipation associated with the electronic degrees of freedom is absent in the above systems, our studies can pave the way for the creation of novel electronic devices for classical and even quantum information processing where the signals can propagate with almost no dissipation.
 V. Zyuzin, A.A. Kovalev, Phys. Rev. Lett. 117, 217203 (2016).
 Y. Shiomi, R. Takashima, E. Saitoh, Phys. Rev. B 96, 134425 (2017)
 B. Li, S. Sandhoefner, A.A. Kovalev, arXiv:1907.10567 (2019)
 B. Li, A. Mook, A. Raeliarijaona, A.A. Kovalev, arXiv:1910.00143 (2019)
Host: Shulei Zhang