Nonlinear antidamping spin-orbit torque originating from intraband transport on the warped surface of a topological insulator

Motivated by recent experiments observing a large antidamping spin-orbit\ntorque (SOT) on the surface of a three-dimensional topological insulator, we\ninvestigate the origin of the current-induced SOT beyond linear-response\ntheory. We find that a strong antidamping SOT arises from intraband transitions\nin non-linear response, and does not require interband transitions as is the\ncase in linear transport mechanisms. The joint effect of warping and an\nin-plane magnetization generates a non-linear antidamping SOT which can exceed\nthe intrinsic one by several orders of magnitude, depending on warping\nparameter and the position of Fermi energy, and exhibits a complex dependence\non the azimuthal angle of the magnetization. This nonlinear SOT provides an\nalternative explanation of the observed giant SOT in recent experiments.\n