Bio: Assoc. Professor Dimi Culcer is a FLEET Chief Investigator located in the School of Physics at UNSW. He graduated with an MPhys from Oxford University in 2000 and obtained his PhD from the University of Texas at Austin in 2005. His research is in topological quantum matter and quantum computing.
Abstract: Spin torques at topological insulator (TI)/ferromagnet interfaces have received considerable attention in recent years with a view towards achieving full electrical manipulation of the spin degree of freedom. The most important question in this field concerns the relative contributions of bulk and surface states to the spin torque, a matter that remains incompletely understood. Whereas the surface state contribution has been extensively studied, the contribution due to the bulk states has received comparatively little attention. I will first discuss spin torques due to TI bulk states and show that they give rise to a spin transfer torque (STT) due to the inhomogeneity of the magnetisation in the vicinity of the interface. This spin transfer torque is somewhat unconventional since it arises from the interplay of the bulk TI spin-orbit coupling and the gradient of the monotonically decaying magnetisation inside the TI. We find, likewise, that there is no spin-orbit torque due to the bulk states on a homogeneous magnetisation, in contrast to the surface states, which give rise to a spin-orbit torque via the Edelstein effect. Whereas we consider an idealised model in which the magnetisation gradient is small and the spin transfer torque is correspondingly small, I will argue that in real samples the spin transfer torque should be sizable and may provide the dominant contribution due to the bulk states. I will show that an experimental smoking gun for identifying the bulk states is the fact that the spin transfer torque has a comparable size for in-plane and out-of-plane magnetisations when the bulk states dominate, distinguishing them from the surface states, which are expected to give a much stronger torque on an out-of-plane magnetisation than on an in-plane magnetisation. I will also discuss our latest insights into the spin-Hall effect arising from TI bulk states. I will show that, contrary to popular belief, we do not expect any intrinsic spin-Hall effect due to the bulk. There is the possibility of an extrinsic spin-Hall effect, but we expect this to be destroyed near the interface, while the possibility also exists for an intrinsic spin-Hall effect to be generated near the interface. In the last part of my talk I will attempt to put together all the pieces of this rather complex puzzle.