Phenomena of spin transfer torques (STTs) have been attracting persistent interests due to promising prospects of STTs in designing nano devices. By using nonequilibrium Green's function method, we derive time-averaged formulism for spin transfer torques in a noncollinear magnetic tunneling system under ac modulation within wide-band-limit approximation. Using these formulas, we further investigate ac spin transfer torques in a carbon-nanotube-based magnetic tunneling system. It shows that under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque on bias still holds, and the regular angular dependence on noncollinear angle is maintained. By introducing photon-assisted tunneling, the bias-induced components of the in-plane and out-of-plane torques can be enhanced by about 10 and 80 times, respectively. Further analysis shows that optimized enhancement can be achieved by using ac driving frequency w=e₀/k, k=1,2,..., where e₀ marks a remote dc transmission peak, and using ac amplitude D such that D/w maximizes a k^th-order Bessel function of the first kind. Out findings suggest that ac modulation is a promising way for electrical manipulation of spin transfer torques, paving the road towards emerging nanoelectronic and spintronic devices utilizing spin transfer torques.

个人简介：Dr. Chen works on theoretical Physics as a postdoctoral fellow in the department of Physics at McGill University, Canada. She received her Ph.D. of condensed matter Physics from Tsinghua University in 2014 and has been conducting her postdoctoral research with Prof. Hong Guo at McGill University since then. Her early research interests were nanoscale heat transport and thermoelectricity. More recently, she has worked on the spin Caloritronics, spin thermoelectrics, valley transport, and time-dependent physics in nanostructures.