Alireza Taghizadeh: Anomalous excitonic signature in nonlinear spin Hall current of monolayer TMDs

posted 27 May 2019, 13:17 by info admin   [ updated 28 May 2019, 04:47 ]
Ålborg University and Technical University of Denmark

The spin Hall effect (SHE) is a physical phenomenon, in which moving spin-up/-down charges are separated and accumulate at opposite boundaries of a surface [1]. The SHE is an extraordinary tool for studying fundamental physics as well as for spintronics, where the spin degree of freedom is manipulated. Monolayer TMDs are intriguing candidates for investigating the SHE, due to their large spin-orbit coupling and long spin relaxation time [2].  
Recently, we theoretically predict the existence of a dc SHE in ordinary monolayer TMDs (1H phase) that emerges from a nonlinear optical process, in which a strong time-dependent field induces a spin current at zero frequency via optical rectification. Two distinct mechanisms contribute to this photo-induced spin current, a purely interband part, and a mixed inter-/intraband contribution. Analogous to the linear optical absorption, excitons modify the SHE spectrum significantly by introducing strong discrete resonances. Remarkably, the direction of the excitonic spin current is inverted by varying the temperature, as the relative strengths of inter- and intraband contributions change.  We provide numerical data for MoS2 and WSe2, but the main findings hold true for other TMDs. Our results pave the way for generating spin currents in ordinary TMDs without the need for doping or external fields. 

[1] M. Dyakonov and V. Perel, Phys. Lett. A 35, 459 (1971).
[2] X. Xu, W. Yao, D. Xiao, and T. F. Heinz, Nat. Phys. 10, 343 (2014).

Alireza Taghizadeh is a postdoc researcher at Center for Nanostructures Graphene (CNG) in Aalborg University (AAU) and Technical University of Denmark (DTU). His main research interests lie in the fields of nanophotonics and physics of low-dimensional systems such as 2D materials and carbon nanotubes. He has a multidisciplinary background in electronics, optics, and physics. He graduated with honor (ranked 1st) in two B.Sc. programs of physics and electrical engineering in 2009. Having accomplished his master thesis in microelectronics, he graduated excellently (ranked 1st) in 2011. He received his Ph.D. degree in 2016 from DTU, where he exclusively studied various nonphotonic devices such nanolasers for optical interconnect applications. In 2015, he had a research stay in U.C. Berkeley, USA, working on a project about optical antenna-enhanced nanoLEDs. After Ph.D. graduation, he continues his researches at DTU as a postdoc for one year, working on novel electromagnetic phenomena in nanoscale systems. Since the beginning of 2017, he is actively doing research in the field of 2D materials, particularly their excitonic effects and nonlinearities. This includes a post-doctoral fellowship at the QUSCOPE center of excellence in AAU, which is followed by another postdoc at the CNG center in DTU and AAU. He is the author of 16 journal articles and 2 international patents.