Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2   Measured Using Ultrafast Spectroscopy
  We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys. 
Cong Mai, Yuriy G. Semenov, Andrew Barrette, Yifei Yu, Zhenghe Jin,   Linyou Cao, Ki Wook Kim, Kenan Gundogdu
cond-mat.mtrl-sci