Coherent spin dynamics of excitons in strained monolayer semiconductors

We develop a model of the coherent exciton spin-valley dynamics in two-dimensional transition metal dichalcogenides under elastic strain. The strain splits the exciton radiative doublet in linearly polarized states. Consequently, it induces an effective magnetic field acting on the exciton pseudospin and causes its precession. As a result, under circularly polarized excitation, the circular polarization of excitons oscillates with time, also a time-oscillating linear polarization appears. We study competition of the strain-induced effective magnetic field with the field caused by the exciton longitudinal-transverse splitting. We uncover different regimes of coherent spin dynamics of two-dimensional excitons. In particular, we show that for sufficiently large strain-induced and longitudinal-transverse splittings two frequencies related to these splittings appear in the exciton circular polarization beats.