Selective control of valley polarization in monolayer transition metal dichalcogenides among multiexcitonic states

In 2018, an inverted valley polarization (VP) for the biexciton emission in monolayer WS2 was experimentally observed [Nagler et al., Phys. Rev. Lett. 121, 057402 (2018)]. Here, we develop a comprehensive model, which incorporates thermal effect and magnetic response of exciton and biexciton complexes with both intra- and intervalley relaxation channels, to reveal the underlying physics. We obtain the inverted biexciton VP, in excellent agreement with the experimental data. Notably, beyond available experimental measurements, our theory also predicts a reversal of VP for excitons associated with the intra- and intervalley thermalization processes, greatly fascinating for selective VP control among distinct excitonic states. These (i) open a pathway toward valley exciton based field-effect transistor and (ii) make possible a concept of "excitonic filter," which resembles spin filter while with functionality of selectively filtering excitonic states, offering an intriguing platform for spintronic and orbitronic as well as valleytronic combined applications. A comparison with other W-based and even Mo-based monolayers of transition metal dichalcogenides including WSe2, MoSe2, and MoS2 is also made.