Threshold voltage as a function of I. The red and black lines indicate the threshold voltage for the polarization switching when V_G > 0 and V_G < 0, respectively. The polarization switching exhibits a strong dependence on I when V_G < 0; the higher I, the larger V_G is required to switch the polarization. Such phenomena are due to the efficient valley-flip scattering as demonstrated in ref. 18. When V_G > 0, however, we suggest a different mechanism, namely a correlation-driven exciton site transition. In this regime, the polarization switching shows a weak I -dependence. See the main text for more details.
