Spontaneous valley splitting and valley pseudospin field effect transistors of monolayer VAgP 2 Se 6 .

Valleytronics has attracted much attention due to its potential applications in the information processing industry. Creation of permanent valley polarization (PVP), i.e. unbalanced occupation at different valleys, is a vital requirement for practical devices in valleytronics. However, the development of an appropriate material with PVP remains a main challenge. Here we used first-principles calculations to predict that the spin-orbit coupling and magnetic ordering allow spontaneous valley Zeeman-type splitting in the pristine monolayer of VAgP2 Se6 . After suitable doping of VAgP2 Se6 , the Zeeman-type valley splitting results in a PVP, similar to the effect of spin polarization in spintronics. The VAgP2 Se6 monolayer has nonequivalent valleys which can emit or absorb circularly polarized photons with opposite chirality. It thus shows great potential to be used as a photonic chirality filter and a circularly polarized light source. We then designed a valley pseudospin field effect transistor (VPFET) based on the monolayer VAgP2 Se6 , akin to the spin field effect transistors. In contrast to the current common transistors, VPFETs carry information of not only the electrons but also the valley pseudospins, far beyond common transistors.