Ultra-wide band gap and large exciton effect in 2D ferrovalley material H-FeCl2.

Ferrovalley materials are valleytronic materials with intrinsic ferromagnetism, in which the presence of spontaneous valley polarization is more conducive to practical applications. The optical properties of ferrovalley are important for selectively exciting electrons at the valley. In this paper, the electronic and optical spectrum of the H-phase FeCl₂monolayer is studied using first-principles calculations as an example. We use hybrid functional HSE06 and GW₀methods with spin-orbit coupling for our calculations, the band gap of H-FeCl₂is about 3.975 and 4.072 eV at K and -K valley, which is significantly larger than that obtained by the PBE method, with a 97 meV valley splitting. It is shown that the monolayer H-FeCl₂is a ferrovalley material with an ultra-wide band gap and large intrinsic valley polarization, which has strong electronic correlation and many-body effects. Calculation of the imaginary part of the dielectric function using GW-BSE method shows that the energy corresponding to the exciton peak is 2.421 and 2.491 eV, much smaller than the GW band gap. The exciton binding energy is about 1.554 and 1.581 eV at K and -K valley, indicating a large exciton effect. And the exciton binding energy of the two valleys are unequal, with a difference of 27 meV. It is found that splitting occurs at the first exciton peak in the ferrovalley material, and the splitting value is inequivalent to the bandgap splitting at the valley, which is instructive for further research as well as application of the valleytronics.