Giant correlation, many body and exciton effects in Janus ferrovalley material H-FeClBr

The family of transition metal dichlorides are recently found to be ferrovalley materials, exhibiting desirable spontaneous valley polarization that is a key to practical applications. In this work, Janus monolayer H-FeClBr is investigated as a case study by performing first-principles calculations. We focus on the giant correlation and many-body and exciton effects that will essentially modulate the electronic, valleytronic, and optical properties. The H-FeClBr presents strong ferromagnetism and spin–orbit coupling, giving rise to large spontaneous valley polarization. Due to the enormous electron correlation of the localized d electrons of the Fe atoms, the energy gap calculated using Hubbard U and hybrid functional HSE06 is extraordinarily widened by about seven times with respect to the Perdew–Burke–Ernzerhof (PBE) counterpart. As a result, the orbitals in valley bands are exchanged, the sign of the valley Berry curvatures is switched, and the valley polarization is reversed. The GW calculations further enhance the gap significantly to about 4 eV, which is close to ten times that of the PBE gap and indicative of very strong many-body effects. The exciton spectrum obtained by solving the Bethe–Salpeter equations reveals colossal electron–hole interaction, giving rise to a giant exciton binding energies of ∼1.54 eV. Corresponding to the optical excitation in the two inequivalent valleys with unequal gaps, there are two split exciton peaks, as opposed to the A and B exciton peaks in MoS2 spectrum, which correspond to the optical transitions in the same valley.