Magnetic field effects and excitonic selection rules in monolayer palladium diselenide as a large-gap quantum spin Hall insulator

Quantum spin hall (QSH) effect, a class of quantum state, is promising for dissipationless transport with topologically protected helical edge state. Based on first-principles calculations, we theoretically demonstrate the topological physics of 1H-PdSe2 with a large gap up to 0.24 eV. The band inversion takes place either among the pz and px,y orbitals of Se atom or due to spin-orbital coupling under compressive strains. The edge modes exist in the energy gap and can be characterized by an effective edge state Hamiltonian. Based on the derived two-band k·p model, we demonstrate the profound phenomena induced by the applied magnetic field effects and optical transitions for chiral fermions, which lead to complicated excitonic selection rules in this topologically nontrivial two-dimensional material. Our results will pave the way for future theoretical and experimental studies on PdX2 (X=S,Se,Te) monolayers.