Multiband k . p theory of monolayer XSe (X=In, Ga)

We derive the multiband k . p Hamiltonian for an electron in monolayer XSe (X = In, Ga) near the Gamma point in the presence of spin-orbit coupling and strain. With four conduction bands and ten valence bands included, the 14-band k . p Hamiltonian is capable of describing the nonparabolic energy band dispersion, strong interband mixing, and strain-induced effects, in good agreement with first-principles calculations. This k . p theory provides a simple and convenient way to understand and manipulate the optical and transport properties of monolayer XSe and their nanostructures by strain and external fields. Interestingly, we find that uniaxial strain can even induce an indirect-to-direct band-gap transition in these two-dimensional materials.