Electronic properties of metal-induced gap states at insulator/metal interfaces: Dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity

Motivated from the experimental observation of metal-induced gap states (MIGS) at insulator/metal interfaces by Kiguchi [Phys. Rev. Lett. 90, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density-functional method. We have found that, on top of the usual evanescent state, MIGS generally have appreciable amplitudes on halogen sites with a p(z)-like character, whose penetration depth (lambda) is as large as half the lattice constant of bulk alkali halides. This implies that lambda, while little dependent on the carrier density in the jellium, is dominated by the energy gap of the alkali halide, and is scaled by the lattice constant, where lambda(LiF)<lambda(LiCl)<lambda(LiI). We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity, especially in a system where similar to10 Angstrom of metal is sandwiched by alkali halide substrates.