Superconductivity from two dimensional interfaces: CuCl/Si, GaP/Si, ZnS/Si*

Two-dimensional (2D) interfaces of hetero-bonded semiconductor superlattices are studied using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. The 2D system, of metal-insulator-metal, is one of the candidate geometries to realize the excitonic mechanism of superconductvity, \footnote{V.L. Ginzburg, Sov. Phys. JETP {\bf 20},1549 (1965)} where $T_C$ can be greatly enhanced over phonon mediation. Epitaxially grown CuCl on Si (111) was reported to exhibit an anomalous diamagnetic susceptibility at 60$\sim$150 K. \footnote{Mattes and Foiles,Physica 135B, 139 (1985)} For all superlattices, 2D metallicity was found at the interfaces due to charge transfer from the polarity mismatch, \footnote{Rhim, Saniz, Yu, Ye, and Freeman, Phys. Rev. B {\bf 76}, 184505 (2007)} as evidenced by their bands, Fermi surfaces, and charge densities. The $T_C$, calculated within the crude RMTA and the McMillan-Hopfield formula, is 0.04$\sim$4.4K for the CuCl/Si case, but vanishes for the other cases. To pursue the excitonic mechanism, we are determining the Kernel function $K(\omega)$, i.e. the average of the effective Coulomb interfaction, with {\bf q} dependent dynamic screening. First results for CuCl/Si show $K(\omega)$ to be attractive for a certain energy range