Definite Experimental Evidence against Intrinsic Electron-Hole Superconductivity in Pure CuCl

Optical absorption measurements on very pure CuCl single crystals, prepared by a novel process, show the exponential drop of the direct optical absorption edge to continue below an absorption coefficient $K\sim {10}^{-2\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. This rules out unambiguously the existence of an indirect-gap absorption below 3 eV, which is not only consistent with band-structure calculations but also excludes pure CuCl from being a suitable candidate for intrinsic high-$T_c$ superconductivity (on the grounds of existing theories).