Electron-phonon interaction at the direct gap of the copper halides

We have measured the effect of changing the copper isotope on the gap of CuCl and found that it decreases with increasing mass, contrary to observations in other tetrahedral semiconductors. Also, the direct gap of CuCl is found to increase anomalously (i.e., strongly nonlinearly) with increasing temperature. We show that both effects are related to electron-phonon renormalization of the electronic structure. A semiempirical theory is presented which explains these anomalies as due to an opposite sign of the effect on the gap of the copper and the halogen vibrations.