Electron-phonon renormalization of the absorption edge of the cuprous halides

We report the isotope effects of copper and bromine on the gap of CuBr, and that of copper on the gap of CuI. The measured isotope effects reveal an anomalous negative shift with increasing copper mass for the three halides. These results allow us to understand the anomalous temperature dependences of the band gap $E_0$ in the copper halides, which we report in the case of CuI. Similarities to the behavior observed for the copper and silver chalcopyrites are also pointed out.