Static and time-dependent optical properties of CuI

We aim to use first-principles electronic-structure theory to explain the static and time-dependent optical properties of the wide-band gap semiconductor CuI, which is a promising candidate for a transparent conducting material. In particular, we aim to clarify the importance of the spin-orbit interaction for the appearance of an above-gap spectral feature that is reported in multiple experiments. Measurements of the linear optical properties that have been reported in the literature agree in attributing that peak in the spectrum to a spin-orbit split-off valence-band state. However, the significant oscillator strength of the peak might raise doubts about its origin due to the spin-orbit interaction. We use a combination of density-functional and many-body perturbation theory to simulate the electronic structure and optical properties including excitonic effects. Our simulations do not reproduce the experimentally reported peak structure and we show that the optical dipole transitions from the corresponding spin-orbit split-off electronic state do not show a polarization dependence. We interpret this as an indication that more direct experimental evidence is needed to support the spin-orbit origin of any feature in this spectral range. We also solve the Boltzmann transport equation to account for electron-phonon scattering as a relaxation mechanism in pump-probe experiments and implement the resulting time-dependent occupation numbers as constraint in simulations of the spectrum. Our predicted pump-probe spectra for this material show that increasing the intensity of the excitation only significantly changes the magnitude of the spectrum values, while increasing the excitation energy leads to changes in the presence of peaks, peak location, and peak width.