Excitonic correlation in the Mott crossover regime in Ge

Abstract

Exciton Mott transition (EMT) in Ge was investigated by using optical-pump and terahertz-probe spectroscopy. From the quantitative analysis of optical conductivity and dielectric function, we evaluated the densities of unbound electron-hole pairs and excitons after the photoexcitation, from which we determined the ionization ratio of excitons \ensuremath{\alpha}. The Mott crossover density region in Ge was elucidated from the density dependence of \ensuremath{\alpha} in the temperature range above the critical temperature of electron-hole droplets. The $1s\ensuremath{-}2p$ excitonic transition energy hardly shifted with increasing density toward the EMT. Combined with the similar results recently observed in bulk Si, we suggest that the robustness of excitonic correlation against the Coulomb screening is a universal feature in bulk semiconductors in the Mott crossover regime.