Direct Raman spectroscopy observation of quantum isotope effects in isotopically pure germanium single crystals

High-precision studies of Raman scattering on isotopically pure $^{70}$Ge and $^{74}$Ge single crystals have been performed in the temperature range from $80$ to $296$ K. It has been found that a difference between the normalized Raman frequencies of the $^{70}$Ge and $^{74}$Ge isotopes measured with an accuracy of $0.05$ cm$^{-1}$ at temperatures $T < 100$ K is about $0.12$ cm$^{-1}$, which certainly indicates the manifestation of quantum isotope effects.