Isotope effects on the electronic critical points of germanium: Ellipsometric investigation of the E1 and E1+Δ1 transitionsD. Rönnow, L. F. Lastras-Martínez and M. CardonaEur. Phys. J. B, 5 1 (1998) 29-35DOI: https://doi.org/10.1007/s100510050415