Isotopic dependence of the heat capacity of c-C, Si, and Ge: an ab initio calculation

摘要

The vibrational spectra of crystalline diamond, silicon, and germanium are calculated with a first-principles local density functional scheme using 64 as well as 128 atoms supercells. The (harmonic) dynamical matrices are obtained from linear response theory. The phonon density of states, calculated for various isotopic masses M, are used to evaluate the vibrational heat capacities C_v and their dependence on M at low temperatures. Simple rules obeyed by the isotope effect of C_v are discussed. The results are compared with recent experimental data for Ge. It is hoped that this work will encourage and help to interpret analogous measurements in diamond and silicon with different isotopic compositions.