COMPRESSION EFFECT ON SPECIFIC-HEAT AND THERMAL-EXPANSION OF SI AND GE

The comparison effect on the specific heat C(OMEGA) and thermal expansion coefficient alpha for Si and Ge are studied theoretically using the lattice dynamical method based on pseudo-potentials. The numerical results of C(OMEGA)qh and alpha(qh) in the quasi-harmonic approximation are obtained at low temperatures below room temperature. These temperature-dependent data of C(OMEGA)qh and alpha(qh) are varied under pressure, especially for the thermal expansion coefficient, and the negative thermal expansion appears over the wide temperature region for Si and Ge. At high temperatures, the compression effect on the anharmonic coefficient A for the specific heat C(OMEGA)ah = C(OMEGA)qh (1 + AT) are calculated by using the expression by Trivedi et al., and the obtained values of A increase under compression. Then, anharmonic specific heat C(OMEGA)ah and thermal expansion coefficient alpha(ah) for Si and Ge are presented quantitatively at high temperatures and under compression.