Ab initio simulation of pressure-induced low-energy excitations in amorphous silicon

Extensive experimental and theoretical investigations have made considerable progress toward understanding the nature of vibrational dynamics of amorphous silicon (a-Si). However, the changes in the vibrational spectrum of a-Si with external perturbations need to be explored. An investigation of a-Si under pressure is, therefore, important to understand not only changes in the vibrational spectrum but also the structure of a-Si. a-Si can be prepared in various ways, which yields structures with different physical properties. It has been reported that the vibrational spectrum of a-Si depends on degree of structural disorder, sample preparation, deposition conditions, and hydrogen concentration. The TA/TO intensity ratio and TO linewidth correlate with bond angle distribution of the sample. The coordination defects lead to localized states not only near the high-frequency band edge, but also in the low-frequency tail. There is a correlation between the density of a-Si and the number of localized states ~the number of localized states decreases with increasing density of a-Si). Nakhmanson and Drabold have shown that a fourfold coordinated model of a-Si does not exhibit low-energy excitations and that the presence of voids leads to extra lowenergy states in the vibrational density of states ~VDOS! and a sharp peak in C(T)/T . Recently we showed that a-Si transformed via a firstorder phase transition into an amorphous metallic phase using an ab initio constant pressure relaxation simulation, which is in agreement with experiments. Kelires confirms an amorphous-to-amorphous phase transition in a-Ge and a-Si using an environment dependent interatomic potential ~EDIP! and Tersoff potential. Kelires finds that the transition in both structures proceeds gradually and the freeenergy calculation suggests a first-order transition in a-Si. Since the behavior of a-Si under pressure is known from experimental and theoretical studies, a-Si is an excellent system to further study the phonon spectrum and states under pressure. In this paper, we present the pressure dependence of the lowand high-energy modes in a-Si using an ab initio constant pressure relaxation simulation and lattice-dynamical calculation. We find that pressure softens the low-energy modes and leads to a dramatic decrease in the acousticlike nature of these frequencies. The higher-frequency modes