Optical Response of Quantum Crystals

Abstract

Quantum solids are crystals in which, even at T = 0K, the atoms of the ordered array (or molecules) undergo large rms displacements or zero-point motion (ZPM) about their equilibrium lattice sites¹. The ZPM in these crystals, in comparison to normal crystals, is not small relative to the nearest neighbor distance, R_O. The dynamical aspects of ordinary crystals can be treated classically or quantum mechanically in the quasi-harmonic approximation. Quantum crystals must be treated quantum mechanically because the zero-point energy is comparable to the static lattice energy. This in itself would present no difficulty if potentials were harmonic, however real potentials are highly anharmonic and for quantum crystals the usual per turbative treatment of anharmonicity breaks down. New theoretical approaches which have been developed in the last decade to handle the dynamical problems will be discussed in the first section.