Low-temperature anharmonic lattice deformations near rotator impurities: A quantum Monte Carlo approach.

Abstract

At zero temperature the equilibrium structures of a system consisting of a quantum rotator (${\mathrm{N}}_{2}$) embedded in a relaxing lattice (Ar) surrounding are studied with a variational approach. With symmetric wave functions (para-${\mathrm{N}}_{2}$), we obtain a cubic lattice deformation near the rotator, while with antisymmetric wave functions (ortho-${\mathrm{N}}_{2}$), we obtain a tetragonal lattice deformation forming a stable oriented ground state. At low temperatures, we investigate the properties of this system with a quantum Monte Carlo simulation. On top of the tetragonal deformation the width of the nearest-neighbor oscillations follows classical ``scaling'' laws according to a harmonic approximation, while the static deformation turns out to be anharmonic. The Monte Carlo relaxation of the rotational degree of freedom occurs according to an Arrhenius law with an activation energy much lower than the local energy barriers.