Orientational Order in fcc Solid Ortho-H2. Green's-Function Treatment of the Internal-Field Approximation

Abstract

The technique of temperature-dependent Green's functions is applied to a molecular-field model of the orientational order-disorder transition of ortho-${\mathrm{H}}_{2}$ molecules on a face-centered-cubic (fcc) lattice. The chain of equations for the various Green's functions is decoupled by an approximation similar to the one used by Tahir-Kheli and ter Haar, which enables one to solve the three remaining Green's-function equations of motion. It is shown that the Green's-function approach leads to the same results as the more conventional methods.