Quantum "melting" of orientationally ordered physisorbates

Abstract

Based on path-integral Monte Carlo simulations we investigate systematically the influence of quantum fluctuations on a model for orientational ordering. The quantum anisotropic-planar-rotor Hamiltonian with its herringbone ground state serves as our model for strictly two-dimensional, orientationally ordered linear molecules. We find several distinct regimes as the strength of the quantum fluctuations is increased: (i) a regime with renormalized transition temperature and ground state order parameter which describes systems like N2 physisorbed on graphite; (ii) crossover behavior with residual ground state order but an increased order parameter at higher temperatures; (iii) a regime characterized by vanishing ground state order but significant order in some range of higher temperatures; and (iv) a regime with orientational disorder at all temperatures. This sequence is interpreted in terms of reentrant rotational quantum melting. In addition we probe the range of validity of simple approximation schemes such as the quasiharmonic and the quadratic Feynman-Hibbs effective potential approximations. Both methods fail to describe the full scenario for large quantum fluctuations even on a qualitative level. © 1995 American Institute of Physics.