QUANTUM FLUCTUATIONS DRIVEN ORIENTATIONAL DISORDERING - A FINITE-SIZESEALING STUDY

The orientational ordering transition is investigated in the quantum g eneralization of the anisotropic-planar-rotor model in the low-tempera ture regime. The phase diagram of the model is first analyzed within t he mean-field approximation. This predicts at T=O a phase transition f rom the ordered to the disordered phase when the strength of quantum f luctuations, characterized by the rotational constant Theta, exceeds a critical value Theta(c)(MF). As a function of temperature, mean-field theory predicts a range of values of Theta where the system develops long-range order upon cooling, but enters again into a disordered stat e at sufficiently low temperatures (reentrance). The model is further studied by means of path-integral Monte Carlo simulations in combinati on with finite-size scaling techniques, concentrating on the region of parameter space where reentrance is predicted to occur. The phase dia gram determined from the simulations does not seem to exhibit reentran t behavior; at intermediate temperatures a pronounced increase of shor t-range order is observed rather than a genuine long-range order.