Quantum fluctuations driven orientational disordering: A finite-size sealing study

摘要

The orientational ordering transition is investigated in the quantum generalization of the anisotropic-planar-rotor model in the low-temperature regime. The phase diagram of the model is first analyzed within the mean-field approximation. This predicts at T=O a phase transition from the ordered to the disordered phase when the strength of quantum fluctuations, 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 state at sufficiently low temperatures (reentrance). The model is further studied by means of path-integral Monte Carlo simulations in combination with finite-size scaling techniques, concentrating on the region of parameter space where reentrance is predicted to occur. The phase diagram determined from the simulations does not seem to exhibit reentrant behavior; at intermediate temperatures a pronounced increase of short-range order is observed rather than a genuine long-range order.