Quantum Fluctuations Driven Orientational Disordering: A Finite-Size Scaling Study

TL;DR

This study investigates the quantum orientational transition in a rotor model using mean-field and Monte Carlo methods, revealing discrepancies in reentrant behavior predictions and highlighting the importance of finite-size effects.

Contribution

It provides a detailed finite-size scaling analysis of the quantum orientational transition, challenging mean-field reentrance predictions with simulation results.

Findings

Mean-field theory predicts reentrant phase transitions.

Monte Carlo simulations do not confirm reentrant behavior.

Short-range order increases at intermediate temperatures.

Abstract

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 $T=0$ a phase transition from the ordered to the disordered state when the strength of quantum fluctuations, characterized by the rotational constant $\Theta$, exceeds a critical value $\Theta_{\rm c}^{MF}$. 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.