Theoretical evidence for a reentrant phase diagram in {\em ortho-para} mixtures of solid H$_2$

TL;DR

This paper presents a mean-field formalism for quantum rotors in ortho-para hydrogen mixtures, revealing a reentrant phase transition and the impact of small odd-J concentrations on orientational order.

Contribution

It introduces a novel multi order parameter mean-field approach specifically for ortho-para hydrogen systems, predicting reentrant phase behavior.

Findings

Reentrant orientational phase transition observed in solid H₂ and D₂.

Short-range order persists at low temperatures and increases with temperature.

Small concentrations (1%) of odd-J molecules can induce orientational ordering.

Abstract

We develop a multi order parameter mean-field formalism for systems of coupled quantum rotors. The scheme is developed to account for systems where {\it ortho-para} distinction is valid. We apply our formalism to solid H$_2$ and D$_2$. We find an anomalous {\it reentrant} orientational phase transition for both systems at thermal equilibrium. The correlation functions of the order parameter indicate short-range order at low temperatures. As temperature is increased the correlation increases along the phase boundary. We also find that even extremely small {\it odd-J} concentrations (1%) can trigger short-range orientational ordering.