Orientational Ordering in Monolayers of Ortho-Para Hydrogen

TL;DR

This paper investigates the orientational ordering in monolayers of solid hydrogen, analyzing NMR experimental data with a rotor model to understand local structures and their dependence on crystalline fields.

Contribution

It introduces a two-dimensional rotor model with quadrupolar coupling to explain NMR findings and describes two distinct para-rotational short-range ordered structures.

Findings

Identification of two types of PR short-range order

Quantitative analysis of NMR frequency splitting

Rejection of ferromagnetic-like PR ordering at low concentrations

Abstract

We discuss orientational ordering in monolayers of solid hydrogen in view of recent experimental findings in NMR studies of (\QTR{it}{ortho)}$_{c}$-(\QTR{it}{para)}$_{1-c}$-hydrogen mixtures on boron nitride substrate. Analysis of the temperature-concentration behavior for the observed NMR frequency splitting is given on the basis of a two-dimension ($J=1)_{c}$-($J=0)_{1-c}$-rotor model with the quadrupolar coupling constant $\Gamma_{0}=0.50\pm 0.03$ $K$ and the crystalline field amplitude $V_{0}=0.70\pm 0.10$ $K$ derived from experiment. The two distinct para-rotational (PR) short-range ordered structures are described in terms of the local alignment and orientation of the polar principal axis, and are shown to be due to the interplay between the positive and negative crystalline fields. It is shown that the observed below the 2D site percolation threshold $c_{p}=0.72$ local structures are rather different from the ferromagnetic-type PR ordering suggested earlier by Harris and Berlinsky.