Experimental studies of hydrogen on boron nitride: II. NMR studies of orientational ordering of H2

Abstract

The authors report the results of NMR studies of thin films of hydrogen adsorbed on hexagonal boron nitride. Orientational ordering is observed below 1 K but the ordering is not complete, and a clear two-component ordering is observed. Molecules are either (i) almost completely ordered with local order parameters {sigma}={l_angle}1{minus}3/2Jz{sup 2}{r_angle} clustered close to a maximum value of {sigma}{congruent}0.94 (comparable to the values for long range ordering in bulk samples at high ortho concentrations), and (ii) a large fraction of the molecules that remain nearly disordered with {sigma}{<=}0.25. The degree of orientational ordering depends on the number of hydrogen layers and on the ortho-hydrogen concentration, and these studies indicate that ordering occurs principally in the first four layers closest to the substrate, with weaker orientational ordering in the outer layers near the free surface even at temperatures as low as 210 mK.