H 2 NMR line shapes for orientational ordering study

Abstract

A peculiar NMR line shape was reported (J. Low Temp. Phys. 114 (1999) 173; Phys. Rev. B 57 (1998) 12595; Phys. Rev. B 55 (1997) R664) for a study of orientational ordering in submonolayer films of H 2 molecules adsorbed on hexagonal boron nitride. This low-temperature (below 200 mK ) NMR line shape was seen for relatively high ortho-H 2 ( o-H 2) concentration (0.48< x[ o-H 2]⩽0.69) and was interpreted as a signature of the hindered rotor state. Preliminary results of recent computer simulations for the NMR line shapes of H 2 molecules, however, indicate that the peculiar NMR line shape may be the result of a combination of three different NMR line structures, and suggest possible well-defined clusters in two dimensions with three different order parameters dominating the orientational ordering. In addition to the hindered rotor line shape, the order-parameter dependence of the quadrupolar glass NMR line shape will be discussed.