An NMR Study of Helium-3 Adsorbed on Hexagonal Boron Nitride

Abstract

A Pulse-NMR study of helium-3 adsorbed on hexagonal boron-nitride (BN) powder has been performed. Structurally very similar to graphite, the exposed basalplanes present a very smooth, ideal adsorbing surface and lack its undesirable strong anisotropic diamagnetism. The relaxation times T1 and T2 of helium-3 have been measured as a function of coverage, temperature and frequency. A variety of two dimensional phases have been observed including: a fluid, commensurate solid, incommensurate solid plus a separate crystallite edge film. 2D melting in the incommensurate solid and an order-disorder transition in the commensurate solid have been observed. Evidence for a low temperature, low coverage fluid+commensurate solid coexistence which transforms to a single phase at higher temperatures plus a possible domain-wall phase at higher coverages has been identified. Coupled magnetic relaxation between the helium-3 film and substrate boron-11 spins has been noted. Boron-11 relaxation times have been measured against coverage and temperature. Heteronuclear relaxation is particularly important in the commensurate phases where it can dominate homonuclear spin-lattice relaxation, providing a powerful new probe of the low coverage phases. Based on the detailed theory of coupled magnetic dipolar relaxation a model has been developed which quantitatively describes all the important features of the data many of which are unique to the BN/3He system. Presented separately in chapter 8, it concludes the magnetic properties of registered helium 3 spins are dominated by 14N�� 3He cross relaxation processes, mediated by the €14N quadrupole splitting at FQ(14N) and driven by exchange motion in the film. Using a computer for unattended, real-time experimental control has allowed substantial quantities of high quality relaxation data to be taken. Off-line, automated, numerical analysis of raw spin-echo and processed data has been extensively used. Modelling relaxation data with a stretched-exponential function, h(t) = h(0) exp(ta/T1,2) has provided a exceptionally sensitive indicator of physical changes in the film.