Orientational Ordering in Ultra-Thin Films of Quantum Rotors on Boron Nitride

Abstract

We report the results from NMR studies of submonolayer and monolayer films of solid hydrogen (corresponding to the expected \(\sqrt 3 \times \sqrt 3 \) commensurate and incommensurate solid, respectively) on a well defined insulating substrate (boron nitride). The orientational ordering of the ortho-H2molecules (J=1 quantum rotors) was explored for ortho-H2concentrations 0.3<X<0.74 and temperatures 0.05<T<4.2 K. The phase diagram for submonolayer hydrogen films (75% of full monolayer) has been identified in terms of X and T from clear structural changes of the NMR spectra, including the observation of a new 2D orientational quadrupolar glass ordering for 0.48<X<0.69 and 0.2≤T≤0.6 K. Two distinct long range orientationally ordered phases are identified for monolayer hydrogen films: an expected pinwheel lattice structure for high X and an expected herringbone structure for low X.