NMR Studies of ³He Films on Boron Nitride

Abstract

Abstract.We report the results of NMR studies of the dynamics of 3 He adsorbed on hexagonal boron nitride. These studies can identify the phase transitions of the 2D films as a function of temperature. A thermally activated temperature dependence is observed for 2.6 < T < 8 K compared to a linear temperature dependence for 0.7 < T < 2.6 K. This linear dependence is consistent with that expected for thermal diffusion in a fluid for coverages of 0.4 -0.6 of a monolayer. IntroductionHelium-three monolayer and submonolayer films are of great interest because the different phases and microscopic dynamics that occur are dominated by quantum effects that lead to new phenomena in reduced dimensions at low temperatures [1,2]. In addition the films are ideal for testing our understanding of the quantum dynamics and spin ordering in two dimensions from first principles [3]. At low temperatures it had been postulated that the large quantum mechanical zero-point kinetic energy of the 3 He atoms could be sufficient to prevent the system from condensing as a 2D liquid in some scenarios [4], leading to a unique quantum gas at extremely low temperatures. Experimental studies by Bhattacharyya and Gasparini [5,6] offered evidence for such a phase but the majority of experiments [7][8][9] and the most recent theoretical treatments [10][11][12] do not support the existence of a low temperature gas phase. Recent thermodynamic measurements by Sato et al. [13,14], however, for 3 He adsorbed on bare graphite (and on 4 He plated graphite), have provided new evidence in support of a self-bound state for very low coverages at very low temperatures. We have carried out NMR studies at much higher coverages of 0.41 and 0.62 of a full monolayer and at higher temperatures to determine the microscopic dynamics of the 3 He atoms and the mechanisms of diffusion at coverages near the fluid-solid phase transition region [12,15]. At these coverages NMR studies by Crane and colleagues [16] for 3 He on BN showed evidence of a mixed coverage of fluid and solid components with the solid melting near 1.6K to form a dense fluid state with appreciable orientational order imposed by the substrate. We have carried out studies using a much higher NMR frequency which should help distinguish between the fluid and solid components.In addition to measuring the diffusion in the fluid phase, NMR studies can be used to determine the spin exchange rates in the solid phases. Below monolayer coverage, the 3 He monolayer forms a commensurate √ 3 × √ 3 solid structure that provides a unique testing ground for the multi-particle exchange model [17,18] invoked to explain the magnetic properties of bulk 3 He and 3 He films [15,19,20]. For example, coverages less than the ideal commensurate structure lead to a reduced probability of 3-particle exchange compared to the 2-particle exchange and

Prediction for two spatially modulated superfluids: $^4$He on fluorographene and on hexagonal BN

“…Our study is for 4 He, not 3 He, but at first sight those experiments seem in contradiction with our results because it is difficult to understand how 3 He could have a stable √ 3 × √ 3R30 • phase while we find that for the heavier mass 4 He such phase is unstable. A first comment is that the mentioned NMR measurements 25,26 give evidence that at a special coverage the 3 He atoms are in registry with those of hBN but there is no direct evidence that this ordered phase of 3 He corresponds indeed to a coverage of 1/3 of the adsorption sites as in the √ 3 × √ 3R30 • phase. On the basis of adsorption isotherms this NMR special coverage can be reconciled 28 with the √ 3 × √ 3R30 • phase only by advocating the presence of ill controlled strongly binding sites and edge effects.…”

“…We are not aware of experimental study of the phase diagram of 4 He adsorbed on hBN while some studies have been performed for 3He [25][26][27] . From these NMR measurements the authors conclude that at low temperature 3 He forms an ordered state and this is identified as the √ 3 × √ 3R30 • phase.…”

“…On the basis of adsorption isotherms this NMR special coverage can be reconciled 28 with the √ 3 × √ 3R30 • phase only by advocating the presence of ill controlled strongly binding sites and edge effects. Also the presence of a liquid component 26 , in addition to the solid one, over a large range of temperature and coverage is difficult to understand if the √ 3 × √ 3R30 • phase were to lowest energy state of the monolayer as it is in the case of graphite. Our results show that the √ 3 × √ 3R30 • phase of 4 He on a monolayer of hBN is unstable, this does not exclude that a different commensurate ordered phase might be stable at a coverage larger than 1/3.…”

“…Our results show that the √ 3 × √ 3R30 • phase of 4 He on a monolayer of hBN is unstable, this does not exclude that a different commensurate ordered phase might be stable at a coverage larger than 1/3. A second comment is that these measurements [25][26][27] are performed on powder of hBN and it is known 28 that the adsorption properties have a significant dependence on the preparation method of the powder. In any case the single platelets in such powders have thickness of a fraction of micron so they are formed by many layers of the basal plane.…”