Role of Substrate Symmetry in Nucleating Solid Helium

Abstract

The continuous nucleation of bcc solid 3 He from the liquid phase due to the presence of a MgO substrate is detected. For superfluid 4 He there is no evidence of continuous nucleation of either the hep or the bcc solid from the substrate. The experiment contrasts the influence of the square lattice of MgO with the triangular lattice of a basal-plane graphite substrate. The phenomenon of pore condensation for solid 3 He is discussed.PACS numbers: 67.80. Cx, 68.45.Da In this paper we present new results which compare the onset of solidification of 3 He and 4 He on a magnesium oxide substrate, which has cubic symmetry, with that already reported 1 * 2 for solidification on Grafoil, which has a hexagonal symmetry. The Grafoil experiments had shown that hep 4 He undergoes continuous solid nucleation whereas, surprisingly, bcc 3 He did not nucleate similarly. Since the van der Waals forces between the substrate and either isotope are identical, and this force is considered 1 to be responsible for the superpressurization of the first few helium layers on the substrate, we were led to suggest that the substrate symmetry (hexagonal graphite encouraging the hep 4 He) is the determining factor in the onset of solidification. To test this conclusion we repeated the experiments with a MgO substrate 3 * 4 which has a cubic symmetry. For this substrate we observed uniform nucjeation of the bcc 3 He whereas 4 He does not show any signs of surface solidification in either hep or bcc regions.We should emphasize that the experiment on Grafoil alone did not prove the relevance of substrate symmetry. During the condensation of the first layer of He on Grafoil, it is known from adsorption studies 5 that initially an ordered state is reached in which the helium atoms are registered with the graphite lattice. Subsequently, as the coverage is increased, a close-packed triangular two-dimensional lattice is formed, with a spacing incommensurate with the graphite lattice. It can then be argued that this layer, which is formed by either isotope, is the natural base for the continuing growth of hep 4 He, whereas it would not encourage the further growth of a bcc solid. The present experiments disprove the universality of this model, and demonstrate the importance of a matching substrate symmetry. Further experimental evidence from direct observation of the growth of aligned 4 He crystals on the Grafoil substrate will also be presented to support the conclusion.The experimental evidence for the onset of solidification is deduced from the measurement of equilibrium points along isopycnals 6 in the vicinity of the bulk melting curve. There appears a substantial difference between the isopycnal and the isochore for the bulk liquid when surface solidification occurs, in the presence of a substrate of sufficiently large surface area. The present experiments used a 2.9-g MgO sample prepared from compressed smoke at the University of Washington. 3 Its properties have been measured by Dash etal. 4 who showed that it produces krypton step like adsor...

Absence of continuous growth for bcc and hcp solid 3He on Grafoil

“…Thus, it seems to be clear that, even under an external pressure from the liquid phase of 3He, there is no continuous formation of solid layers of 3He on Grafoil. These results contradict the conclusions of Landau and Eckstein [4,8] that continuous nucleation takes place when the symmetry of the monolayer matches the symmetry of the adjacent bulk solid lattice. Following this argument, the missing symmetry between the triangular monolayer [13,14] and the bcc 3He solid can prevent nucleation as observed in our experiment.…”

“…nor at the phase boundary between the bcc and the hcp solid. If the deviations of isopycnals from the bulk ones (Ap) were of the same order of magnitude as, for example, in the system 4He-Grafoil or 3He -MgO (Ap = 0.12 bar) [4], they would have been easily detected in the present experiment with an accuracy of bp of 0.005 bar. We conclude, consequently, that there is no evidence for continuous growth of a uniform surface solid near a Grafoil substrate in the hcp region also.…”

“…Thus, following the classification of adsorption of Dash [5] and Peierls [6], these two systems have been found to be of Class I (uniform deposits at all thicknesses). On the other hand, thermodynamic [4,7] and neutron scattering investigations [7] of 4He on the cubic structure MgO and thermodynamic measurements of 3 He on Grafoil [8] in the temperature range from 30 mK to 320 mK show no evidence for continuous nucleation of a surface solid, classifying these systems as Class II (bulk nucleation after a thin uniform deposit).…”

“…Recently, the nucleation of surface solid from the liquid phase of 3He and 4He on a hexagonal substrate (Grafoil) and a cubic substrate (MgO) has been studied. Continuous nucleation was established for the system 4He-Grafoil from measuring isopycnals [1,2], taking neutron diffraction patterns [1] and by attempts to over-pressurize liquid 4He near the phase transition curve [3], and for 3He on MgO by recording isopycnals [4]. Thus, following the classification of adsorption of Dash [5] and Peierls [6], these two systems have been found to be of Class I (uniform deposits at all thicknesses).…”

Liquid and solid 4He near a graphite wall

“…It is a quantum adsorbed system, the density of which varies from that of a compressed solid at the wall to that of a liquid a few atomic distances away. For example, the structure of the solid layer depends on the crystalline symmetry of the underlying substrate (1). In the case of 3~e , the magnetic properties of the surface layer can be strongly modified by the substrate (2, 3).…”