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Register a new userSuperfluid and supersolid phases of 4He on the second layer of graphite
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We revisited the phase diagram of the second layer of 4He on top of graphite using quantum Monte Carlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a 7/12 registered phase. The 4/7 commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.
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Flexural mode vibrations of miniature piezoelectric tuning forks (TF) are known to be highly sensitive to superfluid excitations and quantum turbulence in $mathrm{^3He}$ and $mathrm{^4He}$ quantum fluids, as well as to the elastic properties of solid $mathrm{^4He}$, complementing studies by large scale torsional resonators. Here we explore the sensitivity of a TF, capable of simultaneously operating in both the flexural and torsional modes, to excitations in the normal and superfluid $mathrm{^4He}$. The torsional mode is predominantly sensitive to shear forces at the sensor - fluid interface and much less sensitive to changes in the density of the surrounding fluid when compared to the flexural mode. Although we did not reach the critical velocity for quantum turbulence onset in the torsional mode, due to its order of magnitude higher frequency and increased acoustic damping, the torsional mode was directly sensitive to fluid excitations, linked to quantum turbulence created by the flexural mode. The combination of two dissimilar modes in a single TF sensor can provide a means to study the details of elementary excitations in quantum liquids, and at interfaces between solids and quantum fluid.
We model the superfluid flow of liquid helium over the rough surface of a wire (used to experimentally generate turbulence) profiled by atomic force microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that the sharpest features in the surface induce vortex nucleation both intrinsically (due to the raised local fluid velocity) and extrinsically (providing pinning sites to vortex lines aligned with the flow). Vortex interactions and reconnections contribute to form a dense turbulent layer of vortices with a non-classical average velocity profile which continually sheds small vortex rings into the bulk. We characterise this layer for various imposed flows. As boundary layers conventionally arise from viscous forces, this result opens up new insight into the nature of superflows.
Surface waves on both superfluid 3He and 4He were examined with the premise, that these inviscid media would represent ideal realizations for this fluid dynamics problem. The work on 3He is one of the first of its kind, but on 4He it was possible to produce much more complete set of data for meaningful comparison with theoretical models. Most measurements were performed at the zero temperature limit, meaning T < 100 mK for 4He and T ~ 100 {mu}K for 3He. Dozens of surface wave resonances, including up to 11 overtones, were observed and monitored as the liquid depth in the cell was varied. Despite of the wealth of data, perfect agreement with the constructed theoretical models could not be achieved.
This is a Reply to Nemirovskii Comment [Phys. Rev. B 94, 146501 (2016)] on the Khomenko et al, [Phys.Rev. B v.91, 180504(2016)], in which a new form of the production term in Vinens equation for the evolution of the vortex-line density $cal L$ in the thermal counterflow of superfluid $^4$He in a channel was suggested. To further substantiate the suggested form which was questioned in the Comment, we present a physical explanation for the improvement of the closure suggested in Khomenko et al [Phys.Rev. B v. 91, 180504(2016)] in comparison to the form proposed by Vinen. We also discuss the closure for the flux term, which agrees well with the numerical results without any fitting parameters.
We study the elasticity of perfect 4He at zero-temperature using the diffusion Monte Carlo method and a realistic semi-empirical pairwise potential to describe the He-He interactions. Specifically, we calculate the value of the elastic constants of hcp helium C_{ij} as a function of pressure up to 110 bar. It is found that the pressure dependence of all five non-zero C_{ij} is linear and we provide accurate parametrization of each of them. Our elastic constants results are compared to previous variational calculations and low-temperature measurements and in general notably good agreement is found among them. Furthermore, we report T = 0 results for the Gruneisen parameters, sound velocities and Debye temperature of hcp 4He. This work represents the first of a series of computational studies aimed at thoroughly characterizing the response of solid helium to external stress-strain.
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