Search results for "Helium-4"
showing 10 items of 44 documents
The growth of atomically rough 4He crystals
1986
We have studied the growth of atomically rough bcc and hcp4He crystals from the superfluid phase for temperaturesT>0.9 K. The growth coefficient displays a temperature dependence which can be represented bym 4 K∝ $$e^{\Delta E/k_B T} $$ . The parameter ΔE is found to be in close agreement with the energy gap of rotons, suggesting that these thermal excitations dominate the growth kinetics. Besides, the absolute value of the growth coefficient depends on crystal orientation, with an anisotropy for the hcp phase of about a factor of 2.5 between the $$\left\{ {10\bar 10} \right\}$$ and {0001} planes.
Nonlocal effects in superfluid turbulence: Application to the low-density- to high-density-state transition and to vortex decay
2005
We discuss a phenomenological equation for the evolution of vortex tangle in counterflow superfluid turbulence, which takes into account the influence of the nonlocal effects, introducing into the original equation of Vinen two simple corrective terms dependent on a nonvanishing ratio between the average separation between vortex lines and the diameter of the channel. The equation allows one to describe, in relatively good agreement with experimental results, the two turbulent regimes present in counterflow superfluid turbulence and the transition between them. The decay rate of the vortex line density L, when the heat flux is suddenly turned off, is also investigated; due to the simplicity…
Refrigeration of an Array of Cylindrical Nanosystems by Flowing Superfluid Helium
2016
We consider the refrigeration of an array of heat-dissipating cylindrical nanosystems as a simplified model of computer refrigeration. We explore the use of He II as cooling fluid, taking into account forced convection and heat conduction. The main conceptual and practical difficulties arise in the calculation of the effective thermal conductivity. Since He II does not follow Fourier’s law, the effective geometry-dependent conductivity must be extracted from a more general equation for heat transfer. Furthermore, we impose the restrictions that the maximum temperature along the array should be less than (Formula presented.) transition temperature and that quantum turbulence is avoided, in o…
Contribution of the normal component to the thermal resistance of turbulent liquid helium
2015
Previous results for the velocity profile of the normal component of helium II in counterflow are used to evaluate the viscous contribution to the effective thermal resistance. It turns out that such a contribution becomes considerably higher than the usual Landau estimate, because in the presence of vortices, the velocity profile is appreciably different from the Poiseuille parabolic profile. Thus, a marked increase in the contribution of the normal component to the thermal resistance with respect to the viscous Landau estimate does not necessarily imply that the normal component is turbulent. Furthermore, we examine the influence of a possible slip flow along the walls when the radius of …
Sub-MeV dark matter and the Goldstone modes of superfluid helium
2019
We show how the relativistic effective field theory for the superfluid phase of helium-4 can replace the standard methods used to compute the production rates of low momentum excitations due to the interaction with an external probe. This is done by studying the scattering problem of a light dark matter particle in the superfluid, and comparing to some existing results. We show that the rate of emission of two phonons, the Goldstone modes of the effective theory, gets strongly suppressed for sub-MeV dark matter particles due to a fine cancellation between two different tree-level diagrams in the limit of small exchanged momenta. This phenomenon is found to be a consequence of the particular…
Solvation of triplet Rydberg states of molecular hydrogen in superfluid helium
2004
We report ab initio interaction potentials, transition dipole moments, and radiative lifetimes for the four lowest triplet states of ${\mathrm{H}}_{2}:$ $b$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ $c$ ${}^{3}{\ensuremath{\Pi}}_{u},$ $a$ ${}^{3}{\ensuremath{\Sigma}}_{g}^{+},$ and $e$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ and their response to the perturbation due to approaching ground state He atom. Hybrid density functional\char21{}quantum Monte Carlo calculations employing the ab initio interaction potentials are then used for calculating the liquid structure around the molecular excimers in bulk superfluid ${}^{4}\mathrm{He}.$ Calculations demonstrate a wide variety of possible solvation …
Coupling of heat flux and vortex polarization in superfluid helium
2020
We consider a macroscopic description of the mutual influence between heat flux and vortex polarization in superfluid helium, in which the vortices produce a lateral deviation of the heat flux, and the heat flux produces a lateral drift of vortices. This coupling is a consequence of a microscopic Magnus force and mutual friction force between the vortices and the flow of excitations carrying the heat. We keep track of these effects with simplified macroscopic equations, and we apply them to second sound propagation between rotating concentric cylinders and to spatial distribution of polarization across a rectangular channel with vortices polarized orthogonally to the channel in the presence…
Determination of the helium-4 mass in a Penning trap
2001
We have measured the cyclotron frequencies of He+, H+ 2 and D+ 2 ions in a room temperature Penning trap. The resonances were detected destructively by a time-of-flight technique. The statistical uncertainty of the resonance frequencies was generally below 1 ppb. A detailed account of measures to minimize systematic frequency shift is presented. Using the accepted values for the proton and deuteron mass we obtain a value for the 4He mass: M(4 He) = 4.0026032489(22) (0.5 ppb). It is in agreement with the accepted value.
LiquidHe4andHe3at negative pressure
1992
The critical pressures {ital P}{sub {ital c}} at which liquid {sup 4}He and {sup 3}He each become macroscopically unstable are determined from two kinds of microscopic calculations that reproduce the equation of state. The behavior of the sound velocity {ital c} around this critical pressure is analyzed, and the critical exponent in {ital c}{proportional to}({ital P}{minus}{ital P}{sub {ital c}}){sup {nu}} is found to be {nu}=1/4. A comparison with empirical analysis is also done.
Effective thermal conductivity of superuid helium: Laminar, turbulent and ballistic regimes
2016
Abstract In this paper we extend previous results on the effective thermal conductivity of liquid helium II in cylindrical channels to rectangular channels with high aspect ratio. The aim is to compare the results in the laminar regime, the turbulent regime and the ballistic regime, all of them obtained within a single mesoscopic formalism of heat transport, with heat flux as an independent variable.