37 research outputs found
Critical dislocation speed in helium-4 crystals
Our experiments show that in He crystals, the binding of He
impurities to dislocations does not necessarily imply their pinning. Indeed, in
these crystals, there are two different regimes of the motion of dislocations
when impurities bind to them. At lowdriving strain and frequency
, where the dislocation speed is less than a critical value (45
m/s), dislocations and impurities apparently move together. Impurities
really pin the dislocations only at higher values of . The critical
speed separating the two regimes is two orders of magnitude smaller than the
average speed of free He impurities in the bulk crystal lattice.We obtained
this result by studying the dissipation of dislocation motion as a function of
the frequency and amplitude of a driving strain applied to a crystal at low
temperature. Our results solve an apparent contradiction between some
experiments, which showed a frequency-dependent transition temperature from a
soft to a stiff state, and other experiments or models where this temperature
was assumed to be independent of frequency. The impurity pinning mechanism for
dislocations appears to be more complicated than previously assumed
Layering transitions in superfluid helium adsorbed on a carbon nanotube mechanical resonator
Helium is recognized as a model system for the study of phase transitions. Of
particular interest is the superfluid phase in two dimensions. We report
measurements on superfluid helium films adsorbed on the surface of a suspended
carbon nanotube. We measure the mechanical vibrations of the nanotube to probe
the adsorbed helium film. We demonstrate the formation of helium layers up to
five atoms thickness. Upon increasing the vapour pressure, we observe
layer-by-layer growth with discontinuities in both the number of adsorbed atoms
and the speed of sound in the adsorbed film. These hitherto unobserved
discontinuities point to a series of first-order layering transitions. Our
results show that helium multilayers adsorbed on a nanotube are of
unprecedented quality compared to previous works. They pave the way to new
studies of quantized superfluid vortex dynamics on cylindrical surfaces, of the
Berezinskii-Kosterlitz-Thouless phase transition in this new geometry, perhaps
also to supersolidity in crystalline single layers as predicted in quantum
Monte Carlo calculations.Comment: 10 pages, 3 figure
Anomaly in the stability limit of liquid helium 3
We propose that the liquid-gas spinodal line of helium 3 reaches a minimum at
0.4 K. This feature is supported by our cavitation measurements. We also show
that it is consistent with extrapolations of sound velocity measurements.
Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar
behavior for the spinodal of water and related it to a change in sign of the
expansion coefficient alpha, i. e. a line of density maxima. Helium 3 exhibits
such a line at positive pressure. We consider its extrapolation to negative
pressure. Our discussion raises fundamental questions about the sign of alpha
in a Fermi liquid along its spinodal.Comment: 5 pages, 3 figure