843 research outputs found
Steps and facets at the surface of soft crystals
We consider the shape of crystals which are soft in the sense that their
elastic modulus is small compared to their surface tension , more
precisely where is the lattice spacing. We show that
their surface steps penetrate inside the crystal as edge dislocations. As a
consequence, these steps are broad with a small energy which we calculate. We
also calculate the elastic interaction between steps a distance apart,
which is a repulsion. We finally calculate the roughening temperatures
of successive facets in order to compare with the remarkable shapes of
lyotropic crystals recently observed by P. Pieranski et al. Good agreement is
found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.
Low frequency elastic measurements on solid He in Vycor using a torsional oscillator
Torsional oscillator experiments involving solid He confined in the
nanoscale pores of Vycor glass showed anomalous frequency changes at
temperatures below 200 mK. These were initially attributed to decoupling of
some of the helium's mass from the oscillator, the expected signature of a
supersolid. However, these and similar anomalous effects seen with bulk
He now appear to be artifacts arising from large shear modulus changes
when mobile dislocations are pinned by He impurities. We have used a
torsional oscillator (TO) technique to directly measure the shear modulus of
the solid He/Vycor system at a frequency (1.2 kHz) comparable to that
used in previous TO experiments. The shear modulus increases gradually as the
TO is cooled from 1 K to 20 mK. We attribute the gradual modulus change to the
freezing out of thermally activated relaxation processes in the solid helium.
The absence of rapid changes below 200 mK is expected since mobile dislocations
could not exist in pores as small as those of Vycor. Our results support the
interpretation of a recent torsional oscillator experiment that showed no
anomaly when elastic effects in bulk helium were eliminated by ensuring that
there were no gaps around the Vycor sample.Comment: Accepted by Journal of Low Temperature Physic
Non-linear effects and shock formation in the focusing of a spherical acoustic wave : Numerical simulations and experiments in liquid helium
The focusing of acoustic waves is used to study nucleation phenomena in
liquids. At large amplitude, non-linear effects are important so that the
magnitude of pressure or density oscillations is difficult to predict. We
present a calculation of these oscillations in a spherical geometry.
We show that the main source of non-linearities is the shape of the equation
of state of the liquid, enhanced by the spherical geometry. We also show that
the formation of shocks cannot be ignored beyond a certain oscillation
amplitude. The shock length is estimated by an analytic calculation based on
the characteristics method. In our numerical simulations, we have treated the
shocks with a WENO scheme. We obtain a very good agreement with experimental
measurements which were recently performed in liquid helium. The comparison
between numerical and experimental results allows in particular to calibrate
the vibration of the ceramics used to produce the wave, as a function of the
applied voltage.Comment: 20 pages, 26 figures. Submitted to The European Physical Journal
Dislocation networks in helium-4 crystals
The mechanical behavior of crystals is dominated by dislocation networks,
their structure and their interactions with impurities or thermal phonons.
However, in classical crystals, networks are usually random with impurities
often forming non-equilibrium clusters when their motion freezes at low
temperature. Helium provides unique advantages for the study of dislocations:
crystals are free of all but isotopic impurities, the concentration of these
can be reduced to the ppb level, and the impurities are mobile at all
temperatures and therefore remain in equilibrium with the dislocations. We have
achieved a comprehensive study of the mechanical response of 4He crystals to a
driving strain as a function of temperature, frequency and strain amplitude.
The quality of our fits to the complete set of data strongly supports our
assumption of string-like vibrating dislocations. It leads to a precise
determination of the distribution of dislocation network lengths and to
detailed information about the interaction between dislocations and both
thermal phonons and 3He impurities. The width of the dissipation peak
associated with impurity binding is larger than predicted by a simple Debye
model, and much of this broadening is due to the distribution of network
lengths.Comment: accepted by Phys. Rev.
Thermally assisted quantum cavitation in solutions of 3He in 4He
We have investigated the quantum-to-thermal crossover temperature T* for
cavitation in liquid helium mixtures up to 0.05 3He concentrations. With
respect to the pure 4He case, T* is sizeably reduced, to a value below 50 mK
for 3He concentrations above 0.02. As in pure 4He, the homogeneous cavitation
pressure is systematically found close to the spinodal pressure.Comment: Typeset using Revtex, 9 pages and 4 figure
Unraveling the Landau's consistence criterion and the meaning of interpenetration in the "Two-Fluid" Model
In this letter we show that it is possible to unravel both the physical
origin of the Landau's consistence criterion and the specific and subtle
meaning of interpenetration of the "two fluids" if one takes into account that
in the hydrodynamic regime one needs a coarse-graining in time to bring the
system into local equilibrium. That is, the fuzziness in time is relevant for
the phenomenological Landau's consistency criterion and the meaning of
interpenetration. Note also that we are not questioning the validity of the
"Two-Fluid" Model.Comment: 8 pages, affiliation added, typos corrected, final version published
in Eur. Phys. J.
Ground-state properties and superfluidity of two- and quasi two-dimensional solid 4He
In a recent study we have reported a new type of trial wave function
symmetric under the exchange of particles and which is able to describe a
supersolid phase. In this work, we use the diffusion Monte Carlo method and
this model wave function to study the properties of solid 4He in two- and quasi
two-dimensional geometries. In the purely two-dimensional case, we obtain
results for the total ground-state energy and freezing and melting densities
which are in good agreement with previous exact Monte Carlo calculations
performed with a slightly different interatomic potential model. We calculate
the value of the zero-temperature superfluid fraction \rho_{s} / \rho of 2D
solid 4He and find that it is negligible in all the considered cases, similarly
to what is obtained in the perfect (free of defects) three-dimensional crystal
using the same computational approach. Interestingly, by allowing the atoms to
move locally in the perpendicular direction to the plane where they are
confined to zero-point oscillations (quasi two-dimensional crystal) we observe
the emergence of a finite superfluid density that coexists with the periodicity
of the system.Comment: 16 pages, 8 figure
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