39 research outputs found
Low frequency acoustics in solid He at low temperature
The elastic properties of hcp He samples have been investigated using low
frequency (20 Hz to 20 kHz) high sensitivity sound transducers. In agreement
with the findings of other workers, most samples studied grew very
significantly stiffer at low temperature; Poisson's ratio was observed to
increase from 0.28 below 20 mK to at 0.7 K. The span of the
variation of varies from sample to sample according to their thermal and
mechanical history. Crystals carefully grown at the melting curve show a
different behavior, the change in taking place at lower and being
more abrupt
Bottom-loading dilution refrigerator with ultra-high vacuum deposition capability
A Kelvinox 400 dilution refrigerator with the ability to load samples onto
the mixing chamber from the bottom of the cryostat has been combined with an
ultrahigh-vacuum (UHV) deposition chamber equipped with molecular beam sources.
The liquid helium cooled sample transfer mechanism is used in a manner that
allows films to be grown on substrates which are kept at temperatures of order
8K with chamber pressures in the 10^-9 to 10^-10 Torr range. This system
facilitates the growth of quench-condensed ultrathin films which must always be
kept below ~ 12K in a UHV environment during and after growth. Measurements can
be made on the films down to millikelvin temperatures and in magnetic fields up
to 15 T.Comment: 10 pages text, 1figur
Rayleigh-Taylor instability of crystallization waves at the superfluid-solid 4He interface
At the superfluid-solid 4He interface there exist crystallization waves
having much in common with gravitational-capillary waves at the interface
between two normal fluids. The Rayleigh-Taylor instability is an instability of
the interface which can be realized when the lighter fluid is propelling the
heavier one. We investigate here the analogues of the Rayleigh-Taylor
instability for the superfluid-solid 4He interface. In the case of a uniformly
accelerated interface the instability occurs only for a growing solid phase
when the magnitude of the acceleration exceeds some critical value independent
of the surface stiffness. For the Richtmyer-Meshkov limiting case of an
impulsively accelerated interface, the onset of instability does not depend on
the sign of the interface acceleration. In both cases the effect of
crystallization wave damping is to reduce the perturbation growth-rate of the
Taylor unstable interface.Comment: 8 pages, 2 figures, RevTe
Hydrodynamic instability during non-uniform growth of a helium crystal
We analyze an analog of the hydrodynamic Rayleigh-Taylor instability for the
liquid-solid phase interface under non-uniform growth of the solid phase. The
development of the instability starts on conditions of an accelerated interface
growth and if the magnitude of acceleration exceeds some critical value. The
plane and spherical shapes of the interface are considered. The observation of
the instability can be expected for helium crystals in the course of their
abnormal fast growth.Comment: Revtex, 5 pages, 3 figure
On the spectrum of facet crystallization waves at the smooth 4He crystal surface
The wavelike processes of crystallization and melting or crystallization
waves are well known to exist at the 4He crystal surface in the rough state.
Much less is known about crystallization waves for the 4He crystal surface in
the smooth well-faceted state below the roughening transition temperature. To
meet the lack, we analyze here the spectrum of facet crystallization waves and
its dependence upon the wavelength, perturbation amplitude, and the number of
possible facet steps distributed somehow over the wavelength. All the
distinctive features of facet crystallization waves from conventional waves at
the rough surface result from a nonanalytic cusplike behavior in the angle
dependence for the surface tension of smooth crystal facets.Comment: 7 pages, 3 figures, 1 tabl