423 research outputs found
Wetting of Superfluid 4He by Liquid 3He
We have investigated optically the spreading of He3 on top of the He4-rich solution in phase-separated helium-mixture films, 20-50 ÎŒm thick. In equilibrium, the He3 layer wets the He4-rich phase completely, but nearly circular or stripelike pools of He3-rich phase are stabilized instead when He4 atoms are condensed to the liquid sample at the rate 10 exp 15-5 Ă 10 exp 15 atoms/cm2s. For the contact angle we obtain about 10 mrad, which suggests a fractional change of the He3 surface tension by a factor of 10 exp â5 from the equilibrium value.Peer reviewe
Observation of a New Surface State on 4He Crystal Interfaces
The equilibrium shape of hcp 4He crystals has been studied at temperatures 0.05â€Tâ€0.7 K by means of a high-precision optical interferometer. We find that the profile of the interfacial boundary, close to an almost horizontal c-facet, has a well-defined slope discontinuity separating two angular regions with different behavior of the surface stiffness αÌ. For surfaces tilted by an angle ÏâČ100 ÎŒrad with respect to the c-facet, we obtain αÌ=Îș/Ï, contrary to the linear angular dependence predicted by current theories. Our results on four large crystals yield Îș=(11±3)Ă10 exp â4T erg/ cm exp 2 K.Peer reviewe
Evidence of 4He Crystallization via Quantum Tunneling at mK Temperatures
We have investigated creation of 4He crystals from the superfluid phase at the temperature range 2 mKâ1.0 K. Statistical nucleation-event distributions in overpressure were found to be broad, asymmetric, and temperature independent below 100 mK. Our statistical analysis agrees with a theoretical model suggesting that solid formation is driven by macroscopical quantum-mechanical fluctuations from a seed preexisting in a cavity on the wall.Peer reviewe
Cooled video camera for optical investigations below 1 mK
An optical imaging system for milliKelvin temperatures has been developed based on a regular B/W surveillance camera (25 frames/s), with its CCD sensor inside the 4âK vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed.Peer reviewe
Observations on Superfluid Meniscus in Rotating 3He-B
The parabolic superfluid meniscus has been seen for states in which only (1) the normal or (2) the superfluid component of 3He-B rotates. A reduced, temperature-dependent meniscus (1) was formed at small speeds ΩâČ0.21 rad /s. A deeper-than-expected meniscus (2) was observed after a rapid halt of the cryostat, when the normal fluid stops during a short relaxation time, followed by slow decay of the superfluid circulation. The depth of the meniscus (2) was explained by a reactive radial force between the rotating superfluid and the stationary normal liquid.Peer reviewe
Facet Growth of 4He Crystals at mK Temperatures
We have investigated growth of c facets in good quality helium crystals with screw dislocation densities 0â20 cm exp â2 along the c axis. Three distinct regimes of growth were observed. One of them can be explained by spiral growth provided that kinetic energy of moving steps and their tendency to localization at large driving forces are taken into account. In the absence of screw dislocations we find burstlike growth unless the speed is less than 0.5 nm/s, in which case anomalous, intrinsic growth of facets is detected.Peer reviewe
Observation of Dispersion in the J=2+ Collective Modes of He-3-B by Nonlinear Acoustic Spectroscopy
Parametric excitation yields new results, which cannot be obtained by conventional linear spectroscopy, on the collective modes in superfluid He-3-B. In our ultrasonic experiments, performed in a chamber with four quartz crystals, we have excited the real squashing (J = 2+) modes by two simultaneous sound waves which are mutually either parallel, orthogonal, or antiparallel. Qualitatively these results are in agreement with the theoretical dispersion relation of the J = 2+ modes, and quantitatively we have been able to extract the collective-mode velocities from the line splittings in zero magnetic field
Non-universal equilibrium crystal shape results from sticky steps
The anisotropic surface free energy, Andreev surface free energy, and
equilibrium crystal shape (ECS) z=z(x,y) are calculated numerically using a
transfer matrix approach with the density matrix renormalization group (DMRG)
method. The adopted surface model is a restricted solid-on-solid (RSOS) model
with "sticky" steps, i.e., steps with a point-contact type attraction between
them (p-RSOS model). By analyzing the results, we obtain a first-order shape
transition on the ECS profile around the (111) facet; and on the curved surface
near the (001) facet edge, we obtain shape exponents having values different
from those of the universal Gruber-Mullins-Pokrovsky-Talapov (GMPT) class. In
order to elucidate the origin of the non-universal shape exponents, we
calculate the slope dependence of the mean step height of "step droplets"
(bound states of steps) using the Monte Carlo method, where p=(dz/dx,
dz/dy)$, and represents the thermal averag |p| dependence of , we
derive a |p|-expanded expression for the non-universal surface free energy
f_{eff}(p), which contains quadratic terms with respect to |p|. The first-order
shape transition and the non-universal shape exponents obtained by the DMRG
calculations are reproduced thermodynamically from the non-universal surface
free energy f_{eff}(p).Comment: 31 pages, 21 figure
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