135 research outputs found
Possible role of 3He impurities in solid 4He
We use a quantum lattice gas model to describe essential aspects of the
motion of 4He atoms and of 3He impurities in solid 4He. This study suggests
that 3He impurities bind to defects and promote 4He atoms to interstitial sites
which can turn the bosonic quantum disordered crystal into a metastable
supersolid. It is suggested that defects and interstitial atoms are produced
during the solid 4He nucleation process where the role of 3He impurities (in
addition to the cooling rate) is known to be important even at very small (1
ppm) impurity concentration. It is also proposed that such defects can form a
glass phase during the 4He solid growth by rapid cooling.Comment: 4 two-column Revtex pages, 4 figures. Europhysics Letters (in Press
Absence of Pressure-Driven Supersolid Flow at Low Frequency
An important unresolved question in supersolid research is the degree to
which the non-classical rotational inertia (NCRI) phenomenon observed in the
torsional oscillator experiments of Kim and Chan, is evidence for a
Bose-condensed supersolid state with superfluid-like properties. In an open
annular geometry, Kim and Chan found that a fraction of the solid moment of
inertia is decoupled from the motion of the oscillator; however, when the
annulus is blocked by a partition, the decoupled supersolid fraction is locked
to the oscillator being accelerated by an AC pressure gradient generated by the
moving partition. These observations are in accord with superfluid
hydrodynamics. We apply a low frequency AC pressure gradient in order to search
for a superfluid-like response in a supersolid sample. Our results are
consistent with zero supersolid flow in response to the imposed low frequency
pressure gradient. A statistical analysis of our data sets a bound, at the 68%
confidence level, of 9.6 nm/s for the mass transport velocity
carried by a possible supersolid flow. In terms of a simple model for the
supersolid, an upper bound of 3.3 is set for the supersolid
fraction at 25 mK, at this same confidence level. These findings force the
conclusion that the NCRI observed in the torsional oscillator experiments is
not evidence for a frequency independent superfluid-like state. Supersolid
behavior is a frequency-dependent phenomenon, clearly evident in the frequency
range of the torsional oscillator experiments, but undetectably small at
frequencies approaching zero.Comment: 6 pages, 5 figure
The effect of disorder on the critical temperature of a dilute hard sphere gas
We have performed Path Integral Monte Carlo (PIMC) calculations to determine
the effect of quenched disorder on the superfluid density of a dilute 3D hard
sphere gas. The disorder was introduced by locating set of hard cylinders
randomly inside the simulation cell. Our results indicate that the disorder
leaves the superfluid critical temperature basically unchanged. Comparison to
experiments of helium in Vycor is made.Comment: 4 pages, 4 figure
Heat Capacity of ^3He in Aerogel
The heat capacity of pure ^3He in low density aerogel is measured at 22.5
bar. The superfluid response is simultaneously monitored with a torsional
oscillator. A slightly rounded heat capacity peak, 65 mu K in width, is
observed at the ^3He-aerogel superfluid transition, T_{ca}. Subtracting the
bulk ^3He contribution, the heat capacity shows a Fermi-liquid form above
T_{ca}. The heat capacity attributed to superfluid within the aerogel can be
fit with a rounded BCS form, and accounts for 0.30 of the non-bulk fluid in the
aerogel, indicating a substantial reduction in the superfluid order parameter
consistent with earlier superfluid density measurements.Comment: 4 pages, 5 figure
Vortex Fluid State below an Onset Temperature T_0 of Solid 4He
Detailed studies of AC velocity Vac and temperature dependence of torsional
oscillator responses of solid 4He are reported. A characteristic onset
temperature T0 about 0.5 K is found, below which a significant Vac dependent
change occurs in the energy dissipation for the sample at 32 bar. A Vac
dependence of the non-classical rotational inertia fraction, NCRIF also appears
below about T0. This value of T0 excludes the possible explanation of
supersolid by liquid superfluidity in grain boundaries or other liquid related
origins. The log(Vac) linear dependence was found in NCRIF. Furthermore, this
linear slope changes in proportion to 1/T 2 for 40 < Vac < 400 micro-m/s, then
crosses over to about 1/T for larger Vac. We discuss properties of the vortex
fluid state proposed by Anderson above Tc, below T0.Comment: 4pages,4 figure
Study of Supersolidity and Shear Modulus Anomaly of 4He in a Triple Compound Oscillator
The recently discovered shear modulus anomaly in solid 4He bears a strong
similarity to the phenomenon of supersolidity in solid 4He and can lead to the
period shift and dissipative signals in torsional oscillator experiments that
are nearly identical to the classic NCRI signals observed by Kim and Chan. In
the experiments described here, we attempt to isolate the effects of these two
phenomena on the resonance periods of torsion oscillators. We have constructed
a triple compound oscillator with distinct normal modes. We are able to
demonstrate that, for this oscillator, the period shifts observed below 200 mK
have their primary origin in the temperature dependence of the shear modulus of
the solid 4He sample rather than the formation of a supersolid state
The density dependence of the transition temperature in a homogenous Bose flui
Transition temperature data obtained as a function of particle density in the
He-Vycor system are compared with recent theoretical calculations for 3D
Bose condensed systems. In the low density dilute Bose gas regime we find, in
agreement with theory, a positive shift in the transition temperature of the
form . At higher densities a maximum is
found in the ratio of for a value of the interaction parameter,
na, that is in agreement with path-integral Monte Carlo calculations.Comment: 4 pages, 3 figure
Ideal Bose gas in fractal dimensions and superfluid He in porous media
Physical properties of ideal Bose gas with the fractal dimensionality between
D=2 and D=3 are theoretically investigated. Calculation shows that the
characteristic features of the specific heat and the superfluid density of
ideal Bose gas in fractal dimensions are strikingly similar to those of
superfluid Helium-4 in porous media. This result indicates that the geometrical
factor is dominant over mutual interactions in determining physical properties
of Helium-4 in porous media.Comment: 13 pages, 6 figure
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