19 research outputs found
Observation of non-classical rotational inertia in bulk solid 4He
In recent torsional oscillator experiments by Kim and Chan (KC), a decrease
of rotational inertia has been observed in solid 4He in porous materials and in
a bulk annular channel. This observation strongly suggests the existence of
"non-classical rotational inertia" (NCRI), i.e. superflow, in solid 4He. In
order to study such a possible "supersolid" phase, we perform torsional
oscillator experiments for cylindrical solid 4He samples. We have observed
decreases of rotational inertia below 200 mK for two solid samples (pressures P
= 4.1 and 3.0 MPa). The observed NCRI fraction at 70 mK is 0.14 %, which is
about 1/3 of the fraction observed in the annulus by KC. Our observation is the
first experimental confirmation of the possible supersolid finding by KC.Comment: 6 pages, 3 firures, submitted to J. Low Temp. Phys. (Proceedings of
QFS2006
Zero-point vacancies in quantum solids
A Jastrow wave function (JWF) and a shadow wave function (SWF) describe a
quantum solid with Bose--Einstein condensate; i.e. a supersolid. It is known
that both JWF and SWF describe a quantum solid with also a finite equilibrium
concentration of vacancies x_v. We outline a route for estimating x_v by
exploiting the existing formal equivalence between the absolute square of the
ground state wave function and the Boltzmann weight of a classical solid. We
compute x_v for the quantum solids described by JWF and SWF employing very
accurate numerical techniques. For JWF we find a very small value for the zero
point vacancy concentration, x_v=(1.4\pm0.1) x 10^-6. For SWF, which presently
gives the best variational description of solid 4He, we find the significantly
larger value x_v=(1.4\pm0.1) x 10^-3 at a density close to melting. We also
study two and three vacancies. We find that there is a strong short range
attraction but the vacancies do not form a bound state.Comment: 19 pages, submitted to J. Low Temp. Phy
Search for supersolidity in 4He in low-frequency sound experiments
We present results of the search for supersolid 4He using low-frequency,
low-level mechanical excitation of a solid sample grown and cooled at fixed
volume. We have observed low frequency non-linear resonances that constitute
anomalous features. These features, which appear below about 0.8 K, are absent
in 3He. The frequency, the amplitude at which the nonlinearity sets in, and the
upper temperature limit of existence of these resonances depend markedly on the
sample history.Comment: Submitted to the Quantum Fluids and Solids Conf. Aug. 2006 Kyot
The glassy response of solid He-4 to torsional oscillations
We calculated the glassy response of solid He-4 to torsional oscillations
assuming a phenomenological glass model. Making only a few assumptions about
the distribution of glassy relaxation times in a small subsystem of otherwise
rigid solid He-4, we can account for the magnitude of the observed period shift
and concomitant dissipation peak in several torsion oscillator experiments. The
implications of the glass model for solid He-4 are threefold: (1) The dynamics
of solid He-4 is governed by glassy relaxation processes. (2) The distribution
of relaxation times varies significantly between different torsion oscillator
experiments. (3) The mechanical response of a torsion oscillator does not
require a supersolid component to account for the observed anomaly at low
temperatures, though we cannot rule out its existence.Comment: 9 pages, 4 figures, presented at QFS200
BCC vs. HCP - The Effect of Crystal Symmetry on the High Temperature Mobility of Solid He
We report results of torsional oscillator (TO) experiments on solid He at
temperatures above 1K. We have previously found that single crystals, once
disordered, show some mobility (decoupled mass) even at these rather high
temperatures. The decoupled mass fraction with single crystals is typically 20-
30%. In the present work we performed similar measurements on polycrystalline
solid samples. The decoupled mass with polycrystals is much smaller, 1%,
similar to what is observed by other groups. In particular, we compared the
properties of samples grown with the TO's rotation axis at different
orientations with respect to gravity. We found that the decoupled mass fraction
of bcc samples is independent of the angle between the rotation axis and
gravity. In contrast, hcp samples showed a significant difference in the
fraction of decoupled mass as the angle between the rotation axis and gravity
was varied between zero and 85 degrees. Dislocation dynamics in the solid
offers one possible explanation of this anisotropy.Comment: 10 pages, 5 figures, to appear in Journal of Low Temperature Physics
- special issue on Supersolidit
A glassy contribution to the heat capacity of hcp He solids
We model the low-temperature specific heat of solid He in the hexagonal
closed packed structure by invoking two-level tunneling states in addition to
the usual phonon contribution of a Debye crystal for temperatures far below the
Debye temperature, . By introducing a cutoff energy in the
two-level tunneling density of states, we can describe the excess specific heat
observed in solid hcp He, as well as the low-temperature linear term in the
specific heat. Agreement is found with recent measurements of the temperature
behavior of both specific heat and pressure. These results suggest the presence
of a very small fraction, at the parts-per-million (ppm) level, of two-level
tunneling systems in solid He, irrespective of the existence of
supersolidity.Comment: 11 pages, 4 figure
Tunnelling defect nanoclusters in hcp 4He crystals: alternative to supersolidity
A simple model based on the concept of resonant tunnelling clusters of
lattice defects is used to explain the low temperature anomalies of hcp 4He
crystals (mass decoupling from a torsional oscillator, shear modulus anomaly,
dissipation peaks, heat capacity peak). Mass decoupling is a result of an
internal Josephson effect: mass supercurrent inside phase coherent tunnelling
clusters. Quantitative results are in reasonable agreement with experiments.Comment: 13 pages, 5 figure
Two-body correlations and the superfluid fraction for nonuniform systems
We extend the one-body phase function upper bound on the superfluid fraction
in a periodic solid (a spatially ordered supersolid) to include two-body phase
correlations. The one-body current density is no longer proportional to the
gradient of the one-body phase times the one-body density, but rather it
depends also on two-body correlation functions. The equations that
simultaneously determine the one-body and two-body phase functions require a
knowledge of one-, two-, and three-body correlation functions. The approach can
also be extended to disordered solids. Fluids, with two-body densities and
two-body phase functions that are translationally invariant, cannot take
advantage of this additional degree of freedom to lower their energy.Comment: 13 page
On The Mobile Behavior of Solid He at High Temperatures
We report studies of solid helium contained inside a torsional oscillator, at
temperatures between 1.07K and 1.87K. We grew single crystals inside the
oscillator using commercially pure He and He-He mixtures containing
100 ppm He. Crystals were grown at constant temperature and pressure on the
melting curve. At the end of the growth, the crystals were disordered,
following which they partially decoupled from the oscillator. The fraction of
the decoupled He mass was temperature and velocity dependent. Around 1K, the
decoupled mass fraction for crystals grown from the mixture reached a limiting
value of around 35%. In the case of crystals grown using commercially pure
He at temperatures below 1.3K, this fraction was much smaller. This
difference could possibly be associated with the roughening transition at the
solid-liquid interface.Comment: 15 pages, 6 figure
Defects and glassy dynamics in solid He-4: Perspectives and current status
We review the anomalous behavior of solid He-4 at low temperatures with
particular attention to the role of structural defects present in solid. The
discussion centers around the possible role of two level systems and structural
glassy components for inducing the observed anomalies. We propose that the
origin of glassy behavior is due to the dynamics of defects like dislocations
formed in He-4. Within the developed framework of glassy components in a solid,
we give a summary of the results and predictions for the effects that cover the
mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of
the glassy response of solid He-4. Our proposed glass model for solid He-4 has
several implications: (1) The anomalous properties of He-4 can be accounted for
by allowing defects to freeze out at lowest temperatures. The dynamics of solid
He-4 is governed by glasslike (glassy) relaxation processes and the
distribution of relaxation times varies significantly between different
torsional oscillator, shear modulus, and dielectric function experiments. (2)
Any defect freeze-out will be accompanied by thermodynamic signatures
consistent with entropy contributions from defects. It follows that such
entropy contribution is much smaller than the required superfluid fraction, yet
it is sufficient to account for excess entropy at lowest temperatures. (3) We
predict a Cole-Cole type relation between the real and imaginary part of the
response functions for rotational and planar shear that is occurring due to the
dynamics of defects. Similar results apply for other response functions. (4)
Using the framework of glassy dynamics, we predict low-frequency yet to be
measured electro-elastic features in defect rich He-4 crystals. These
predictions allow one to directly test the ideas and very presence of glassy
contributions in He-4.Comment: 33 pages, 13 figure