27 research outputs found
The Anomalous Behavior of Solid He in Porous Vycor Glass
The low temperature properties of solid He contained in porous Vycor
glass have been investigated utilizing a two-mode compound torsional
oscillator. At low temperatures, we find period shift signals for the solid
similar to those reported by Kim and Chan \cite{ref1}, which were taken at the
time as evidence for a supersolid helium phase. The supersolid is expected to
have properties analogous to those of a conventional superfluid, where the
superfluid behavior is independent of frequency and the ratio of the superfluid
signals observed at two different mode periods will depend only on the ratio of
the sensitivities of the mode periods to mass-loading. In the case of helium
studies in Vycor, one can compare the period shift signals seen for a
conventional superfluid film with signals obtained for a supersolid within the
same Vycor sample. We find, contrary to our own expectations, that the signals
observed for the solid display a marked period dependence not seen in the case
of the superfluid film. This surprising result suggests that the low
temperature response of solid He in a Vycor is more complex than previously
assumed and cannot be thought of as a simple superfluid.Comment: 4 pages, 5 figure
Probing the Upper Limit of Nonclassical Rotational Inertia
We study the effect of confinement on solid 4-He's nonclassical rotational
inertia (NCRI) in a torsional oscillator by constraining it to narrow annular
cells of various widths. The NCRI exhibits a broad maximum value of 20% for
annuli of approximately 100 micrometer width. Samples constrained to porous
media or to larger geometries both have smaller NCRI, mostly below about 1%. In
addition, we extend Kim and Chan's blocked annulus experiment to solid samples
with large supersolid fractions. Blocking the annulus suppresses the
nonclassical decoupling from 17.1% below the limit of our detection of 0.8%.
This result demonstrates the nonlocal nature of the supersolid phenomena. At 20
mK, NCRI depends on velocity history showing a closed hysteresis loop in
different thin annular cells.Comment: 5 pages, 4 figure
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
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