arXiv subjects

John D. Reppy

Publications and source records attributed to John D. Reppy.

12 recordsLinked to original sources

arXiv2015

Multiple Mode Torsional Oscillator Studies and Evidence for Supersolidity in Bulk 4He

The discovery by Kim and Chan (KC) of an anomalous decrease in the period of torsional oscillators (TO) containing samples of solid $^4$He at temperatures below 0.2 K was initially interpreted as a superfluid-like decoupling of a fraction of the solid moment of inertia from the TO. These experiments appeared to confirm the thirty-year-old theoretical prediction by Chester, Andreev, and Leggett of the existence of a low-temperature Bose-condensed supersolid phase in solid $^4$He. The initial results of KC lead to a flurry of experimental and theoretical activity and their results for bulk $^4$He samples were soon confirmed in a number of laboratories. The early excitement in this field, however, has been tempered by the realization that an anomaly in the shear modulus of solid helium $^4$He may explain most if not all of the period shifts observed in the early TO experiments. In principle, it is possible to distinguish experimentally between shear modulus induced period shifts and period shifts resulting from other physical mechanisms through the use of multiple frequency torsional oscillators. In this paper we shall discuss our recent results on bulk solid $^4$He with double and triple mode TOs, using both open cylindrical cells and annular cells. In these experiments, we have observed a small frequency independent contribution to the period shift signal such as might be expected in the presence of a supersolid phase. Since the interplay between the elastic properties of the solid and the mechanics of TOs can be subtle and depends on the specific design of the TO, we shall discuss in detail the mechanics of the TOs employed in our measurements applying both analytic and finite element methods (FEM) for the analysis.

cond-mat.other↗

arXiv2014

Evidence for Supersolidity in Bulk Solid $^4$He

We report low temperature measurements of bulk solid $^4$He in a two-frequency compound torsional oscillator with both annular and open cylinder sample geometries. The oscillators were designed to suppress period shifts arising from all known elastic effects of solid $^4$He. At temperatures below 0.25 K, period shift signals similar to those reported by Kim and Chan [Science {\bf 305}, 1941 (2004)] were observed, albeit two orders smaller in magnitude. A sizable fraction of the observed signals are frequency-independent and consistent with the mass-decoupling expected for supersolid $^4$He. This result is in stark contrast with recent works on Vycor-solid-$^4$He system and suggests that a small supersolid fraction on the order of $1 \times 10^{-4}$ may indeed exist in bulk solid $^4$He.

cond-mat.other↗

arXiv2013

In Pursuit of the Elusive Supersolid

The excitement following the initial report of supersolid behavior for $^4$He embedded in porous Vycor glass has been tempered by the realization that many of the early supersolid observations were contaminated by effects arising from an anomaly in the elastic properties of solid $^4$He. In an attempt to separate dynamic elastic effects from a true supersolid signal, we employed a torsional oscillator with two eigen frequencies to study the $^4$He-Vycor system. We found that frequency dependent elastic signals can entirely account for the observed period shift signals. Although, we conclude that supersolid does not exist for the $^4$He-Vycor case, the question of its presence in bulk samples remains open. In our current experiments we apply the two-frequency test to bulk samples of solid $^4$He. Again we find a frequency dependent contribution arising from elastic effects, however, in some cases we also find a small frequency independent contribution, which may indicate the existence of a remnant supersolid phase. Given the history of this subject such results must be treated with caution.

cond-mat.other↗

arXiv2012

The Anomalous Behavior of Solid $^{4}$He in Porous Vycor Glass

The low temperature properties of solid $^4$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 $^4$He in a Vycor is more complex than previously assumed and cannot be thought of as a simple superfluid.

cond-mat.other↗

arXiv2011

Interpreting Torsional Oscillator Measurements: Effect of Shear Modulus and Supersolidity

The torsional oscillator is the chief instrument for investigating supersolidity in solid 4He. These oscillators can be sensitive to the elastic properties of the solid helium, which show anomalies over the same range of temperature in which the supersolid phenomenon appears. In this report we present a detailed study of the influence of the elastic properties of the solid on the periods of torsional oscillators for the various designs that have been commonly employed in supersolid measurements. We show how to design an oscillator which measures supersolidity, and how to design one which predominantly measures elasticity. We describe the use of multiple frequency TOs for the separation of the elastic and supersolid phenomena.

cond-mat.other↗

arXiv2011

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.

cond-mat.other↗

arXiv2010

A non-superfluid origin of the non-classical rotational inertia in a bulk sample of solid 4He

The torsional oscillator experiments described here examine the effect of disorder on the non-classical moment of inertia (NCRI) of a solid 4He sample. The NCRI increases with increasing disorder, but the period changes responsible for this increase occur primarily at higher temperatures, while at the lowest temperature the oscillator period remains relatively unaffected. This result is contrary to expectations based on a supersolid scenario and points to a non-superfluid origin for the NCRI.

cond-mat.other↗

arXiv2009

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$\times 10^{-4}$ 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$\times 10^{-6}$ 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.

cond-mat.other↗

arXiv2008

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.

cond-mat.other↗

arXiv2007

Observation of Classical Rotational Inertia and Nonclassical Supersolid Signals in Solid 4He below 250 mK

We have confirmed the existence, as first reported by Kim and Chan, of a supersolid state in solid 4He at temperatures below 250 mK. We have employed a torsional oscillator cell with a square cross section to insure a locking of the solid to the oscillating cell. We find that NCRI signal is not a universal property of solid 4He, but can be eliminated through an annealing of the solid helium sample. This result has important implications for the nature of the supersolid state.

cond-mat.other↗

arXiv2007

Annealing in solid $^4$He

We have used a torsional oscillator with square cross section and a resonance frequency of 185 Hz to confirm the nonclassical rotational inertia (NCRI) discovered by Kim and Chan\cite{1,2}. We have also found a strong correlation between the NCRI signal and a high dissipation Q$^{-1}$ of 4 $\times$ 10$^{-6}$ of the oscillation above the transition temperature. Here, we present preliminary results of the annealing process in $^{4}$He at a pressure of 26 bar. When holding the temperature constant above 1 K we have observed a immediate rise in the period and a slow decay of the dissipation. The equilibrium value of Q$^{-1}$ decreases with increasing temperature.

cond-mat.other↗

arXiv2007

Disorder and the Supersolid State of Solid $^4$He

We report torsional oscillator supersolid studies of highly disordered samples of solid $^4$He. In an attempt to approach the amorphous or glassy state of the solid, we prepare our samples by rapid freezing from the normal phase of liquid $^4$He. Less than two minutes is required for the entire process of freezing and the subsequent cooling of the sample to below 1 K. The supersolid signals observed for such samples are remarkably large, exceeding 20 % of the entire solid helium moment of inertia. These results, taken with the finding that the magnitude of the small supersolid signals observed in our earlier experiments can be reduced to an unobservable level by annealing, strongly suggest that the supersolid state exists for the disordered or glassy state of helium and is absent in high quality crystals of solid $^4$He.

cond-mat.other↗