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We have confirmed the existence, as first reported by Kim and Chan, of a supersolid state in solid 4 He 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 4 He, but can be eliminated through an annealing of the solid helium sample. This result has important implications for our understanding of the supersolid state. He confined within the porous media of porous gold and Vycor glass. KC followed this remarkable discovery with the observation of a supersolid or non-classical rotational inertia (NCRI) signal in bulk solid helium [2]. An important goal, which we have achieved in our experiments, has been to provide an independent confirmation of the KC observations for bulk 4 He. We have also investigated the influence of annealing and sample preparation on the existence of the supersolid state. We find that the supersolid state is not a universal property of solid 4 He, but that it is possible through crystal annealing near the melting curve to create samples that show only classical rotational inertia (CRI).In these measurements we have employed the torsional oscillator technique developed at Cornell over the past several decades [4]. Oscillators with two different sample geometries were used in the experiments reported in this letter. The first had a cylindrical sample geometry with an internal volume of 2.7 cm 3 and operated at a frequency of 253 Hz, while the second had a cubic geometry with a volume of 1.4 cm 3 and operated at 185 Hz. The helium used in the experiments was commercial well-grade helium similar to that used by KC in their bulk solid helium work [2]. This helium has a stated 3 He impurity level of 0.2 -0.3 ppm.Our earliest runs were made with the cylindrical cell. Data obtained with this cell at a pressure of 27 bar showed an onset of the supersolid signal at a temperature near 0.2 K, with a maximum signal amplitude at 30 mK of 0.6 % of the total period shift seen upon forming the solid in the cell. The magnitude of this signal is in good agreement with the observations of KC made under similar conditions and provides a confirmation of their results. In an attempt to improve the size of our NCRI signal we increased the sample pressure toward 55 bar. The cell failed before reaching this pressure.A diagram of our second cell is shown in Fig. 1. The design is similar to that of our first cell with several significant modifications. First, we have converted the The torsion cell's motion is excited and detected capacitively. The AC voltage on the detection electrodes serves as reference signal for a lock-in amplifier to keep the oscillation in resonance. At 4 K, the mechanical quality factor is 9 x 10 5 , and the resonance frequency is 185 Hz.interior of the cell to a nearly cubic geometry by epoxying a Tellurium-Copper insert into the cylindrical volume of the Beryllium-Copper oscillator. The insert was machined to provide a cubic ...

The influence of quenched disorder on the critical behavior of superfluid He is studied by confinement of the helium to the pores of three diflerent porous media: Vycor, xerogel, and aerogel glasses. In each case a well defined power-law behavior for the superfluid density is observed as the transition temperature is approached. In Vycor the superfluid critical exponent is bulklike, whereas strikingly diff'erent critical exponents are seen in the other two media.

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. Following the discovery by Kim and Chan (KC) [1, 2] of the supersolid or nonclassical rotational inertia (NCRI) state of bulk solid 4 He, several independent groups using the same torsional oscillator technique have confirmed the KC supersolid results. These include the Japanese groups of Shirahama et al. [3], working at Keio University, and Kubota et al. [4] at the ISSP, as well as our group [5] at Cornell University. In these early experiments, the solid samples were formed by the blocked capillary technique. In this method the fill line to the cell is first allowed to freeze ensuring that solidification in the cell occurs under a condition of constant average density. This technique is known to produce relatively disordered polycrystalline samples. The signals observed in the early experiments were small representing, at most, a few percent of the total solid helium mass. The Cornell experiments [5] also demonstrated that the supersolid signal could be substantially reduced through annealing of the sample. In some cases the annealing process appeared to eliminate the supersolid signal; i.e., it reduced the supersolid fraction below the 0.05% level of experimental detection. This signal reduction upon annealing strongly suggested that sample disorder plays an important role in supersolid phenomena. This inference is supported by more recent work by Chan's group [6] where high quality crystals were grown under conditions of constant pressure. The supersolid signals observed for these constant pressure samples were somewhat smaller in magnitude than those obtained for more disordered samples created in the same cell by the blocked capillary method.Recently, there has been a growing consensus in the theoretical community [7,8,9,10] that an ideal hcp helium crystal will not exhibit the supersolid phenomenon, but rather, some form of disorder such as vacancies, interstitials, superfluid grain boundaries [11,12], or perhaps a glassy or superglass phase [13,14] is required for the existence of the supersolid state. A summary of the current theoretical literature is given in a recent review [15]. FIG. 1: Torsional oscillator:The motion of the torsion bob is excited and detected electrostatically. A Straty...

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The superfluid properties of 4He absorbed in porous media are discussed. Emphasis is given to the nature of the superjluid transition for helium contained in a variety of different porous structures. Recent heat capacity and superfluid measurements for helium in Vycor glass and other porous media are presented in some detail. The onset of superfluidity as a function of adsorbed helium coverage at zero temperature, or "boson localization," is touched on briefly. The problem of dissipation of superflow in porous media, especially in the vicinity of the superfluid transition, is discussed.

Superfluid 4 He films adsorbed on porous Vycor glass have been studied in the low-density limit where the superfluid interparticle spacing is more than an order of magnitude greater than the atomic hard-core diameter. For the lowest transition temperatures observed, the influence of critical fluctuations near T c is much reduced and a range of crossover to behavior more characteristic of the dilute Bose gas is observed.PACS numbers: 67.40.-w, 67.70.+n Stimulated by recent theoretical and experimental advances in the study of low-density, spin-polarized hydrogen, 1 there has been renewed interest in the phenomenon of Bose condensation and the possibility of observing superfluidity in a dilute weakly interacting Bose gas. In this Letter we report new results from a different approach to the experimental realization of a dilute Bose gas, one that utilizes a low-density 4 He system.Our approach has been to make systematic observations of the temperature dependence of the superfluid mass of low-density 4 He films adsorbed on porous Vycor glass. Earlier work on the 4 He-Vycor system 2 " 4 has demonstrated superfluidity for a range of transition temperatures varying over nearly two orders of magnitude. For the lowest transition temperatures previously observed, 3 the effective superfluid coverage was only a few percent of a monolayer, and the average spacing between mobile superfluid particles was considerably greater than the atomic scattering length. We wished to test the proposition that, if the superfluid coverage were sufficiently reduced, superfluid behavior characteristic of the dilute Bose gas might be observed. The data that we report in this Letter give, for the first time, a definite indication that a crossover to a regime of dilute Bose gas behavior has been seen.The dense three-dimensional (3D) superfluids, bulk 4 He, liquid 3 He-4 He mixtures, and higher coverage superfluid films adsorbed on Vycor, all show superfluid behavior that is strongly influenced by order-parameter fluctuations. The temperature variation for the superfluid mass obeys the well known "two-thirds" power law in the neighborhood of the transition temperature. In contrast, fluctuations are usually assumed to be unimportant in the case of the dilute, weakly interacting Bose gas. For the dilute gas, the superfluid mass is expected to follow a temperature dependence close to that of the condensate density in the ideal Bose gas and thus to vary linearly with T c -T as the transition temperature, T c , is approached.In the present experiment we have determined the temperature dependence of the superfluid mass for an extended range of 4 He coverages adsorbed on Vycor glass. As the density of the superfluid was systematically reduced, we have looked for indications of a crossover from the "two-thirds" power-law behavior seen for the dense superfluids to the more linear temperature variation of the superfluid mass expected for the dilute, weakly interacting Bose gas.The experimental technique employed in the present work is similar to that re...

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