Joshua T. West

Nonclassical rotational inertia in helium crystals.

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.

Effect of 3He impurities on the nonclassical response to oscillation of solid 4He.

We have investigated the influence of impurities on the possible supersolid transition in 4He by systematically enriching isotopically pure samples with 3He. The addition of 3He broadens the onset of nonclassical rotational inertia and shifts it to higher temperature, suggesting that the phenomenon is correlated with the condensation of 3He atoms onto the dislocation network in solid 4He.

Torsional oscillator and synchrotron X-Ray experiments on solid 4He in aerogel.

X-ray diffraction experiments show that solid 4He grown in aerogel is highly polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only approximately 3 x 10(-4), and the onset temperature is approximately 100 mK.

4He crystal quality and rotational response in a transparent torsional oscillator

We have studied natural purity

Supersolid Behavior in Confined Geometry

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Role of shear modulus and statistics in the supersolidity of helium

He single crystals and polycrystals between 10 and 600 mK using a torsional oscillator with a 2 cm

Probing phase coherence in solid helium using torsional oscillators of different path lengths

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Effect of

rigid cell made of sapphire with a smooth geometry. As the temperature was lowered, we observed sample dependent but reproducible resonant frequency shifts that could be attributed to a supersolid fraction of order 0.1%. However, these shifts were observed with single crystals, not with polycrystals. Our results indicate that, in our case, the rotational anomaly of solid helium is more likely due to a change in stiffness than to supersolidity. This interpretation would presumably require gliding of dislocations in more directions than previously thought.

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We have carried out torsional oscillator and heat capacity measurements on solid 4He samples grown within a geometry which restricts the helium to thin (150 microm) cylindrical disks. In contrast with previously reported values from Rittner and Reppy of 20% nonclassical rotational inertia for similar confining dimensions, 0.9% nonclassical rotational inertia (consistent with that found in bulk samples and samples embedded in porous media) was observed in our torsional oscillator cell. In this confined geometry, the heat capacity peak is consistent with that found in bulk solid samples of high crystalline quality.