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Recent measurements of James Day and John Beamish have revealed that the shear modulus of solid 4He increases up to 10% as the temperature is reduced from 200 mK to 20 mK. The temperature dependence of the shear modulus value closely tracks the temperature dependence of the period in the torsional oscillator experiments of Kim and Chan. Day and Beamish interpret their results by the behavior of dislocations and trace 3He impurities in the solid 4He.

Fig. 2. Measurements on the melting curve of 4He. Upper part: Deviation of the melting pressure from the best T 4 fits measured with 4He of regular purity (80 ppb of 3He) and ultra pure 4He (0.3 ppb of 3He); the solid line is pressure in the liquid 4He at a constant volume. Lower part: Deviation of the difference between the melting pressure in the liquid 4He at a constant volume from the best T 4 fit. Symbols refer to the same crystals as in the upper part. All curves are offset for clarity.

In our melting curve measurements above 320 mK temperature we also detect the roton contribution to the specific heat of superfluid 4He. We extract a value of 6.8 K for the roton energy gap, in agreement with other methods. Because we did not observe any vacancy contribution in the melting pressure of 4He, we can set a lower limit of ~5.5 K for their activation energy. In addition, we have measured the thermal expansion coefficient of superfluid 4He in the range from 10 to 700 mK with a precision of 10-7 K-1. This is two orders of magnitude better resolution than in the previous measurements. Unexpectedly, we found large deviations from the results of Grilly and Mills and Sydoriak.

SEARCH FOR NEW TYPES OF FACETS IN 4He — EVIDENCE FOR [10-12] AND [11-20] FACETS

Harry Alles, Heikki Junes, Matti Manninen, and Igor Todoshchenko

Collaborator: Alexander Parshin

After the melting curve measurements we have searched for new facets in 4He. Only three types of facets have been detected on hcp 4He crystals. The difficulty in observing higher order facets in 4He results from their small step energies. The critical overpressure above which the facets roughen is proportional to the square of the step energy. Therefore the higher order facets can be seen only very close to the equilibrium conditions where the facets are still small. The facet size can be

Annual Report 2007

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