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During last two years the INTERFACE group has been concentrated on the studies of 4He crystals. In order to search for the supersolid transition anomaly, accurate measurements of the melting curve of 4He were carried out. In addition, we have also searched for new facets on 4He crystals and measured the step energies of different facets.

MEASEREMENTS ON THE MELTING CURVE OF 4He — NO SIGN OF SUPERSOLID TRANSITION

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

Collaborators: Alexander Parshin (Kapitza Institute for Physical Problems, Moscow, Russia) and Viktor Tsepelin (University of Lancaster, Lancaster, UK)

During first half of 2007 we completed our measurements on the melting curve of 4He which were started in the end of 2005. We have measured the melting pressure of both commercial and ultra pure 4He with the accuracy of about 0.5 bar. We have found that in the temperature range from 10 to 320 mK the melting pressure of 4He follows the T4 law due to phonons (see Fig. 1). Therefore the anomaly which was observed in our earlier measurements below 80 mK (Fig. 2, upper part) turned out to be due to the temperature dependence of the elastic modulus of Be-Cu from which our pressure gauge is made of. In order to get rid of this effect, we had to calibrate the sensitivity of our pressure gauge by measuring the variation of the pressure in liquid 4He at a constant volume. Our final results are presented in the lower part of Fig. 2.

Fig. 1. Melting pressure of ultra pure 4He below T = 0.32 K. The line is not a fit to the data but consists of 1000 raw data points!

Recently, Moses Chan and his team measured the heat capacity of solid 4He. After subtracting off the phonon T 3 contribution they found a broad peak in the heat capacity centered near 75 mK, and attributed it to the supersolid transition. The extra heat capacity under the peak would correspond to a deviation of about 5 bar from the T 4 law in our melting curve measurement of 4He. The discrepancy with our results in Fig. 2 can be due to different sample quality: Our measurements were done for single crystals with good quality, while the samples in the heat capacity measurements were grown by blocked capillary method, known to lead to polycrystalline samples with poor quality.

Annual Report 2007

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