Alexander Parshin

Melting curve of 4He: No sign of a supersolid transition down to 10 mK

We have measured the melting curve of 4He in the temperature range from 10 to 400 mK with the accuracy of about 0.5 micro bar. Crystals of different quality show the expected T4 dependence in the range from 80 to 400 mK without any sign of the supersolid transition, and the coefficient is in excellent agreement with available data on the sound velocity in liquid 4He and on the Debye temperature of solid 4He. Below 80 mK, we have observed a small deviation from T4 dependence, which, however, cannot be attributed to the supersolid transition, because instead of decrease the entropy of the solid rather remains constant, about 2.5 x 10(-6) R.

Morphology and growth kinetics of 3He crystals below 1 mK

The shapes as well as the growth and melting properties of bcc-3He single crystals have been investigated with a low temperature Fabry-Pérot interferometer. Eleven types of facets were clearly identified during slow crystal growth at the temperature of 0.55 mK, where the solid is in the antiferromagnetically ordered u2d2 phase. The growth rates of the individual facets have been measured and the results indicate significant growth anisotropy. The observed linear dependence of the growth velocity on the driving force shows that facets grow due to the presence of screw dislocations, while the step velocity is limited by the spin wave velocity due to the strong interaction of the moving step with magnons in the solid. The measured growth rates of the facets and the assumed growth mechanism gave us the unique opportunity to obtain the step free energies for ten different types of facets observed during a single growth sequence. The dependence of the free energy of the step on the step height is compared with predictions of the weak- and strong-coupling models. Our results suggest that 3He crystals have rather strong coupling of the liquid/solid interface to the crystal lattice and that the step-step interactions are of elastic origin.

Interferometric studies of interfaces at milliKelvin temperatures

A brief review of our recent work on interfaces in quantum systems at millikelvin temperatures is presented. In this paper, we concentrate on high-resolution interferometry on superfluid/solid interface in4He. Our results show a novel surface transition at small inclination angles off the c-axis, slow facet growth which cannot be assigned to the regular screw-dislocation-mediated mechanism, and fast spiral growth moderated by step localization.

Faceting and growth kinetics of 3He crystals

Abstract We review and discuss our recent results on faceting and growth kinetics of 3 He crystals imaged with a Fabry–Perot multiple-beam interferometer at our lowest temperature of 0.55 mK . More than ten different types of facets were identified on the growing crystals and the growth rates of the individual facets were measured. Some further possible experiments are also suggested.

Faceting of 3He crystals

Abstract We report on our optical observations on BCC 3 He crystals in the temperature range of 8– 55 mK . With our interferometric technique we were able to identify at least three different types of facets, (1 1 0), (1 0 0) , and (2 1 1) , at temperatures up to 55 mK . Previously, only the (1 1 0) facet was observed at such high temperatures.

SEARCH FOR THE STABILIZING EFFECT OF MAGNETIC FIELD ON A GAS OF HE*2 EXCITATIONS IN LIQUID HELIUM PHASES

The absorption spectra of triplet He2*metastable molecules in normal liquid3He under various pressures have been observed for the first time. Molecules are generated as a result of recombination of positive and negative ions extracted from discharge regions near sharp tips operated in field-emission/field-ionization modes. We have shown experimentally that this method allows one to create measurable concentrations of triplets without essential overheating of normal liquid in the cell.The decay of the molecules after the current through the cell is turned off have been studied in various magnetic fields up to 5.5 T. Experimental values of the mutual recombination coefficient agree with the theory of diffusion-limited recombination of excimers and show no effect of the magnetic field. The magnetic relaxation time is found to exceed 200 msec under all conditions studied.

Spiral growth of c-facets in4He crystals at mK-temperatures

We have made detailed investigations on spiral growth of c-facets in helium crystals with dislocation densities ∼10 cm−2. Our results at high temperaturesT=100–200 mK agree quite well with the classical parabolic power law for spiral growth, whereas a linear dependence on the driving pressure is observed at 2–20 mK. This is a consequence of the inertia of elementary steps in the situation when damping due to phonons is small. At high velocities, all our data display a tendency to saturation which is interpreted as an indication of step localization at large driving forces.

Observation of a new surface state on4He crystal interfaces

AbstractThe equilibrium shape ofhcp4He crystals has been studied at temperatures 0.05≤T≤0.7 K by means of a high precision optical interferometer. We find that the profile of the interfacial boundary, close to an almost horizontal c-facet, has a well-defined slope discontinuity separating two angular regions with different behavior of the surface stiffness

Observation of higher order facets on 3He crystals.

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