Igor Todoshchenko

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.

Absence of low temperature anomaly on the melting curve of 4He

The melting pressure and pressure in the liquid at a constant density of ultrapure 4He (0.3 ppb of 3He impurities) have been measured with an accuracy of about 0.5 μbar in the temperature range from 10 to 320 mK. The measurements show that the anomaly on the melting curve below 80 mK, which was recently observed [I. A. Todoshchenko et al., Phys. Rev. Lett. 97, 165302 (2006)], is entirely due to an anomaly in the elastic modulus of Be-Cu from which our pressure gauge is made. Thus, the melting pressure of 4He follows the T4 law due to phonons in the whole temperature range from 10 to 320 mK without any attribute of a supersolid transition.

Spectroscopic Studies of the Triplet-state 4He 2 * Molecules in Superfluid Helium Up to the Solidification Pressure

We report detailed investigations of the absorption spectra of the triplet excimer helium molecules in superfluid helium in a wide pressure range. The width of the (0–0) a → c absorption band shows peculiar dependence on helium pressure with maximum near 24 bar. Studies of dynamic properties of molecules in the controlled geometry allow for the first time to perform independent measurements of their mutual decay coefficient, light absorption cross section and probability of molecules formation in the ions recombination process.

Quartz tuning fork as a probe of surface oscillations

Quartz tuning forks are high-quality mechanical oscillators widely used in low temperature physics as viscometers, thermometers, and pressure sensors. We demonstrate that a fork placed in liquid helium near the surface of solid helium is very sensitive to the oscillations of the solid-liquid interface. We developed a double-resonance read-out technique, which allowed us to detect oscillations of the surface with an accuracy of 1 A in 10 s. Using this technique, we have investigated crystallization waves in 4He down to 10 mK. In contrast to previous studies of crystallization waves, our measurement scheme has very low dissipation, on the order of 20 pW, which allows us to carry out experiments even at sub-mK temperatures. We propose to use this scheme in the search for crystallization waves in 3He, which exist only at temperatures well below 0.5 mK. The suggested technique can also be used for accurate displacement detection in a large variety of systems.

Experimental setup for the observation of crystallization waves in 3He

At low temperatures helium crystals can grow and melt so fast that a melting-freezing wave may propagate along the liquid-solid interface. In 4He these crystallization waves have been observed at temperatures below 0.5K. The required temperature for the observation of the crystallization waves in 3He is predicted to be about 0.2 mK. In order to reach such low temperature at the melting pressure of 3He, special care has to be taken in the design of the experimental cell. Here we present the draft of the experimental cell which is designed for observation of the crystallization waves in 3He.

Faceting on 3He crystals

It has been predicted by Landau that, ideally at low temperatures, crystals should show many different types of facets, i.e., flat smooth faces on their surface, but this so-called “devils staircase” phenomenon has been difficult to observe experimentally. In this paper we describe our recent experiments, in which altogether 11 different types of facets have been identified on growing 3He crystals at the temperature of 0.55 mK by using a unique low-temperature Fabry–Pérot interferometer. Previously only 3 types of facets had been seen in this system. We have also measured the growth velocities of different facets, and our interpretation of the obtained results yields the conclusion that 3He has much stronger coupling of the liquid–solid interface to the crystal lattice than has been expected. After an introduction we present a short theoretical background about the equilibrium crystal shape and the roughening transitions, which is followed by the description of our experimental results and discussion.

Optical spectra of the triplet molecules4He 2 * in superfluid helium in a magnetic field

We report study of absorption spectra fora→b anda→c transitions of the triplet excimer molecules4He2* in superfluid helium in the temperature range 1.4–2.1 K and in magnetic fields of 0–5.5 T. The lifetime of the excimers was determined by the rate of their mutual recombination and was 2 order of magnitude longer than in previous measurements. It was found that even at the maximum lifetime of 50 ms the molecules were not completely thermalized with respect to the rotational and vibrational degrees of freedom. The vibrational relaxation time was determined for the first time. It was found that the rotational relaxation is significantly faster than the vibrational relaxation. No expected suppression of the excimer recombination due to the molecules polarization in the magnetic field was found.

Crystallization Waves (In Memory of Alexander Parshin)

Melting-freezing wave is a peculiar phenomenon taking place on a surface of quantum crystal. This wave can propagate due to the extremely fast growth dynamics of the surface and due to the non-dissipative mass transport provided by the superfluid surrounding a crystal. This short paper reviews the studies of crystallization waves starting from their theoretical prediction and the first experimental observation by Alexander Parshin and his colleagues.

On the growth dynamics of 4He crystals near the first roughening transition

The first roughening transition of a surface of hcp 4He crystals was carefully studied in Paris in 1980s. By investigating the growth dynamics of the (0001) facet, the free energy of an elementary step was measured in the close vicinity of the transition and a good agreement was found with the theory of critical fluctuations developed by Nozieres and Gallet. We believe, however, that the interpretation of the growth data near the roughening transition made by the Paris group is not self‐consistent. We argue that with the step energies they obtained, assuming that the growth is due to the process of 2D‐nucleation of terraces, another growth mechanism provided by screw dislocations should be much more effective.