Viktor Tsepelin

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.

Vibrating Wire Measurements in Superfluid 3He at the Melting Curve Down to 0.53 mK

Measurements of the vibrating wire spectrum have been carried out in superfluid 3He along the melting curve down to 0.53mK. We have observed that at temperatures below 0.3 Tc the width of the mechanical resonance of the wire decreases exponentially with 1/T, indicating the ballistic regime of collisions with quasiparticles. The value of the superfluid energy gap was found to be (1.99±0.05)Tc, in good agreement with the values obtained from heat capacity measurements. The vibrating wire was thereby calibrated for further experiments at temperatures below 0.5mK, where the sensitivity of the melting curve thermometry becomes rather poor.

Direct Observation of (110), (100) and (211) Facets on 3He Crystals

We present the results of our recent observations on 3He crystals grown from the superfluid phase at 0.55 mK. The crystal images were obtained with a low-temperature multiple-beam interferometer. The angles between the crystal facets were measured by employing a phase-shift technique and true 3D shapes of the crystals were reconstructed on the basis of the obtained information. Three different types of facets (110), (100) and (211) were clearly visible in these experiments.

Breaking the superfluid speed limit in a fermionic condensate

An experiment reports the unexpected behaviour of an object in uniform motion in superfluid helium-3 above the Landau critical velocity — the limit above which it can generate excitations at no energy cost.

Visualizing Pure Quantum Turbulence in Superfluid 3He: Andreev Reflection and its Spectral Properties.

Superfluid 3He-B in the zero-temperature limit offers a unique means of studying quantum turbulence by the Andreev reflection of quasiparticle excitations by the vortex flow fields. We validate the experimental visualization of turbulence in 3He-B by showing the relation between the vortex-line density and the Andreev reflectance of the vortex tangle in the first simulations of the Andreev reflectance by a realistic 3D vortex tangle, and comparing the results with the first experimental measurements able to probe quantum turbulence on length scales smaller than the intervortex separation.

The Annihilation of Two Phase Interfaces in Superfluid 3He: Simulated Brane Annihilation in the Laboratory

The structure of the order parameter in superfluid 3 He mimics closely the analogous structure of the “metric” of the Universe. Since the superfluid may exist in several phases we can, using a magnetic field gradient, set up coherent phase boundaries in the superfluid which are the closest laboratory analogues we have to cosmological branes. We find that if we annihilate two such branes (an AB and a BA interface) then we find that the annihilation leaves relic defects in the superfluid texture. These results can help to “validate” those braneworld scenarios where brane annihilation is thought similarly to lead to the formation of topological defects such as cosmic strings.

Observations on Faceting of 3He Crystals at T=0.55 mK

We report on our recent observations on growing 3He crystals in which altogether eleven different types of facets were found. The crystals were imaged with a novel low-temperature Fabry-Pérot interferometer. Intensity based analysis methods combined with a phase shift technique were used to identify the observed facets.

Optical observation of 3He crystals at mK-temperatures

We report on our new optical setup which employs multiple-beam interferometry for measurements on He-3 crystals. Our interferometer enables us to observe the global shape of the crystal as well as the fine details of the solid/liquid interface on a scale of less than 5 mu m. Phase measurement allows accurate studies of growth and melting kinetics. We present the first images of He-3 crystals obtained with this setup.

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.