Andrey Penzev

Ac vortex-dependent torsional oscillation response and onset temperature T0 in solid 4He.

Detailed studies of the AC velocity Vac and T dependence of torsional oscillator responses of solid He are reported. A characteristic onset temperature T0 ∼ 0.5 K is found, below which a significant Vac dependent change occurs in the energy dissipation for the sample at 32 bar. A Vac dependence of the ”non-classical rotational inertia” fraction(NCRIF) also appears below ∼ T0. This value of T0 excludes the possible explanation of supersolid by liquid superfluidity in grain boundaries or other liquid related origins. The log(Vac) linear dependence was found in NCRIF. Furthermore, this linear slope changes in proportion to 1/T 2 for 40 < Vac < 400 μm/s, then crosses over to ∼ 1/T for larger Vac. We discuss properties of the vortex fluid proposed by Anderson above Tc and below T0.

Threshold Effect During Dissolution of 3He Inclusions in Solid 4He

A pressure jump has been found at the onset of the dissolution of bcc inclusions in separated solid3He - 4He mixture if the crystal is overheated above a certain critical value. This effect can be explained in the framework of a multistage dissolution process model.

Supersolid 4He under DC rotation: Energy dissipation for slow and fast temperature sweeps

In this report, we describe torsional oscillator (TO) measurements under DC rotation of a 49 bar 4He-sample, at Vac = 0.2 mm/s. We investigated the behavior of the energy dissipation (~ ΔQ-1) as well as the supersolid fraction for different rotation velocities (0 < Ω < 1.256 rad/s) and Vac = 0.06, 0.2 mm/s in the temperature range of from 47 mK to 150 mK, using fast (~3h) and slow (~ 55h) temperature sweeps. Variation of Ω did not influence the supersolid fraction. The performed analyzes have shown a linear increase of the ΔQ-1 vs. Ω for high Vac = 0.2 mm/s (for fast and slow temperature sweeps). In the case of Vac = 0.06 mm/s, no change of the energy dissipation was observed up to Ω = 0.628 rad/s for the low temperature limit. These results suggest a possible explanation for the energy dissipation in supersolid 4He and its proportionality to the number of low dimensional vortices influenced by torsional oscillations. The 3D-vortex lines created by DC rotation can dissipate their energy by interaction with low dimensional vortices. Their dynamics under rotation are characterized by long relaxation times.

Dynamics of Quantized Vortices in a Torsional Oscillator under Rotation: Proposed Experiments in Supersolid 4He

Recently there have been reports of superfluidity in solid 4He and possibly in other solids. One of the common features of these systems is their small superfluid density. This would imply a rather long three dimensional (3D) coherence length. Sub‐monolayer superfluid 4He films condensed on 3D connected surfaces show 3D superfluid transitions, in some cases with a rather small 3D superfluid density and a long 3D coherence length, which implies a large 3D vortex core size. We have previously detected 3D vortex lines in a 3D superfluid made of a sub‐monolayer 4He film condensed on the pore surface of large pore diameter porous glass, by measuring the energy dissipation in a torsional oscillator under rotation. We propose application of this method to the new supersolids.

The influence of small impurities of 4He on the melting curve of 3He

The melting curve of 3He containing a small 4He impurity is measured in the temperature range 20–600 mK. It is found that the coordinates of the minimum of the melting curve are shifted, that hysteresis appears, and that the slope of the melting curve at low temperatures is changed. The data obtained agree with a calculation that takes into account the change in the coordinates of the minimum due to the entropy of mixing of the 3He–4He solution. The results of the experiment are used to estimate the error arising when the melting curve of 3He is used to determine the temperature.

Phase Separation Line for Solid Mixtures of 3He in 4He

The excess pressure due to the phase separation of solid mixtures of 3He in 4He held at a constant volume was measured and used for constructing the phase separation diagram of this system. We obtained high-quality homogeneous samples of the solid mixtures after several cycles of cooling down and heating up the two-phase crystal. This gave reliable and reproducible experimental data without hysteresis efects. We compared the phase diagram line obtained with various theoretical approaches, which describe the phase separation of the helium isotope mixtures. The regular solution model can not describe the experimental data well and neither can the asymmetrical Mullins model. Good agreement is observed only with the theory of Edwards and Balibar which takes into account the difference between the crystal symmetry (hcp and bcc) of the coexisting phases.

Melting and Crystallization of 3He Inclusions in Two-phase Solid 3He-4He Mixtures

The peculiar features of the phase diagram for the 3He-4He system make it possible to melt the 3He inclusions formed during phase separation of the mixture by further cooling and to crystallize them in subsequent heating. The kinetics of these processes is studied on a sample with a molar volume of 20.54 cm3/mole (P=31.7 bar) using pressure measurements. The time dependence of the crystal pressure P(t) is measured on cooling at a rate of ∼10 mK/h followed by heating. The dependence P(t) has two distinct rises in pressure, the first rise being associated with the phase separation of the mixture and the second one with the melting of the 3He inclusions formed. It is shown that the melting of the 3He inclusions is almost complete after the fast cooling and the observed pressure jump is in good agreement with the corresponding change in the molar volume. The repeated crystallization of the inclusions is found to give rise to a large pressure gradient near the boundary of the inclusions, suppressing quantum diffusion considerably. This may result in an incomplete crystallization of the inclusions. The experimentally observed difference between the initial and final pressure in the sample corresponds to the fact that approximately 20% of the 3He remains in the liquid state.

Kinetics of Liquid 3He Droplets in Solid 4He Matrix

Precise measurements of pressure in the crystal at constant volume were used to obtain the data on growth and dissolution kinetics of liquid 3He droplets formed as a result of isotopic phase separation of solid 3He-4He Mixtures. We studied several crystals with an initial 3He concentration of 2.05% in the pressure range of 26–27 bar. It is shown that the growth of the liquid droplets during the stepwise cooling of the two-phase crystal is correctly described by the superposition of two exponential processes: diffusion decomposition with a small time constant and strain relaxation with a big time constant. The strain layer near the droplet boundaries is due to a great difference in molar volume between the droplets and the matrix, and leads to a plastic deformation of the matrix and to a non-equilibrium 3He concentration in the matrix. Under such conditions quantum diffusion is significantly suppressed and 3He atom transport occurs only as the strain is relaxed.

Phase Separation Kinetics of Solid 3He-4He Mixtures

The kinetics of isotopic phase separation in solid mixture of3He in4He with the initial concentration 2.05 % at various molar volumes has been investigated by precise pressure measurements. It has been shown that during both stepped and fast cooldown into the metastable region the equilibrium of coexisting phases is described by the exponential law with a characteristic time constant τ, The value of τ is found to decrease as the molar volume increases and the temperature lowers. It confirms that the growth of the3He-rich phase is connected with nonthermally activated (quantum) diffusion in the gas of delocalized3He quasiparticles. The obtained experimental results can be described only qualitatively by current kinetic theory of binary quantum solid mixtures. The conditions permitting the realization of the isotopic phase separation during the time observed in the experiment are analyzed. The effective quantum diffusion coefficient providing required3He atoms transport is about an order of magnitude higher than the corresponding value measured in NMR experiments. These conditions are probably fulfilled at the big concentration gradient which takes place at isotopic phase separation. The corresponding kinetic theory should be developed.