Vladimir Kovacik

Superfluidity and quantized vortex studies under rotation up to 4 Hz at mK and 1 Hz at sub-mK temperatures

Abstract Two new high-speed rotating cryostats at the ISSP, University of Tokyo, capable of operating at mK temperatures, are currently employed for studies of superfluidity and quantized vortex states in 4 He and 3 He . A stable high-Q torsional oscillator is used for the study of superfluidity in low-density 4 He films adsorbed on 3-D connected porous substrates. At Tc, the critical phenomena of 3-D Bose superfluids is seen. Under rotation a new type of vortex state is observed. High-resolution NMR studies of superfluid 3 He , contained in capillary arrays of two different diameters, are made as a function of rotational speed. In these experiments, single vortex intrusion events as well as texural modification under rotation are clearly resolved.

The ISSP Tokyo rotating cryostat

The rotating dilution refrigerator has been built up and tested to study topology and dynamics of vortices in thin helium films in porous glasses. The torsional oscillator method is considered as the basic technique to be used. We are able to rotate the cryostat with angular velocity Ω up to 6.3 rad/s and resolve changes of about 0.3 mrad/s. The 40mK minimum temperature of the cryostat does not change up to the highest angular velocities.

The Superfluid Transition and Vortex Dynamics in Submonolayer Superfluid 4He Films in Porous Glass Under Rotation

A combination of a rotating dilution refrigerator and high-Q torsional oscillator technique has been used to study dynamics of vortices in thin4He films adsorbed on the porous glass (d=1μm pore size). Under rotation an additional dissipation peak with the amplitude proportional to the angular velocity is seen at the middle of the superfluid transition, on the low temperature side of the stationary peak which is present even at Ω=0. We attribute this peak to the 3D Type vortices created in multiply connected4He film by the rotation. Peak shape of the rotation-induced dissipation could be interpreted as a freezing of the 3D vortices well below Tc

AC flow effect on a vortex state in thin4He superfluid films on porous glass

AC flow response of submonolayer superfluid4He films formed on porous glass with 1 μm pore diameter has been studied over more than three decades of the oscillation amplitude or the AC velocity VAC, using a high Q-value torsional oscillator. As the oscillation amplitude increases a clear crossover has been observed, from linear regime where the height and the shape of the dissipation peak at Tc does not depend on the drive level, to nonlinear regime where energy dissipation peak changes the height in proportion to VAC2 at the characteristic velocity of the order of 0.1cm/sec. At much higher VAC, deviation from VAC2 dependence as well as rapid change of the peak shape have been observed.

Thermal conductivity study of thin He films adsorbed in porous glasses with well controlled pore sizes

Thermal conductivity study of thin He films adsorbed in porous glasses with well controlled pore sizes[2] is reported. The measurements are performed in a cell where a torsional oscillator monitors the superfluid density change, for the same films for comparison. Since the heat flux through the superfluid is proportional to the superfluid velocity, we discuss the possible observation of the intrinsic critical velocity vsl = h/ml, inherent in superfluid He films in such porous systems with unit pore length l, as discussed by Minoguchi and Nagaoka[3], which marks the velocity at which phase slippages start to occur over macroscopic scales.

Torsional Oscillator Experiments with High Stability and Reproducibility

We report experiments of the torsional oscillator to observe the superfluid transition in4He films in porous glass (the pore diameter is 1μm). Stability and reproducibility of the oscillator, which quite often is problematic in previous experiments, is essential for a quantitative analysis of observations in different conditions. It follows that the friction of the superfluid films and the energy dissipation of the solid films are derived from comparisons of measurements for different film thickness.

Mass transport in thin superfluid4He films in porous media

Possiblity to utilize thermal conductivity to study superfluid thin4He films on porous glass has been examined in temperature range T=(0.6–1.1)K. The datailed measurements performed for the film in normal state can be qualitatively described by the ideal gas conductance. In the superfluid state at T≈0.6K a sharp increase of the temperature gradient along the sample at a critical heating power has been observed below Tc. This feature is similar to that at T≈1K, where thermal conductivity technique is supposed to be applicable

Rotationally induced dissipation peak and dynamics of 3D vortex lines

Abstract Vortex dynamics of superfluid 4 He films in porous glass has been investigated with torsional oscillator technique under rotation. In addition to the 2D vortex dissipation peak, a new dissipation peak near the superfluid phase transition is found. Its dependence on the rotation velocity as well as on the temperature is studied. The rotation speed dependence of the Q value at the new peak traces no hysteresis through one rotation sweep cycle. This is argued as a mobility of 3D vortex lines which increases with increasing temperature. The temperature dependence of the mobility could be interpreted as melting of the vortex solid.

Rotation-induced vortices in thin4He film on porous substrate

Torsional oscillator experiments with the submonolayer superfluid4He film on the porous glass have been performed in a rotating cryostat. A new additional dissipation peak was observed under the rotation. This peak can be interpreted as a result of the crossover from 2D to 3D nature of the superfluid state and formation of 3D-type vortices in thin4He film on porous substrate.