Alexei Babkin

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.

New Paramagnetic Susceptibility Thermometers for Fundamental Physics Measurements

New paramagnetic susceptibility thermometers have been developed for use in fundamental physics missions in earth orbit. These devices use a SQUID magnetometer to measure the variation in the dc magnetization of a thermometric element that consists of a dilute concentration of manganese in a palladium matrix. Near 2.2 K these new PdMn thermometers have demonstrated a temperature resolution of better than 100 pK/√Hz and a time constant of 50 ms when operated with a 50 K/W thermal resistance to the liquid helium sample. These thermometers have been observed to be remarkably stable, with a drift of less than 10 fK/s. The observed power spectral density of the noise from these thermometers is consistent with separate measurements of the device’s time constant and thermal standoff from the bath. Recently these PdMn materials have been made into thin films and microstructures for use in future studies of quantum liquids, and for possible use in a new class of bolometers and radiometers. These thermometers have been integrated into an experimental cell and thermal isolation network that are adequate to keep stray heats stable to within a few picowatts, with no systematic temperature errors greater than 60 pK, over the course of a planned fundamental physics experiment on Earth orbit.

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.

Decoherence Under a Heat Flux Near the Superfluid Transition in 4He

Measurements of heat transport at the transition from perfect thermal superconductivity to nonlinear heat diffusion in pure 4He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the fast extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Roughening transitions on the helium crystal-superfluid interface

Abstract We have studied the equilibrium shapes of large 4He crystals at two different roughening transitions (TR1 = 1.2 K, TR2 = 0.9 K) by a simple optical technique. The method used provides the temperature and angular dependences of the surface stiffness. The measured surface stiffness appears to be constant in the close vicinities of the roughening transitions (|T−TR| ∼ 0.001−0.05TR, ϕ ∼ 0.005−0.1), in strong disagreement with two current theories, viz., the phenomenological “mean field theory” and the lattice model calculations.

New propagating mode near the superfluid transition in 4He

Abstract We have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q when the helium column is heated from above. Counter-intuitively this new mode, which resembles second sound, propagates on the normal fluid side of the transition, but it exists only when the column of helium is heated from above. Such a new mode had been predicted to exist on the self-organized heat transport state for Q less than about 100 nW / cm 2 . We confirm that this mode exists in this regime, however we also observe that it propagates even when the helium is held away from the self-organized heat transport state.

Self-Organized Heat Transport Near the Superfluid Transition in 4He

Heat transport through a column of superfluid 4He has been observed experimentally to self-organize, resulting in a thermal gradient that exactly matches the gradient in the superfluid transition temperature across the column, leaving the entire sample at a constant distance from the superfluid critical point1. We describe a new experiment that is designed to accomplish three objectives: 1) search for the upper critical heat flux above which self-organized heat transport can no longer occur, 2) measure the heat capacity of the self-organized heat transport state, and 3) test recent theoretical predictions2,3 of the microscopic mechanism that is responsible for this self-organization.

Onset of superfluidity far from equilibrium: dynamical effects on the correlation length

Abstract Nonlinear heat conduction has recently been measured near the superfluid transition in pure 4He at very low heat flux Q. Since both dynamic effects and gravity limit the divergence of the superfluid correlation length near the transition at low-Q, these measurements must be repeated in the microgravity environment in order to observe the dynamic effects in isolation. Comparison of the microgravity data to similar data obtained on Earth will provide experimental insight into the effect of gravity on this nonlinear conduction region at low heat flux where theoretical predictions are lacking. While some measurement advantages exist in the microgravity laboratory, it is the study of the direct effect of gravity on the nonlinear conduction measurements that motivate the microgravity need.

Dynamic and Gravitational Effects on the Correlation Volume: Experimental Methods

We report on the design of a new prototype flight instrument that will be used to repeat previous Earth-based measurements of nonlinear heat transport near the superfluid transition in the microgravity laboratory. Since this nonlinear conductivity is associated with dynamic limitations to the divergent correlation length, and since gravitational acceleration also limits the correlation lengths divergence, we anticipate that the nonlinear conductivity will depend strongly on gravitational acceleration. The apparatus, data taking procedure, systematic corrections, and error sources are discussed here.