Stephen C. Steel

Critical temperature and critical current of thin-film superfluid3He

Superfluid3He film flow over the rim of a copper beaker has been measured. The flow rate was measured as a function of temperature and as a function of depth of3He below the rim or film thickness at the rim. The critical current, calculated from the flow rate, varied as (1 −T/Tcp)3/2 as expected for pair-breaking;Tcp is a film-thickness-dependent critical temperature. However, the magnitude of the current was an order of magnitude smaller than expected for pair-breaking, in agreement with other experiments that have demonstrated a lower dissipation mechanism in superfluid3He. The suppression of the critical temperatureTcp/Tcb, whereTcb=0.93 mK is the bulk3He transition temperature, varied from 0.93 to 0.7 as the film thickness at the rim varied from 120 to 90 nm. These ratios are larger than expected from Ginzburg-Landau or microscopic theory of superfluid3He-B.

3He Film flow: 4He substrate coating effect

Abstract Superfluid 3He film flow out of a copper beaker was measured without and then with a 4He coating on the copper surface. The effect of the 4 He was to convert the substrate from a purely diffuse to a purely specular surface for the 3He quasiparticles.

3He Film Flow

Following our earlier observation of 3He film flow, a new experiment has been designed to avoid possible temperature gradients along the 3He film. The film substrate, a 4 mm stainless steel tube, is physically separate from the level detectors, and the 3He both inside and outside the beaker is cooled with sinter. The onset of film flow is now below 1 mK, as distinct from ~3 mK in our previous work. At our lowest temperature, 0.55 mK, the flow rate decreases almost exponentially with the height of the film, from ~5 mm3/hour at the rim to ~0.1 mm3/hour with the 3He 1.5 mm below the rim. As T approaches Tc the flow rate decreases. The present results suggest flow of the bulk or 3-D superfluid 3He, with the decrease in flow with height resulting from the suppression of the order parameter as the film thickness approaches the coherence length.

3He melting curves in high magnetic fields

Abstract We have done precise measurements of the3He melting curve between 2 and 85 mK in several magnetic fields up to 15 T. We observe good agreement between our measurements and predictions based on mean field theory, using three exchange parameters: Jnn=-0.667 mK, JT=-0.229 mK and Kp=-0.400 mK.

Thermometry in 15 tesla

Abstract In order to measure temperatures and viscosities in high magnetic field, we have compared the behaviour of a conventional NbTi vibrating wire viscometer to the response of a vibrating plate of Nb3 Sn and calibrated them against a field independent glass thermometer up to 15 T. The glass thermometer was calibrated in zero field against the3He melting curve and in 15 T against a region where the solid follows a Brillouin law.

The Effect of Film Thickness and the Substrate on Superfluid 3He Film Flow

The self-emptying beaker technique was used to study the superfluid properties of thin film liquid 3He. Superfluid film flow was observed only below the bulk transition temperature at 0.93 mK. In a separate experiment the minimum film thickness was determined as a function of 3He level in the beaker. Thus, the flow rate, or critical current density, was determined as a function of temperature and film thickness. Extrapolation allowed a determination of the critical temperature (Tc) and zero temperature critical current density Jc (0) as a function of film thickness. Tc agreed with the predicted superfluid-normal phase boundary at 2d/(T) c a where d is the film thickness and ξ(T) is the coherence length. Jc (0) was an order of magnitude smaller than expected for dissipation by pair-breaking. When a 4He monolayer was adsorbed on the surface of the beaker, it suppressed the diffuse scattering of 3He quasiparticles at the substrate boundary, as also observed by Freeman et al. There was no measurable suppression of Tc even for films as thin as 100 nm. With or without the 4He monolayer teere was always an abrupt drop in the flow-rate when the film thinned to about 200 nm. This may be associated with the transition between the expected thick film B-like phase and thin film A-like phase.

Superfluid 3He film flow—A phase transition and a substrate effect

Experiments to see flow of superfluid Helium 3 are reported.(AIP) Experiments to see flow of superfluid Helium 3 are reported.(AIP)

Optical observations of3He crystals down to 1 mK

Abstract We present results obtained with a setup designed for optical experiments on 3 He below 1 mK. The entire optical system is placed inside the vacuum can of a dilution refrigerator and does not require any windows except for those in the sample cell. It was used to obtain the first images of 3 He crystals growing from the superfluid B phase.