Yury Mukharsky

The determination of the energy barrier for phase slips in superfluid4He

The velocity dependence of the energy barrier for vortex creation in microscopic apertures is determined. When compared to results from other laboratories, the energy barrier seems to be a universal function of velocity. This universality suggests that the vortex nucleation process is independent of the microscopic surface structure of the aperture. In DC flow experiments, the vortices are nucleated at rates up to 700kHz. In single phase slip experiments, the rate of nucleation is on the order of 10 Hz. Each of these types of experiments gives the energy barrier in a different velocity regime. The energy barrier has more curvature, as a function of velocity, than can be accounted for by the half-ring model of vortex nucleation.

Rotation measurements with a superfluid 3He gyrometer

Abstract We report the first precision rotation measurements with a superfluid 3 He gyrometer. This device operates in the Josephson non-hysteretic regime. Its practical sensitivity could ultimately compare with those of ring lasers and atom interferometers.

The Experimental Evidence for Vortex Nucleation in 4 He

This update on the problem of vortex nucleation in superflows through micro-apertures follows the recent reviews of the subject by Zimmermann [1] and one of the authors [2]. Recent developments of the model of vortex nucleation involving a vortex half-ring are assessed with an emphasis on the statistical properties of the critical velocity transition. The occurrence of collapses and multiple slips is then discussed in relation with the problem of vortex nucleation.

Current–phase relationship measurements in the flow of superfluid 3He through a single orifice

Abstract We report accurate measurements of the current-phase relationship in the flow of superfluid 3 He through a single orifice. While ideal sinusoidal 2π-periodic Josephson behavior prevails close to T c , increasingly strong π-periodic admixture is usually observed at lower temperatures.

Simulation of a 4He Superfluid Gyrometer with Large Sensing Area

We present the numerical analysis of a Helmholtz resonator-based4He analog of the rf-SQUID. This device has been demonstrated to act as a sensitive gyrometer. The rotation is detected by its effect on the apparent critical velocity of helium superflow through a small aperture. To increase the sensitivity, one must enlarge the sensing loops and use higher excitation frequencies. The behavior of this device is significantly modified when the propagation time of sound along the tube forming the loop becomes comparable with the period of the Helmholtz resonator. Large pseudo-random noise is introduced into the system. The ratio of the signal to this extra noise depends on the mode of the oscillator being used. The sensitivity and optimal performance of the gyrometer are discussed.

Measurements of Critical Current of Superfluid 4He through An Array of Submicron Apertures very near Tλ

Far from the lambda transition the critical flow of superfluid 4He through a small orifice is determined by thermal nucleation of quantized vortices. Between 300 mK and 2 K linearly decreasing critical flow velocity has been observed earlier. As the temperature approaches Tλ the size of the vortex core increases and becomes comparable to that of the orifice. We report here measurements of the critical mass current in this temperature range. An array of 24 3×0.17 μm holes in parallel with a macroscopic parallel path and flexible-diaphragm Helmholtz resonator have been used. The temperature range explored was from ≈80 mK to ≈20 μK below Tλ. Preliminary analysis of the data shows that for a reduced temperature t=(Tλ−T)/Tλ≳1·10−4 the critical current scales approximately as t1.25. Closer to Tλ the critical phase difference across the array becomes comparable to 2π and the results have to be analyzed in terms of Josephson effect. The superfluid density has been measured at the same time as the critical current.

Josephson effect and a π-state in superfluid 3 He

Evidence has been reported for Josephson-type behaviour in the flow of superfluid helium-3 (3He) through an array of 4,225 apertures and for a metastable π-state at low temperature. Here we show that an array of holes cannot be expected to act as a single weak link in such a system, and that these results can be explained without invoking the superfluid analogue of the much-debated ‘π-junction’ in other systems.

The characteristic response curve, I(ΔP), of a3He weak link

Abstract We report measurements of the mass current as a function of pressure difference, I(ΔP), for superfluid 3 He flowing through an aperture whose dimensions are on the order of the temperature dependent coherence length. The goal of the experiment is to see to what extent this weak link behaves like a simple Josephson junction. A model has been developed which predicts I(ΔP) for the case where the weak link is analyzed as a series connection of resistance, inductance and ideal Josephson junction. This model is found to be inadequate to explain the central feature of the data, i.e. the current increases monotonically with ΔP.

Effect of3He impurities on the nucleation of phase slips in superfluid4He

Abstract The critical value of the dc current of superfluid4He through a micro-aperture is found to decrease at low temperatures in the presence of3He impurities. We measure the dependence of the critical current on the pressure difference across the micro-aperture. In the temperature region where the3He affected decrease in the critical current is detected, we measure a large increase in critical current with pressure difference. The effect can be explained by the finite rate of arrival of ballistic3He atoms to a nucleation site. The critical velocity increases at high pressure difference when the rate of phase slippage becomes comparable to the flux of3He atoms.