We have observed that the motion of a vibrating-wire resonator in superfluid 3 He~i? at temperatures below T c /% is associated with a sharp critical velocity, i/ c , above which very high dissipation sets in. We identify this velocity with the Landau critical velocity for pair breaking. That the observed value of v c is a factor of 2 lower than the critical velocity expected from the BCS value of the gap we take as evidence that the gap is suppressed by about 50% of its bulk value near a moving boundary.PACS numbers: 67.50.FiOne of the characteristic properties of a superfluid is the existence of a critical velocity, u L , below which the creation of elementary excitations is not possible. In principle a body should be able to move frictionlessly through the condensate at zero temperature until v L is reached and dissipation processes set in. In practice it has been found impossible to achieve the necessary velocities with anything but microscopic objects. In He II an alternative dissipation mechanism can operate at lower velocities through the creation of vortices, and the Landau limit has only been reached over a limited range of temperatures and pressures by microscopic projectiles in the form of negative ions. 1,2 In the case of 3 He the superfluid is embedded in a highly dissipative quasiparticle gas which conspires to mask any mechanical probing of a critical velocity, and again behavior appearing to show a Landau velocity has only been observed in the case of ions. 3,4 The inverse process, the flow of superfluid through a stationary channel, again shows dissipation in He II at much lower velocities because of the generation of vorticity, but critical currents apparently associated with pair breaking have been observed in 3 He-i? in the Ginzburg-Landau regime. 5,6 Here, however, the measured quantity is a critical volume current which contains not only a velocity but also the superfluid fraction.In the experiments reported here we have studied the motion of two wire resonators in 3 He-£ at very low temperatures. The precise temperature is unknown, since we are able to cool the liquid below the limit of our various thermometers. In this temperature regime, where the quasiparticle density is essentially zero, pair-breaking effects manifest themselves as a very clear critical velocity in the motion of the wire.We have investigated this behavior using two very different vibrating wires in various magnetic fields and at three different pressures. The results are very clear-cut: The maximum velocity of a light vibrating wire cannot significantly exceed a critical velocity even when the driving force is increased by two orders of magnitude. This critical velocity has the same magni-tude for the two wires we have studied which differ in diameter by a factor of 10. From a comparison of the results at different pressures this critical velocity is found to be very accurately proportional to the Landau critical velocity, A//? F , but its magnitude (about 8 mm/s for 0 bar) is smaller than the straightforward Landau val...

No abstract

No abstract

At temperatures below about 0.3T c , heat is transported through 3He by ballistic quasiparticles. This suggests an interpretation of the exp (-Δ/k T) variation of the effective boundary resistance between 3He-B and the coolant in our experiments. While most quasiparticles generated by a heat source are reflected back by collisions with the walls, a small fraction will disappear into the interstices between the sheets of sinter-coated refrigerant and are reabsorbed. Hence the equilibrium quasiparticle density near the heater should be simply proportional to the heat applied, in approximate agreement with experiment. These ideas are confirmed in experiments with a new cell. Quasiparticles generated at the far end of the sinter channels do not penetrate into the experimental volume, and no thermometer response is observed until the lattice of the refrigerant is heated sufficiently for heat to be transported by the alternative channel of electronic conduction.

A filamentary wire resonator will generate quasi-particles in 3He-B when driven beyond a critical velocity. By using such a resonator as a source of quasi-particles and a second wire resonator as a quasi-particle detector, we have been able to emit and detect a ballistic quasi-particle beam over a distance of 3 mm in 3He-B at temperatures below 0.3 Tc. From the preliminary experiment the momentum of the quasi-particle beam can be measured and a direct (but as yet very approximate) measurement made of the quasi-particle mean free path.