Propagation of Collisionless Sound in Normal and Extraordinary Phases of LiquidHe3below 3 mK

Abstract

In this Letter we present measurements of the attenuation and velocity of ultrasound in liquid 3 He at temperatures below 3 raK and at various pressures" In this temperature region and in the presence of solid 3 He, two new phases with extraordinary NMR properties were discovered by Osheroff et al. 1 In this same temperature region Webb et al. 2 observed a second-order phase transition, a discontinuity but not a divergence in the specific heat, over a wide range of pressure in liquid 3 He. This phase transition has been associated with the higher-temperature phase transition found in Ref. 1. The lower temperature phase has not yet manifested itself in our experiments. The present measurements were made in zero magnetic field at a variety of pressures from 140 lb/in 0 2 to near the melting curve and, where possible, at 5, 15, and 25 MHz 0 For these frequencies we have the relation oor > 1, where r 1514 (1972). is a collision time, in the low-temperature region of interest. The conditions for measurement at least approximate those for propagation of zero sound 3 * 4 for T>T C and may approximate at our lowest temperatures the conditions for propagation of a density fluctuation mode in a superfluid neutral Fermi fluid as discussed by Leggett 0 5 The 3 He was cooled using an epoxy-walled, cerium magnesium nitrate (CMN)-filled, demagnetization cell of standard design 6 in which the sonic cell was lodged in a cylindrical cavity in the CMN. Sonic measurements were made using the method of Abel, Anderson, and Wheatley. 7 The nature of the experimental data and their treatment are given by Wheatley 8 (see sect. 2.4). The sound is propagated through the 3 He between two 5-MHz epoxy-backed X-cnt quartz crystals separated by a 0 o 4996-cm-longx0 o 76*cm-i.d o Measurements of attenuation and velocity of sound in the collisionless regime have been made in normal and extraordinary phases of liquid 3 He at several pressures. The attenuation has a frequency-independent component with a sharp maximum just below T c and a frequency-independent component with a BCS-like temperature dependence. The velocity, frequency dependent very near T c , becomes frequency independent at lower T and may be approaching the velocity of first sound. 829

Sound Propagation in SuperfluidHe3near the Polycritical Point

“…We have identified the appearance of superfluidity as the temperature where the attenuation departs from the normal liquid value; this is in agreement with theory for both phases• The data at 23.17 bar [Figo 1(a)] are similar to those observed by Paulson, Johnson, and Wheatley 3 with the exception of the discontinuity in the velocity and attenuation at the B-A transition, which was not observed in Ref. 3. The data in Fig, 1(b) at 21.80 bars show the situation at a pressure only slightly above the PCP where the A and B transitions are now quite close to each other.…”

“…1 Earlier studies of sound propagation along the melting curve 2 and at lower pressures 3 showed a large peak in the attenuation at the normal-superfluid transition accompanied by a sharp, but monotonic, decrease in the sound velocity 3 which appeared to extrapolate to the hydrodynamic first-sound value at still lower temperatures. Sound propagation at the B-A transition has not been studied before at pressures between the polycritical point (PCP) and the melting curve; the phase diagram was established by different experimental probes.…”

“…The propagation of ultrasound is a powerful probe of the low-temperature superfluid phases of 3 He. 1 Earlier studies of sound propagation along the melting curve 2 and at lower pressures 3 showed a large peak in the attenuation at the normal-superfluid transition accompanied by a sharp, but monotonic, decrease in the sound velocity 3 which appeared to extrapolate to the hydrodynamic first-sound value at still lower temperatures.…”

“…The sonic components were contained in an epoxy plug which was screwed into the bottom of the demagnetization cell; the threads were sealed with Nonaq stopcock grease. The sound path was defined by an 0 o 6-cm-long fused quartz annulus with an outside diameter of 1.27 cm and an inside diameter of 0.95 cm; the diameter of the sound path was further reduced to 0.6 cm by an epoxy annulus of nearly the same length in order to reduce the volume of 3 He in the cell. The ends of the quartz spacer were castellated (to minimize spurious transmission through the spacer and to allow thermal contact to the liquid) and ground optically flat (to minimize phase cancelation of the wave front) 0 The sound propagation axis was aligned perpendicular to the cryostat axis,, Appropriately machined epoxy pieces were fitted over the spacer to reduce further the quantity of 3 He; a hole together with various grooves allowed liquid thermal contact with the main reservoir 0 The ends of the spacer were capped by unloaded 20-MHz coaxially plated^-cut quartz transducers; electrical contact and mechanical pressure between the transducers and the spacer were provided by gold-plated beryllium-copper spring contacts; the pairs of electrical leads to the input and output transducers were twisted to minimize crosstalk.…”

Propagation of Fourth Sound in SuperfluidHe3

“…where C x is the velocity of first sound 6 show that p s /p is quite small, varies slowly with temperature, and shows little or no detectable change near the first-order transition at T AB ; this is in possible disagreement with the interpretation of the wire-damping experiments 7 and with a prediction 8 relating 1 -p s /p and zero-sound velocity, but is reasonably close to what one might expect from susceptibility 9 measurements. Our experimental epoxy cell, 2.63 cm long and 2.11 cm in diameter, was packed with cerium magnesium nitrate (CMN) powder (size less than 37 iim) to 80% of crystalline density, the CMN being used as superleak, refrigerant, and thermometer.…”