Dissipation Mechanisms near the Superfluid He3 Transition in Aerogel (original) (raw)

Torsional oscillator studies of the superfluidity of 3He in aerogel

Physica B: Condensed Matter, 1998

We have made simultaneous torsional oscillator and transverse NMR measurements (at ∼ 165 kHz) on 3 He contained within aerogels with nominal densities of 1% and 2% of solid glass. The superfluid transition is seen simultaneously by both techniques and occurs at a temperature which agrees semi-quantitatively with that expected for homogeneous isotropic pair-breaking scattering of 3 He atoms by strands of silica. Values obtained for the superfluid density ρ s in the 2% sample are in reasonable agreement with those observed previously. Coupling of the torsional mode to a parasitic resonance prevented accurate determination of ρ s for the 1% aerogel. We have identified other resonances coupling to the torsional oscillations as sound modes within the helium/aerogel medium.

Transverse sound in aerogel with liquid 4 He

Physica B-condensed Matter, 2003

An experiment was performed to measure transverse sound resonances in a square slab of aerogel filled with liquid 4 He: Resonances have been observed both in the superfluid and normal phase. The dynamics of the system was modelled by combining the equations of two-fluid hydrodynamics of helium with those of elasticity of aerogel. r

High-Frequency Acoustics of 3He in Aerogel

Physical Review Letters, 2000

High-frequency ͑ϳ15 MHz͒ acoustics were performed on 3 He in 98% porous silica aerogel using an acoustic cavity technique. Measurements of the sound attenuation in the normal Fermi liquid and superfluid display behavior quite different from the bulk owing to strong elastic scattering of quasiparticles. The transition from first-to-zero sound is completely obscured with a quasiparticle mean-free path estimated to be in the range of 200-300 nm. No collective mode attenuation peak was observed at or below the superfluid transition.

3 He in 99.5% Porous Aerogel at the Normal-Superfluid Transition

Journal of Low Temperature Physics - J LOW TEMP PHYS, 1998

We have used a torsional oscillator to measure the superfluid density and dissipation near the superfluid transition of3He in aerogel of 99.5% porosity. We used a new cell (constructed at Penn State) for which the aerogel was grown in the pores of a ~100 µm silver sinter. The cell was tested with4He and showed no signs of the second-sound resonances that have interfered with previous torsional oscillator measurements. The measurements with3He, presented here, were taken at pressures of 1.34 and 4.13 bars. We observed values of ?s/? in the T? 0 limit of 0.05 and 0.14 respectively. Our measurements show an increase in the dissipation on warming through Tc. This series of measurements is ongoing and temperature sweeps at various pressures are planned.

Non-linear Mechanical Response of the A-like Phase of Superfluid 3He in Aerogel

Journal of Low Temperature Physics, 2007

We present measurements of the response of the Alike phase of superfluid 3 He in aerogel to an applied flow. The measurements are made using a cylindrical piece of 98% silica aerogel attached to a vibrating wire resonator. The resonator is immersed in superfluid 3 He at low temperatures and relatively high magnetic fields such that the aerogel confined superfluid is in the Alike phase, while the surrounding fluid is in the bulk B-phase. We observe a variety of interesting non-linear and hysteretic effects when the resonator is driven to higher velocities. We present some of our preliminary findings and speculate on their implications.

Observation of a second soundlike mode in superfluid-filled aerogel

The Journal of the Acoustical Society of America, 1991

SuperAuid He is interesting acoustically because it can support more than one mode of sound propagation, and these can be used to study critical properties. Recently there has been interest in superfluidfilled aerogels, but for such compressible materials one does not observe the ordinary (fourth) sound; instead there is a mode intermediate between first and fourth sound and a second-sound-like mode. We present a theory for the modes and the first observation of the aerogel second-sound-like mode, which is important because it propagates near the critical temperature.

Stability of the axial phase of superfluid He3 in aerogel with globally anisotropic scattering

Physical Review B, 2008

It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases of superfluid 3 He contained within highly porous silica aerogel. For example, global anisotropy introduced via uniaxial compression of aerogel might stabilize the axial state, which is called the A-phase in bulk superfluid 3 He. Here we present measurements of the phase diagram of superfluid 3 He in a 98% porous silica aerogel using transverse acoustic impedance methods. We show that uniaxial compression of the aerogel by 17% does not stabilize an axial phase.

Novel sound phenomena in superfluid helium in aerogel and other impure superfluids

Physica B-condensed Matter, 2003

During the last decade new techniques for producing impure superfluids with unique properties have been developed. This new class of systems includes superfluid helium confined to aerogel, HeII with different impurities (D 2 , N 2 , Ne, Kr), superfluids in Vycor glasses, and watergel. These systems exhibit very unusual properties including unexpected acoustic features. We discuss the sound properties of these systems and show that sound phenomena in impure superfluids are modified from those in pure superfluids. We calculate the coupling between temperature and pressure oscillations for impure superfluids and for superfluid He in aerogel. We show that the coupling between these two sound modes is governed either by c∂ρ/∂c or σρ a ρ s (for aerogel) rather than thermal expansion coefficient ∂ρ/∂T , which is enormously small in pure superfluids. This replacement plays a fundamental role in all sound phenomena in impure superfluids. It enhances the coupling between the two sound modes that leads to the existence of such phenomena as the slow mode and heat pulse propagation with the velocity of first sound observed in superfluids in aerogel. This means that it is possible to observe in impure superfluids such unusual sound phenomena as slow pressure (density) waves and fast temperature (entropy) waves. The enhancement of the coupling between the two sound modes decreases the threshold values for nonlinear processes as compared to pure superfluids. Sound conversion, which has been observed in pure superfluids only by shock waves should be observed at moderate sound amplitude in impure superfluids. Cerenkov emission of second sound by first sound (which never been observed in pure superfluids) could be observed in impure superfluids.

Transverse sound in aerogel with liquid

Physica B: Condensed Matter, 2003

Magnetic Field Dependence of the A-like to B-like Transition of Superfluid 3He in Aerogel

Journal of Low Temperature Physics, 2010

Longitudinal ultrasound attenuation of superfluid 3 He in 98% aerogel has been measured at 33 bar and 6.22 MHz in the presence of magnetic fields up to 4.44 kG. The A-like to B-like (A-B like) phase transition in aerogel was identified by a rounded jump in attenuation while sweeping the temperature at a fixed magnetic field perpendicular to the direction of sound propagation. The suppression of the B-like phase was monitored as the magnetic field increased until the A-like phase region extended below our lowest attainable temperature (0.2 mK) at the highest field. In addition, the attenuation in the metastable A-like phase that appears when cooling in zero magnetic field was almost identical to the values observed in the A-like phase in high magnetic field.

Dissipation Mechanisms near the Superfluid He3 Transition in Aerogel (original) (raw)

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