R. König

Acoustic properties of amorphous SiO2 and PdSiCu, and of crystalline Ag, NbTi and Ta at very low temperatures

With the vibrating reed and vibrating wire techniques we have investigated the acoustic properties of vitreous silica (SiO2, Suprasil I) and of amorphous PdSiCu as well as of polycrystalline Ag, NbTi and Ta at frequencies of 100 Hz≦ω/2π<6 kHz and at temperatures of 0.1 mK≦T≦1 K. The relative change of sound velocity Δv/v of SiO2 shows saturation effects, strain amplitude dependence, as well as an unexpected temperature dependence below its maximum atT<50 mK. For PdSiCu we observe that below a certain temperature, which depends on the applied strain, the temperature dependence of the sound velocityv deviates from the logarithmic behavior observed at higher temperatures and reaches an almost constant value atT<1 mK. In the same temperature rangeQ−1 does not remain constant but steadily decreases. The acoustic properties of the two amorphous materials at finite strain show substantial deviations from the standard tunneling model. Some of the observed anomalies can be explained taking into account the change of population of the tunneling systems energy states and a nonlinear relaxation absorption. For polycrystalline Ag we find Δv/v αlnT andQ−1αT1/3 over three decades inT atT<100 mK; it shows low-temperature acoustic properties which are strikingly similar to those of amorphous materials. The temperature and strain dependencies of the acoustic properties of polycrystalline superconducting NbTi and Ta resemble those obtained for SiO2. These results indicate that there are basically no differences in the low-temperature acoustic properties of polycrystals and amorphous materials.

Nonlinear acoustic properties of vibrating wires at very low temperatures

We have investigated the acoustic properties of superconducting NbTi and Ta wires in vacuum at 1 mK≤T≤1 K and at a few kHz. The temperature dependence of the acoustic properties of the wires is similar to that found in amorphous materials: a maximum in the sound velocity and a plateau in the dissipation at temperatures above it. In addition, we have observed a strong influence of the acoustic power on the measured properties in agreement with recent measurements on amorphous SiO2. This strain dependence can be interpreted by a modification of the tunneling model as a change of populationS of the two-level system energy states in non-crystalline materials. We can explain the strain-dependent anomalies as the superposition of three nonlinear effects: the change of population of the tunneling systems energy states, self-heating of the wire, and a nonlinear restoring force. These effects have a strong influence on the lineshape of the resonance curve. In addition, we have investigated the behavior of vibrating wires in liquid3He and liquid3He-4He solutions atT<100 mK. We can show that in superfluid3He-B atT<0.2 mK or in solutions of3He in4He with a small concentration of3He it is impossible to use a vibrating wire as a viscometer without having exact information about its intrinsic properties.

Fermi liquid behaviour of the viscosity of3He-4He mixtures

We have investigated3He-4He mixtures at3He-concentrations 0.98%⩽x⩽9.5% by the vibrating wire technique in the temperature range 1 mK⩽T ⩽ 100 mK and at pressures 0 bar ⩽ p ⩽ 20 bar. In the degenerate regime of the mixtures the Landau theory of Fermi liquids predicts a temperature dependence of the viscosity η proportionalT−2. We report on the first observation of this behaviour at 3 mK⩽T ⩽10 mK for all investigated concentrations and pressures. At temperatures below about 20 mK slip corrections had to be taken into account due to the increase of the quasiparticle mean free path at very low temperatures. The low-temperature cut-off in ηT2 = constant indicates the transition into the ballistic regime of the mixtures, where the mean free path of the quasiparticles exceeds the radius of the vibrating wire. Our results for the pressure dependence of the viscosity as well as for its magnitude show substantial differences from predictions based on pseudopotential theory. However, a calculation of η with the quasiparticle interaction potential of recent solubility measurements in mixtures agrees well with our experimental data, in particular the pressure independence of η.

Refrigeration and thermometry of liquid3He-4He mixtures in the ballistic regime

The ballistic regime of liquid3He-4He mixtures is characterized by a large mean free path λ of the thermal excitations compared to the characteristic dimension of the experiment. We report on investigations of the transport properties of mixtures as well as superfluid3He in the ballistic regime by means of the vibrating wire technique. In order to avoid possible sources of heat leaks into the liquid, the experimental setup was built as far as possible of pure materials only. The contribution of a Ag sinter to the heat leak as well as its influence on the attainable minimum temperature of the mixtures were investigated by performing measurements in two similar setups which differed in the size of the heat exchanger by about one order of magnitude. Moreover, we have used the vibrating wire partly immersed in the superfluid3He-B phase of a phase-separated mixture as a very sensitive, continuously monitoring thermometer for liquid mixtures in their ballistic regime. The achieved minimum temperature of a 6.8%-mixture atp = 0.35 bar and of a 9.5%-mixture atp = 9.8 bar was 130 μK. This value can be considered as an upper limit for the temperature of the mixtures as the damping of the vibrating wire thermometer saturates at this temperature due to its intrinsic properties.

Inter- and intragranular effects in superconducting compacted platinum powders

Compacted platinum powders exhibit a sharp onset of diamagnetic screening at

Internal friction and sound velocity of polycrystalline metals at very low temperatures

T \simeq 1.9

Vibrating-wire measurements in vacuum, liquid3He and liquid3He-4He mixtures

mK in zero magnetic field in all samples investigated. This sharp onset is interpreted in terms of the intragranular transition into the superconducting state. At lower temperatures, the magnetic ac susceptibility strongly depends on the ac field amplitude and reflects the small intergranular critical current density

Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

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A study of vibrating wires for viscometry in high magnetic fields

. This critical current density shows a strong dependence on the packing fraction f of the granular samples. Surprisingly,

Transverse Spin Diffusion in 3He-4He Mixtures—Part I

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