M. Levy

Top-Loading Dilution Refrigerator for High Frequency Measurements

We have modified an Oxford dilution refrigerator TL400 to perform ultrasonic and NMR experiments, with the flexibility of rapidly changing samples. The samples, along with thermometers, are mounted on a copper slug, which in turn is top-loaded onto a copper extension to the mixing chamber. The main co-axial contact is made through a 50 Ω spring-loaded central co-ax which has been tested successfully at frequencies up to 1 GHz; in addition, 12 spring-loaded electrical contacts are made from the side, four of which can also be used as low frequency (≤100 MHz) co-axes. The system has been used to do ultrasonic measurements on a single crystal of the heavy-fermion superconductor UPt3; and on a GaAs-AlGaAs heterojunction, showing the Quantum Hall Effect.

Magnetic field dependence of 600 MHz SAW velocity changes for thin Ni films

The authors derive an expression for the change in SAW (surface acoustic wave) velocity as a function of in-plane magnetic field for thin magnetostrictive films, which compares well with the data for thin Ni films. The proposed dynamic model is based on describing the films in-plane magnetization as being perturbed by the periodic magnetostrictive induced change in the anisotropy field. The dissipation is phenomenologically accounted for by the Gilbert equation. It is shown that the attenuation and velocity changes have the same qualitative field dependence.<<ETX>>

Relaxation behavior of ultrasonic attenuation in YBa/sub 2/Cu/sub 3/O/sub 7/

Temperature-dependent ultrasonic attenuation measurements have been performed on a pressed powder sample of YBa/sub 2/Cu/sub 3/O/sub 7/ with frequencies at 10, 27, and 32 MHz. An attenuation anomaly occurs at temperature close to the superconducting transition temperature T/sub c/ (90 K) for each frequency. It is also found that the magnitudes of these attenuation peaks are of quadratic frequency dependence. With normalized attenuation data and by employing the Debye equation for relaxation type ultrasonic attenuation, the relaxation time is found to be exponentially dependent on temperature. Moreover, the velocity of a 10 MHz sound wave propagating in the sample generally increases with decreasing temperature while exhibiting a possible softening around T/sub c/. These variations of attenuation and velocity at temperatures near T/sub c/ may be associated with the structural distortion of YBa/sub 2/Cu/sub 3/O/sub 7/ and may indirectly be a result of superconducting transition.<<ETX>>

Oscillations in the Attenuation of Surface Acoustic Waves due to Proximity Coupling to a 2D Electron Gas

Oscillations in the attenuation of Surface Acoustic Waves (SAW) was observed when a GaAs-AlGaAs heterojunction was placed in close proximity to the surface on which the SAW was propagating. These oscillations correspond to the oscillations in σxx, as a function of magnetic field, of the 2D electron gas on the interface between the GaAs and the AlGaAs.

Ultrasonic study of vortex viscosity and phase transitions in YBa2Cu3O7 single crystals

A novel ultrasonic technique (sampled cw method) is used to probe into the viscosity of vortex systems in untwinned and twinned YBCO single crystals. Motion of the flux lines in the superconducting sample is induced via coupling with the moving ionic lattice in the bulk of the sample. Hence, the observed dissipation has its origin in the (induced) motion of the vortices with respect to the ionic background of conduction charge carriers. In the experiments which were performed, pronounced field‐dependent attenuation changes were observed which are indicative of transitions from a soft vortex system at low fields to a rigid vortex system at high fields. [Research supported by Office of Naval Research and C. Hucho was supported by Deutsche Forschungsgemeinschaft.] a)Currently at: FB Physik, Augsburg, Germany.

Electrical conduction and surface acoustic wave attenuation in a granular YBa/sub 2/Cu/sub 3/O/sub 7-y/ film

Measurements of sheet resistance and surface acoustic wave (SAW) attenuation were performed on a granular film of the high temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) deposited on a piezoelectric (LiNbO/sub 3/) substrate. Measurements were first performed soon after the film was produced, and repeated after three years aging in air. The resistive behavior, as measured by a four-probe method, was found to have changed significantly with aging, whereas little or no change was observed in the behavior of the SAW attenuation. It will be shown that these two results are not contradictory. A model for an average inter-grain junction is developed which agrees quantitatively with the resistance data. Using the same model, the resistance of square random resistor networks of various sizes are calculated yielding a qualitative agreement with the SAW data.

Ultrasonic studies of superconducting and magnetic transitions in a La/sub 2-x/Sr/sub x/CuO/sub 4/ single crystal

Ultrasonic attenuation and sound velocity were measured at frequencies ranging from 25 MHz to 305 MHz in a large single crystal of La/sub 2-x/Sr/sub x/CuO/sub 4/ with nominal x of about 0.15. The observed features in attenuation and velocity suggest a relaxation process of magnetic origin at about 27 K, and those at /spl sim/37.9 K originate in the superconducting transition and a possible structural instability slightly above it.

Surface acoustic wave investigation of mixed-state phases in YBa/sub 2/Cu/sub 3/O/sub 7/

A technique was developed which allows for surface acoustic wave (SAW) experiments up to 500 MHz in YBCO single crystals and films on piezoelectric as well as non-piezoelectric substrates. Experimental data on a granular YBCO film show a field dependent attenuation peak below T/sub c/ which is indicative of temperature dependent changes of the coupling of the magnetic vortex system to the crystal lattice and might be related to melting or depinning of the vortex lattice.

Surface acoustic wave investigation of phase transitions in YBa2Cu3O7−δ films

Pulsed surface acoustic waves (SAW’s) at 165 MHz are used to study the superconducting transition and an apparent structural phase transition in films of YBa2Cu3O7−δ (YBCO). The structural phase transition occurs in the temperature range 200–230 K and is evidenced by large changes in SAW attenuation and velocity. Small changes are also seen at the superconducting transition temperature. These changes are markedly different when a conventional conductor (e.g., copper) is brought into close proximity to the YBCO film. The results of this investigation are in agreement with the hypothesis that YBCO is piezoelectric below the 200 K transition. [This work was supported by the Office of Naval Research, with C. Hucho supported by Deutsche Forschungsgemeinschaft.]

High‐resolution velocity measurements in the superconducting state of the heavy fermion compound UPt3

Ultrasonic measurements have been used very successfully in investigating multi‐phase superconductivity in the heavy fermion compound UPt3. Further probing of the superconducting properties in UPt3 demands even higher resolution in both ultrasonic velocity and attenuation measurements. In order to achieve this, a new approach was used in measuring ultrasonic velocity and attenuation, in which a digital dynamic control technique was utilized in the conventional pulse‐echo method. With this approach, longitudinal ultrasonic velocity and attenuation of frequencies up to 450 MHz were measured on a single crystal UPt3 sample in the superconducting state. The overall resolution in the velocity measurement is in the order of 0.1 ppm. This enables us to determine a complex superconducting phase diagram for UPt3 with unprecedented accuracy. Superconducting phase boundary lines were traced even within the transition width. The critical property of converging phase transition lines is thus determined unambiguously. ...