Jeffrey Robert Feller

Radiation impedance of resonant ultrasound spectroscopy modes in fused silica

The pressure dependence of the resonance frequency of several resonant ultrasound spectroscopy modes in a sample of fused silica has been measured at UCLA in atmospheres of air, helium, and argon near ambient temperature. For both compressional and torsional modes, the radiation resistance is linearly dependent upon pressure and increases with the molecular mass of the surrounding gas. The effects are larger for breathing modes than for torsional modes. They also increase with the molecular mass of the gas. A radiation impedance model is presented which explains some of these data qualitatively and quantitatively.

The effect of gas loading on the RUS spectra of spheres

Resonant Ultrasound Spectroscopy (RUS) of a spherical sample in a pressurizing gas atmosphere was investigated experimentally and theoretically. Measurements were made on a fused silica sphere in He, Ar, and N2 gases up to pressures of 120 bar. The pressure-dependent shift in the resonant frequency, delta f, and the Q-factor were measured for the S00, S11, and T02 modes. A theoretical model based on acoustic radiation impedance was used to calculate delta f and the radiation-resistance component, Qr, of the Q-factor. Agreement between theory and experiment was good for Qr, but there were discrepancies for delta f. It was found that the theoretical delta f due to gas-loading effects associated with acoustic radiation was very small and consistent with the observed dependence on pressure and gas species for the T02 mode but not for the S00 and S11 modes. We conclude that the T02 mode is the most reliable of these modes to use in measuring third-order elastic constants by RUS.

Surface acoustic wave measurements of YBa2Cu3O7−δ thin films and single crystals

Surface acoustic wave (SAW) velocity and attenuation measurements have been performed on high-temperature superconducting YBa2Cu3O7−δ (YBCO) thin films (deposited on nonpiezoelectric substrates), and YBCO single crystals. A “bridge” technique is employed in which piezoelectric substrates, patterned with interdigital transducers, are bonded to the sample under investigation. The temperature dependence of 168 MHz SAW attenuation in a single crystal near its superconducting transition has been measured in applied magnetic fields up to 1.6 T. Features are seen above the superconducting transition temperature which may be indicative of a transition in the electric polarization of the material. A field-dependent attenuation peak, seen in the superconducting state, is discussed in terms of the theory of thermally activated vortex motion.

Instrumentation for cryogenic microwave cavity resonance measurements

We describe instrumentation and measurement procedures for performing microwave measurements on small metallic samples at dilution refrigerator temperatures using the microwave cavity perturbation technique. The quality factor Q of the lead-plated cavity was of order of 105 at temperatures below the superconducting transition temperature of lead. Microwaves were coupled into and out of the resonator through two room-temperature positionable semirigid coaxial lines, each terminated in a small-loop antenna. We describe in detail the arrangement of the apparatus used to tune to, and lock onto, a cavity resonance and the strategy used to categorize various resonance modes. One of the main features of this microwave spectrometer is the application of the FM detection method to measure changes in both the frequency and the quality factor of the sample-loaded cavity. The procedures for converting these quantities to the real and imaginary components of the surface impedance are described. As an application of th...

ULTRASONIC SPECTROMETERS FOR CONDENSED MATTER STUDIES AT VERY HIGH MAGNETIC FIELDS

Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites. The first of these, designed for use with the dc magnets, is a heterodyne spectrometer with the capabilities of high velocity resolution and near-simultaneous measurements at several frequencies (typically near transducer harmonics). The other two spectrometers are designed for the short-pulsed magnets, with the capability of fast data acquisition. These spectrometers have been used in the study of metamagnetism in the heavy fermion materials.

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.

Characterization of thin films of YBa2Cu3O7−δ using an interdigital radio frequency proximity probe technique

A capacitively coupled interdigital probe technique is described whereby the electrical characteristics of thin films at arbitrary frequencies in the range from about 100 kHz to beyond 1 GHz may be studied. Patterning of the film, deposition of electrodes, and bonding of electrical leads are unnecessary, so that all measurements are nondestructive and noninvasive. Preliminary measurements of the superconducting transition in thin films of the high-temperature superconductor YBa2Cu3O7−δ in magnetic fields up to 1.5 T are presented and discussed. They show a large field-dependent peak, attributable to tunneling effects, just below the onset temperature. Anomalies are also seen at higher temperatures, providing evidence of a structural phase transition.

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.]