Carsten Hucho

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.

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.

Ultrasonic studies of superconducting and magnetic transitions in La2 − xSrxCuO4 single crystal

Abstract Ultrasonic attenuation and sound velocity were measured at frequencies ranging from 25 to 305 MHz in a large single crystal of La 2 − x Sr x CuO 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 ≈ 37.9 K orginate in the superconducting transition and a possible structural instability slightly above it.

Evidence for vortex transport by surface acoustic waves in a high‐Tc superconductor

The interaction of surface acoustic waves (SAW) with the magnetic vortex system in YBa2Cu3O7 is investigated. A 100nm YBCO film is deposited on a piezoelectric substrate and structured for electrical 4‐point measurements. Interdigital transducers are fabricated on the same substrate. When applying an external magnetic field perpendicular to the film surface a SAW‐induced dc‐voltage perpendicular to the acoustic sound‐path is observed. This is interpreted as consequence of directed dragging of vortices by the SAW induced dynamic pinning structure [1]. The piezoacoustic wave acts as a conveyor for mobile flux quanta. Additional ac‐dc‐conversion as result of the nonlinear current‐voltage characteristics close to the superconducting transition temperature can be resolved and separated. In order to observe the sound‐induced vortex motion directly, the flux distribution is analyzed magnetooptically. Magnetooptic imaging allows for time resolved analysis of flux distribution. Quantitative analysis of changes in ...

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.

Ultrasonic attenuation and sound velocity changes in a superconducting La2-xSrxCuO4 single crystal

The ultrasonic attenuation and relative sound velocity measured at frequencies up to 305 MHz in the superconducting state of a La2-xSrxCuO4 single crystal. For longitudinal sound waves propagating along the c-axis, the attenuation increases with increasing temperature with a small peak at 27 K and a large peak at about 37 K. These two peaks, especially the small one, become more obvious at higher frequencies. Simultaneously, the velocity exhibits a jump of about 100 ppm around the superconducting transition. At about 27 K, there is another discontinuity in the sound velocity of about 20 ppm, which corresponds to the small peak in the attenuation measurement.

Radiation impedance of RUS modes in fused silica and KCl.

The pressure dependence of the resonance frequency of several RUS modes in samples of fused silica and KCl has been measured at UCLA in atmospheres of air, He, and Ar near ambient temperature. For both compressional and torsional modes the radiation resistance appears to be 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. A model will be presented which will attempt to fit some of these data both qualitatively and quantitatively. [Work supported by the Office of Naval Research.]

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.]