Moises Levy

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.

Ultrasonic Bonds for Use at Liquid‐Helium Temperatures

A process is described for sealing quartz crystals to metals with epoxy resin which has proven to be very dependable for low‐temperature work.

Acoustoelectric interaction in high Tc films

Surface acoustic wave (SAW) may acoustoelectrically interact with a superconducting film deposited on the surface of the piezoelectric substrate which carries the SAW. The polarization fields accompanying the SAW induce currents in the normal conducting film producing attenuation via Ohmic losses which are subsequently quenched when the film becomes superconducting. In a homogeneous film, the SAW attenuation will be proportional to the resistivity of the film even through the superconducting transition. In a granular film percolation effects broaden both the resistive and attenuation transitions. However, if the number of grains per SAW wavelength is sufficiently small, then the attenuation transition is broadened even more than the resistive transition, and the acoustoelectric attenuation in the normal state is larger than it would be for a homogeneous film with the same sheet resistivity. An acoustoelectric attenuation percolation model has been developed which may yield information about the granularit...

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

It is shown that temperature-dependence ultrasonic attenuation data of YBa/sub 2/Cu/sub 3/O/sub 7/ at various frequencies exhibit anomalies at temperatures close to T/sub c/. These attenuation maxima are found to be the result of a relaxation mechanism added on top of an unusual attenuation background. It is proposed that the grain boundary motions induced by the structural distortion and the propagation of sound waves enhances the energy dissipation around T/sub c/. Whether this structural distortion is the consequence of the onset of a superconducting state remains undetermined. It is also possible that either a tunneling effect of the acoustoelectric effect contributes to sound energy dissipation. The temperature dependence of ultrasonic velocity shows a softening around T/sub c/ which may be an intrinsic property of high-temperature superconductors. >

Surface Acoustic Wave Investigation of Superconducting Films

Abstract : The ultrasonic attenuation coefficient of 2 GHz surface waves in a 300 A aluminum film has been measured in the normal and superconducting state. A zero temperature energy gap of (kB(TC) is found for the Al film. Microwave electromagnetic attenuation measurements in the film indicate critical fluctation effects near Tc. However, the evidence for the contribution of fluctuation effects to the ultrasonic attenuation data of this short mean free path al film is not conclusive. The mean free path determined from the electron phonon interaction term obtained from the ultrasonic measurement appears to be at least an order of magnitude larger than the mean free path obtained from a measurement of the electrical conductivity of this thin granular aluminum film. This large difference is believed to indicate that the mean free path determined ultrasonically is for the Al metal while the one determined electrically is for the Al-Al2O3 matrix structure. (Author)

Simple Modification of Attenuation Equipment to Measure Velocity

A simple modification for making velocity measurements using existing ultrasonic attenuation equipment is described.

Metals, superconductors, phase transitions and Izzy

Seven of thethifiy-two PhDstudents supewised by Professor Isadore Rudfick studied electronphonon interaction, phase trmsitions mdsuperconductivity inmetds.

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.]

Magnetic field studies of the superconducting transition in (La1−xSrx)2CuO4 single crystal by ultrasonic measurements

The temperature dependence of the ultrasonic attenuation and the relative sound velocity were measured in a single crystal of (La1−xSrx)2CuO4 (x=0.075, nominal) with dimensions of about 4×4×4 mm3, in a temperature range of 4.2 to 50 K. The superconducting transition of this sample measured by ac susceptibility is at 36.6 K. The frequency of longitudinal sound waves propagating along the c axis was extended up to 305 MHz. The attenuation shows two peaks, one at about 27 K and the other at around 36.7 K, while the sound velocity exhibits softenings of about 20 and 80 ppm at the corresponding temperatures, respectively. Magnetic fields of 11 KOe were applied in the a‐b plane to study the effects of the field on both the temperatures and the heights of these features in attenuation and velocity. The results, combined with the frequency dependences of these features, suggest that the transition at 36.7 K may possibly be produced by both a structural transition and the superconducting transition. In addition, r...

Measurements of ultrasonic attenuation and sound velocity in a single crystal of (La1−xSrx)2CuO4

Measurements of the ultrasonic attenuation and sound velocity were made at frequencies of 32 and 87 MHz in a large single crystal of (La0.925Sr0.075)2CuO4 (∼4×4×4 mm3), which exhibits a sharp (ΔT∼1 K) superconducting transition at 37 K. For longitudinal waves propagating along the c axis, the sound velocity shows a sharp softening, Δv/v∼100 ppm, concurrent with the sharp superconducting transition in the susceptibility measurements. Following this transition, the susceptibility exhibits a small tail extending to 27 K, where a smaller softening, Δv/v∼20 ppm, has been observed in the velocity measurements. Accompanying the ∼100‐ppm velocity drop, there is a signature in the attenuation; following this feature, the attenuation changes as the temperature is decreased further, with a small kink in the attenuation at 27 K. Additional results of the dependence of the attenuation and sound velocity in external magnetic fields, orientated in the a‐b plane, will be presented. [Work supported by ONR.]