M.-F. Xu

Longitudinal sound measurements on UPt3 in a magnetic field

Abstract A new feature (a peak) is found in the magnetic field dependence of the longitudinal ultrasound attenuation in UPt 3 . Below the peak, the attenuation varies linearly with field. Above the peak Δα = α ( H c 2 )- α ( H ) ∼( H c 2 - H ) 2 . The peak, at the change over from linear to parabolic behavior, may correspond to a phase transition in the vortex lattice.

Oscillations in the acousto-electric proximity coupling to a 2D electron gas

Abstract We report the first results on proximity coupling of surface acoustic waves to a two dimensional electron gas. A GaAs-AlxGa1−xAs heterojunction was placed in close proximity to the surface of a piezoelectric substrate on which a surface acoustic wave was propagated. Oscillations in the attenuation of the surface wave, corresponding to the Shubnikov-de Haas oscillations in σxx, were observed as a function of applied magnetic field. Break down of the σxx = 0 state is seen at moderate power levels for the SAW. This is interpreted as either large current effects or due to spatial modulation of the Landau levels.

Ultrasonic attenuation in sinter-forged high Tc superconductor YBa2Cu3O7 − δ

Abstract We have measured ultrasonic absorption in sinter-forged high T c YBa 2 Cu 3 O 7 − δ . Owing to the anisotropic properties of the sinter-forged samples, longitudinal ultrasonic attenuation measurements showed very strong anisotropy when waves were propagated parallel and perpendicular to the forging axis. Sound absorption was found to increase with decreasing temperature for shear waves travelling parallel to the forging axis.

Hysteresis in ultrasonic attenuation in UPt3 in low magnetic fields

Abstract Ultrasonic measurements on the heavy fermion superconductor UPt 3 indicate evidence of the possibility of two or more superconducting phases in a magnetic field. At low fields there is a large hysteresis in the attenuation which was interpreted by Muller et al. as a metastable phase. We report on similar measurements, where in addition to a hysteresis in the attenuation there is an associated hysteretic temperature change. We believe the hysteresis in attenuation to be the result of a temperature increase due to flux jumps. A lower critical field of the order of 10 mT is estimated.

Elastic constant anomalies in sinter-forged high-Tc superconductor YBa2Cu3O7 − δ

Abstract The ultrasound velocity in the sinter-forged YBa2Cu3O7 − δ superconductor was measured and the results showed evident anomalies as well as anisotropies. Both longitudinal and shear wave velocity along the c axis (the forging axis) showed a distinct thermal hysteresis from 65 K to 260 K. The velocity change ΔV V from room temperature to 4 K was 1.5%. The sound velocity along the “basal plane” reached a maximum around 160 K. No distinct thermal hysteresis was observed. ΔV V from room temperature to 4 K was 1%. All of these velocities were measured while sweeping the magnetic field up to 9 T at fixed temperatures of 4 K and 86 K respectively. No velocity change was observed. The shear wave velocity behaved similarly at 9 T during the thermal cycling between room temperature and 4 K.

Ultrasonic investigation of the high Tc, superconducting Ba1−xKxBiO3 system

Ultrasonic Attenuation and velocity measurements have been performed in Ba 1−x K x B1O 3 , with x-0.35 and x-0.4. In the x-0.4 system, for transverse waves, it was observed that the attenuation drops below T c almost linearly and the velocity has a kink right at T c In magnetic field sweeps at 17 K, the velocity decreases and the attenuation increases slightly in high fields, suggesting a vortex lattice structure change in the superconducting state. In the x-0.35 system, with transverse waves, the attenuation appears to have a nearly exponential decrease below T c .

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. >

Field‐dependent behavior of longitudinal ultrasonic attenuation in UPt3

A possible phase transition is evident in the field dependent behavior of the ultrasonic attenuation in UPt3. In the low field region the behavior is linear and in the high field region close in Hc2 the behavior is quadratic in h’=(Hc2−H)/Hc2. At the crossover there is a small peak in the ultrasonic attenuation. The temperature dependence of the coefficients a(T), for the linear behavior, and b(T), for the quadratic behavior are obtained.

Magnetization and sound velocity measurements near the antiferromagnetic transition in the heavy fermion system UPt3

Abstract The heavy fermion superconductor UPt3 has a superconducting transition at 0.5 K and an antiferromagnetic transition at 5 K. We present measurements in the vicinity of the antiferromagnetic transition of both the sound velocity and magnetisation.

Ultrasonic attenuation measurements on the heavy fermion superconductor UPt3

The superconducting pairing mechanism in the heavy fermion superconductor UPt3 is unconventional, as evidenced from any of its low‐temperature properties. Recent ultrasonic absorption, heat capacity, and magnetization measurements show a multiplicity of phases in the H‐T plane, suggesting an exotic order parameter. The experimental results will be reviewed. [Work supported by ONR and NSF.]