A. Schenstrom

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.

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.

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.

Frequency dependent breakdown of the dissipationless state in the quantum Hall effect

Abstract We report the results of a study of the frequency and power dependence of the surface acoustic wave attenuation produced by the 2DEG on the interface of a GaAs-Al x Ga 1−x As heterojunction. The minimum attenuation, seen at integer number of filled Landau levels, increased with increasing power in the surface wave, first at higher numbers of filled levels and at higher frequencies, even though ω ⪡ ω c . This is interpreted as due to heating, resulting from absorption of energy from the surface wave by the electronic states with localization lengths of the order of the acoustic wavelength.

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.

Ultrasonic Investigation of Novel Superconducting Systems

Ultrasonic attenuation measurements are reported for the superconducting, normal, and mixed states of the heavy Fermion superconductors UPt3 and URu2Si2 and in the alloys of the ternary superconductors Er1-x HoxRh4B4. A peak in attenuation in the mixed state of UPt3 may be indicative of collective excitations of the flux line lattice. A broad maximum in attenuation is found below the superconducting transition temperature of URu2Si2. An increase in attenuation in the superconducting phase of the reentrant ternary superconductors with x = 0.6 and 0.813 is ascribed to increased spin phonon interaction induced by superconducting screening of crystalline electric fields.

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 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.]

Magnetic-field-dependent sound attenuation in UPt3

Abstract We report measurements of the attenuation of transverse sound along the c axis of UPt 3 . The peak in attenuation, which has previously been observed in longitudinal sound measurements vs . magnetic field in the superconducting state, is absent in the transverse sound measurements. We also present results on the magnetic-field-dependent attenuation in the normal state, up to 8 T. The attenuation increases with magnetic field, approximately as H 2 .