H. P. Baum

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.

Low magnetic field dependence of A.C. susceptibility in superconducting YBa2Cu3O7−δ

Abstract We have performed high resolution low magnetic field dependent ac susceptibility measurements on a single phase superconducting YBa 2 Cu 3 O 7−δ sample. Below the onset temperature T c , a knee in the susceptibility and a peak in the dissipation curve were observed upon cooling but not upon warming after field sweeps at low temperatures. It is suggested that weak links may be responsible for this response. For the field dependent diamagnetic curves, the position of a major change in slope at very low fields may be identified as the critical field of the weak links, which may be fit by H 1 = H 1 (0) [1 − ( T / T c ) 2 ]; the position of a small slope change at higher fields may be identified as H c1 for the grain and which may be fit by H c 1 = H c 1 (0) [1 − ( T / T c ) 2 ], where H c 1 (0) = 460 Oe .

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.

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.

Shift in maximum of sound attenuation with magnetic field in UPt3

We have measured ultrasonic attenuation and ac susceptibility simultaneously on a UPt3 single crystal, in various constant magnetic fields at 293 MHz, and in zero field at several frequencies. The temperature of the attenuation maximum Tmα appears lower than (or very close to) the susceptibility transition temperature Tcχ. A breakpoint is defined, which occurs always at a lower temperature Tbp than Tmα. The shift of this breakpoint, Tcχ −Tbp, exhibits a small frequency dependence.

Possible Phase Transition in UPt3 in a Magnetic Field

Longitudinal sound attenuation was measured in a magnetic field in UPt3. A possible phase transition is indicated by the appearance of a small peak in the attenuation at about 0.6 Hc2. Below this peak, at low fields, the attenuation αs/αn varies linearly with the magnetic field H; close to Hc2, the attenuationhas a parabolic dependence αs/αn~(Hc2-H)2. The peak, at the change over from linear to parabolic behavior may correspond to a phase transition in the vortex lattice.