M-F. Xu

The Structure of Surfaces

The structure of a surface influences many of its important properties. For example, the chemical, electronic, and vibrational properties of surfaces are affected by small changes (i.e., 0.01 to 0.05 A) in the surface bond lengths. The surfaces of most semiconductors and of some noble metals are reconstructed, producing new and important properties of the surface region. Indeed, modern surface science is anchored on the ability of novel microscopic and spectroscopic techniques to yield quantitative information about surface structure.

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 .

Ultrasonic attenuation anomaly of TlCaBaCuO at superconducting transition

Abstract For longitudinal as well as transverse waves, a frequency independent ultrasonic attenuation anomaly was observed at temperature close to the superconducting transition temperature T c (103K) of a polycrystalline TlCaBaCuO. Temperature derivative of shear wave velocity at 10 MHz shows a discontinuity around T c . These variations of attenuation and velocity near T c may be intrinsic to high temperature superconductors.

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.

Electron-Phonon Scattering and Structure Analysis

We have calculated the inelastic cross section of the Ni(001) surface phonons S4 and S6 for an extended energy range of 50–250 eV. The calculated results indicate regions of energy where the S6 mode has a cross section comparable to that of S4. The theoretical results are confirmed by measurements which observed the S6 mode at the predicted energies. The simultaneous measurement of S4 and S5 modes allows a discrimination between different structural models for the Ni(001) surface. From the S4/S6 intensity ratio, we determined a 1.7-3.3% contraction in the surface interlayer spacing of Ni(001).