Oxide electronics

Abstract:

The crystal structure of the newly discovered double-Hg-layer copper oxide superconductor (Hg, Pr)2Ba2(Y, Ca)Cu2O8-δ was studied as a function of chemical doping using neutron and electron diffraction and high-resolution transmission electron microscopy (HREM). Rietveld refinements of the structural parameters from neutron powder diffraction data indicate that the oxygen site O3 on the mercury plane is highly defective, being both partially occupied and displaced from the high-symmetry position. The variable concentration of oxygen vacancies partially compensates for the cation doping and, together with the O3 displacement field, makes some of the Hg atoms acquire an unusual pyramidal coordination. HREM images confirm that the structure is of the '2212' type, with very few defects. In some grains, faint superstructure reflections were evidenced by electron diffraction, suggesting that both the oxygen vacancies and the O3 displacement field may order at least on a local scale. © 1995.

Abstract:

We present a novel method for the investigation of surface acoustic wave (SAW) fields with nanometer resolution by a scanning acoustic force microscope (SAFM). The detection of ultrasound by a force microscope is connected with the nonlinear dependence of the force on the distance between the tip and the surface. Due to this nonlinearity there is an effective shift of the mean position of the tip if one modulates the gap distance by a propagating SAW. Furthermore, the surface charges influence the tip deflection. By operating the SAFM in the dynamic mode the wave amplitude as well as the charge distribution can be measured at common surface structures. The measurements were carried out by a contact and a non-contact mode scanning force microscope (SFM). The SAWs were excited by an interdigital transducer (IDT) on a YZ-LiNbO3 crystal cut at 30.75 MHz (contact) and 39.5 MHz (non-contact). The amplitude of the SAW was modulated by a low frequency (5 Hz and 50 kHz). The low frequency oscillation of the average of the tip-to-surface distance was measured by lock-in technique in dependence on the amplitude and the frequency of the SAW. The measurements of the amplitude of the SAW and the charge distribution were carried out within the IDT.

Abstract:

We have realized planar tunnel junctions using both Y123 and Y124 single crystals as base electrodes and Pb as counterelectrode. The crystals, with T c =90 K (Y123) and T c =80 K (Y124), have been chemically etched and natural barriers were formed by a short air exposure. We have compared the tunneling spectra obtained on both systems. The features at about±18mV of the Y123 compound are still observed in the Y124, while the low-bias structures at ±5 mV are not present in the tunneling characteristics of the Y124 system. Linearity of the background conductance is also observed, with less steep slopes in Y124 that might indicate a reduced interlayer coupling in this system. © 1994 Societá Italiana di Fisica.