J. G. Bednorz

Reproducible switching effect in thin oxide films for memory applications

Thin oxide films with perovskite or related structures and with transition metal doping show a reproducible switching in the leakage current with a memory effect. Positive or negative voltage pulses can switch the resistance of the oxide films between a low- and a high-impedance state in times shorter than 100 ns. The ratio between these two states is typically about 20 but can exceed six orders of magnitude. Once a low-impedance state has been achieved it persists without a power connection for months, demonstrating the feasibility of nonvolatile memory elements. Even multiple levels can be addressed to store two bits in such a simple capacitor-like structure.

Flux trapping and superconductive glass state in La/sub 2/CuO/sub 4-//sub y/:Ba

Susceptibility and magnetic-moment measurements from 1.9 to 35 K in magnetic fields up to 1.5 T in powder samples of La/sub 2/CuO/sub 4-//sub y/:Ba are reported. The diamagnetism observed in the zero-field--cooled state is considerably larger than under field cooling. The former is metastable like the magnetic moment induced after switching the field off. These observations indicate the existence of a superconductive glass state.

Current-driven insulator–conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals

Materials showing reversible resistive switching are attractive for today’s semiconductor technology with its wide interest in nonvolatile random-access memories. In doped SrTiO3 single crystals, we found a dc-current-induced reversible insulator–conductor transition with resistance changes of up to five orders of magnitude. This conducting state allows extremely reproducible switching between different impedance states by current pulses with a performance required for nonvolatile memories. The results indicate a type of charge-induced bulk electronic change as a prerequisite for the memory effect, scaling down to nanometer-range electrode sizes in thin films.

Susceptibility Measurements Support High-Tc Superconductivity in the Ba-La-Cu-O System

The magnetic susceptibility of ceramic samples in the metallic BaLaCuO system has been measured as a function of temperature. This system had earlier shown characteristic sharp drops in resistivity at low temperatures. It is found that, for small magnetic fields of less than 0.1 T, the samples become diamagnetic at somewhat lower temperatures than the resistivity drop. The highest-temperature diamagnetic shift occurs at (33 ± 2) K, and may be related to shielding currents at the onset of percolative superconductivity. The diamagnetic susceptibility can be suppressed with external fields of 1 to 5 T.

Microwave Magneto-Surface Impedance of High-Tc Superconductors

Magnetic-field-induced microwave absorption in sintered high-Tc copper oxide superconductors is found to be proportional to the surface area of samples and not to their volume, establishing that absorption takes place near the sample surface. The surface resistance and reactance increase with magnetic field as a result of energy loss through fluxons driven by superconducting currents. When the magnetic field is modulated, absorptive and dispersive signals arise from modulation of the complex surface impedance through processes induced by the changing magnetic fields.

Microwave study of the critical state in high-Tc superconductors

Abstract The signals obtained from microwave absorption at the surface of high-Tc superconducting oxides in a magnetic field depend sensitivily on the amplitude of the magnetic-field modulation and the sign of the field sweep. They are insensitive to the modulation frequency and the rate of the field sweep. These effects are shown to have their origin in the properties of the superconducting critical state with pinning and depinning of fluxons during a modulation cycle.

Flux Trapping and Superconductive Glass State in La2CuO4–y:Ba

Susceptibility and magnetic-moment measurements from 1.9 to 35 K in magnetic fields up to 1.5 T in powder samples of La2CuO4-y:Ba are reported. The diamagnetism observed in the zero-field-cooled state is considerably larger than under field cooling. The former is metastable like the magnetic moment induced after switching the field off. These observations indicate the existence of a superconductive glass state.

Screw dislocation mediated growth of sputtered and laser-ablated YBa2Cu3O7−δ films

By imaging the as-grown surfaces of sputtered and laserablated YBa2Cu3O7−δ films with scanning tunneling microscopy (STM), we have directly observed spiral-shaped growth terraces which emanate from screw dislocations. The density of screw dislocations was observed to decrease with increasing growth temperature and substrate misorientation. The surface structures observed by STM together with cross-sectional transmission electron microscope (TEM) images provide insights into the mechanisms of crystal growth operative during the formation of YBa2Cu3O7−δ films grown using these two widespread techniques.

Identification of epitaxial Y2O3 inclusions in sputtered YBa2Cu3O7 films: Impact on film growth

Y2O3 inclusions with typical sizes between 100 and 300 nm3, densely distributed (1016 cm−3) in sputtered YBa2Cu3O7 (YBCO) films on SrTiO3 substrates, have been identified by high‐resolution electron microscopy. The precipitates exhibit either cuboid or needlelike shapes and grow epitaxially within and on top of YBCO. The dominant orientation relationship corresponds to a situation where the two‐dimensional lattices are nearly coincident in the interfacial (001) plane. These precipitates may contribute to the generation of screw and edge dislocations. In addition, they provide a large number of potential pinning sites for magnetic flux lines, which may contribute to the observed high critical current densities.

Large electric field effects in YBa2Cu3O7−δ films containing weak links

The influence of applied electric fields on the transport properties of weak links in YBa2Cu3O7−δ films has been investigated. It is found that the critical temperature Tc0 of epitaxial films containing weak links can be reduced by 10 K with applied electric fields of 6 MV/cm. In one case, Tc0 shifts of up to 25–30 K were observed. These are by far the largest Tc shifts reported for any superconducting system.