Properties of lift-off structured high Tc microbridges

Microbridges and DC SQUIDs (superconducting quantum interference devices) were fabricated, using a lift-off technique, from RF sputtered YBaCuO films on MgO single-crystal substrates. Microwave measurements at 9 GHz on microbridges and the magnetic-field dependence of their critical current reveal wide bridge behaviour up to temperatures near the maximum operating temperature of the bridge. Mostly, a linear dependence of the critical current on the temperature is found, which is connected with high intrinsic 1/f noise if the bridge is constant-current-biased slightly above the critical current. In some bridges and DC SQUIDs, regimes with a temperature dependence proportional to (1-T/Tc)1.5 are found. In this case the 1/f noise level is much smaller and SQUID modulation can be followed to about 65

Introduction of ramp-type technology in HTS quasiparticle injection devices

Injection of quasiparticles with an energy larger than the superconducting gap into a superconducting strip results in breaking of Cooper-pairs and hence the suppression of the superconducting properties. Experiments using planar injection devices made of HTS materials with various barrier materials showed current gains varying from 2 up to 15 at 77 K. By changing the junction size and therefore the superconducting volume the current gain could be increased. A further reduction of the junction volume is very difficult using the planar device geometry. However, by applying the ramp-type technology it is possible to reduce the junction volume by at least one order of magnitude and a further increase in current gain is expected. Another advantage of this technology is the formation of in-situ barriers and electrodes and hence a better control of the junction characteristics should be possible, also the compatibility with the processes involved making RSFQ devices can be interesting for later applications. We have fabricated ramp-type injection devices, using various types of barriers. Characterization of these devices has been performed and the results of these experiments will be presented and discussed

Preparation and properties of all high Tc SNS-type edge DC SQUIDs

High-Tc SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa2Cu3Ox and PrBa2 Cu3O. These layers are c-axis oriented, and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The IcRn product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. The voltage modulation for various bias currents investigated at 4.2 K noise measurements were performed. Details on the fabrication and measurements are presente

Multichannel heart scanner based on high-Tc SQUIDs

A 7-channel magnetometer for magnetocardiography based on high-T c SQUIDs has been realized. This magnetometer is used for test experiments in the development of a multichannel high-Tc SQUID based heart-scanner for clinical applications. The intrinsic noise level of the channels in the 7-channel system is typically 120 fT/¿(Hz) down to 1 Hz. Magnetocardiograms were recorded inside a magnetically shielded room. Introductory experiments were performed on the suppression of noise by combining magnetometers to form planar gradiometers. The noise suppression that can be established appeared to be limited by the imbalance of the gradiometric configuration, which is roughly 2%. This relatively poor balance of the system is caused by inaccuracies in the transfer functions of the individual SQUID magnetometers, and by deviations from the planar geometr

Fabrication of 45 degrees template grain boundary junctions using a CaO lift-off technique

45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO2 template layer and an YBa2Cu3O7 top layer. To minimize the damage to the MgO surface, which will occur if the CeO2 is structured using ion milling, the CeO2 layer has been structured using the CaO lift-off technique. Electrical measurements of these junctions as a function of temperature, microwave irradiation and magnetic field will be discussed in this paper

HTS quasiparticle injection devices with large current gain at 77 K

Recent progress on the development of planar QP-injection devices using YBCO and STO as an epitaxial injection barrier will be discussed. The main problem for HTS injection devices is to grow reliably a well defined, ultra-thin tunneling barrier suitable for QP tunneling. For this purpose, we used inverted cylindrical magnetron sputtering to first optimize the smoothness of our YBCO films by controlling tightly an relevant sputtering conditions. We are able to prepare smooth (001) YBCO films on (001) STO substrates on a routine basis with an average roughness varying between 1 and 2 nm. With these flat YBCO films both planar as well as grain boundary junctions were fabricated using epitaxial STO barriers between 2 and 8 nm thick and a 50 nm of Au counter electrode. Planar junctions with 6 nm STO barriers were in most cases fully insulating, in some cases, a current gain of up to 7.4 at 77 K was obtained. For 3 nm STO barriers, the highest current gain was 15 at 81 K. The injection results also show a scaling behavior with junction size. Based on the present materials development and device understanding, we consider a current gain of up to 20 at 77 K possibl

Fabrications aspects of microwave devices, including ramp-type high-Tc Josephson junctions and log-periodic antenna's

We describe the development of high-Tc Josephson junction devices for applications at millimeter wave frequencies. These devices consist of ramp type YBCO/PBCO/YBCO Josephson junctions that are equipped with a noble metal log-periodic antenna. Growth conditions of all layers, as well as etching, cleaning and annealing procedures are being optimized, to guarantee well-defined device properties. Lowering the deposition temperature of the thick PBCO layer strongly improved its isolating properties, which is of extreme importance for good reproducability of junction fabrication. Special attention is being focused on the optimization of the contact of noble metal to YBCO as well its adhesion to the substrate. Best results are obtained using sputtered gold contacts, after a soft Ar ion sputter clean treatment of the damaged YBCO surface, followed by an anneal procedure

Subharmonic Shapiro steps in high‐Tc Josephson junctions

We studied the response of high‐Tc biepitaxial grain boundaryjunctions to 100 GHz radiation in the presence of a magnetic field. Integer as well as subharmonic constant voltage steps are observed, even at one‐fifth of the voltage separation between integer (Shapiro) steps. Our results indicate that this behavior is due to the synchronized motion of Josephsonvortices along the junction. We show that this effect is directly related to the width of the junction, and not to an array of weaker regions in the barrier

Response of YBCO/PCBO/YBCO ramp type Josephson junctions to near MM wave irradiation

A high Tc Josephson device for high frequency detection applications is being developed, consisting of an YBCO/PBCO/YBCO ramp type junction and a broad band log-periodic antenna. In this contribution we present the response of such a device to (near) mm wave irradiation. Shapiro steps have been observed up to very high voltage values - nearly 4 mV at 10 K, at the maximum of the radiation power. The modulation of the step amplitudes shows very good resemblence with the predictions from the Resistively Shunted Junction model