G.J. Gerritsma

Controlled preparation of all high-Tc SNS-type edge junctions and DC SQUIDs

High-Tc SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa2Cu3Ox and PrBa2Cu3Ox. 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. Details on the fabrication and measurements are presented.

YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson ramp junctions

A detailed study of the fabrication process, current voltage (I‐V) characteristics, and Josephson and normal‐state properties of the YBa2Cu3Ox(YBCO)/PrBa2Cu3Ox(PBCO)/YBCO ramp junctions is presented. The I‐V characteristics can be well described by the resistively shunted junction model. It was found that the critical current Ic and the normal‐state conductance 1/Rn scale linearly with the junction area, whereas Ic, the excess current Iex, and IcRn products decrease with increasing barrier thickness. These junctions with cross‐sectional area A have a good controllability, low capacitance, and high values of IcRn and RnA products. The coherence length ξn of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of the microwave power as well as the modulations of critical current Ic(H) with applied magnetic field are shown. A modulation depth of more than 95% has been observed. Small proximity effect parameters and jun...

Optimization of a chemo-optical surface plasmon resonance based sensor

A surface plasmon based chemooptical sensor has been optimized by the use of computer simulation programs. Calculated and experimentally measured performances are in good agreement, showing the value of the simulation tool.

Characteristics of advanced YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox edge type junctions

This letter describes the scaling behavior and Josephson properties of improved YBCO/PBCO/YBCO edge‐type junctions. The critical current, normal‐state resistance, and IcRn product scale with barrier thickness and junction area. The coherence length of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of microwave power, as well as the current modulation with applied magnetic field, have been studied: well‐developed Shapiro steps at 10 GHz have been observed, and the modulation of Ic(H) shows Fraunhofer‐like behavior with 95% Ic suppression.

Ramp‐type junction parameter control by Ga doping of PrBa2Cu3O7−δ barriers

We analyzed the transport of charge carriers across PrBa2Cu3−xGaxO7−δ (PBCGO) barriers as a function of barrier thickness, Ga‐doping level, temperature, and bias voltage. It was found that by Ga doping, the normal state resistance (Rn) of the junctions was systematically increased, while the critical current (Ic) remained constant. We argue that pair transport takes place by direct tunneling, whereas the quasiparticles have access to channels formed by one or more localized states inside the barrier. By Ga doping the I cRn products were increased, up to 8mV at 4.2K for junctions with 8nm thick PrBa2Cu2.6Ga0.4O7−δ barriers.

Response of YBaCuO thin-film microbridges to microwave irradiation

Microbridges with widths of about 10 μm were lift‐off structured from rf‐sputtered YBaCuO thin films and irradiated with microwaves at different temperatures. The bridges contain only a few grains with a typical size of 4 μm and are fully c‐axis oriented. The observed current‐voltage characteristics exhibit sharp constant voltage steps up to 71 K. An oscillatory dependence of the critical current I0 and the step heights 2In is observed, clearly revealing Josephson‐like behavior. From the grain structure, the critical current, and the microwave response, it is very likely that grain boundaries as superconductor‐normal conductor‐superconductor (SNS) contacts dominate the behavior of the bridges.

Ramp type HTS Josephson junctions with PrBaCuGaO barriers

Ramp type Josephson junctions have been fabricated using DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as electrode material and PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// with x=0, 0.10 and 0.40 as junction barriers. Barrier thickness lie between 6-30 nm. Several junctions without barrier were made in order to find ways to minimize the damage of the ramp interface. In total about 40 chips were fabricated each containing several junctions and their I-V characteristics measured for various temperatures down to 4.2 K. Only those junctions showing clear RSJ-like curves were selected to be analyzed. In some cases we also measured I/sub c/ as a function of a small applied field and obtained a clear Fraunhofer pattern, but there is a tendency to flux trapping as evidenced by LTSEM. It was found at 4.2 K that the critical current density J/sub c/ scales with the specific resistance R/sub n/A as J/sub c/=C/sub bar/(R/sub n/A)/sup -m/ (m=1.8/spl plusmn/0.5). The barrier material dependent constant C/sub bar/ increases with x, whereas, for a given d, J/sub c/ is constant and R/sub n/A increases.<<ETX>>

Epitaxial YBa2Cu3Ox thin films on sapphire with a PrBa2Cu3Ox buffer layer

Highly c‐axis‐oriented YBa2Cu3Ox thin films have been grown on (1102) sapphire substrates by a modified off‐axis magnetron rf‐sputtering technique. To prevent the interaction and to improve the lattice match between film and substrate a thin layer of 10–60 nm of PrBa2Cu3Ox was used as a buffer layer. The diffusion process was effectively blocked and the epitaxy was enhanced. The Jc (77 K) has been measured to be 4×105 A/cm2 which is significantly higher than the Jc of films directly grown on Al2O3 (∼8×104 A/cm2). The surface resistance of such films at 11 GHz and 4.2 K was measured to be 80 μΩ for 50‐nm thin films. These results are comparable to the best results reported so far on sapphire with other buffer layers.

Preparation and properties of all high T/sub c/ SNS-type edge DC SQUIDs

High-T/sub c/ SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa/sub 2/Cu/sub 3/O/sub x/ and PrBa/sub 2/Cu/sub 3/O. 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 I/sub c/R/sub n/ 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 presented.

Transport processes in YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox ramp type Josephson junctions

A study of the YBCO/PBCO/YBCO ramp junctions with and without PBCO barrier shows that the Josephson and normal state behavior of these structures are determined by the thickness of the PBCO barrier and its nature. The boundary resistance and depression of the YBCO superconducting parameters near the interface do not strongly affect the junction characteristic. For thicknesses of 8 to 20 nm of the PBCO barrier the Josephson coupling is established through the high resistive barrier and the behavior of the junctions is better described by a SNINS model than by a SNS weak link model. Proximity effect, resonant tunneling and strong pair breaking mechanisms are discussed to explain the experimental characteristics. Good agreement with the experimental dependence of the IcRn product on the temperature and on the PBCO barrier thickness was obtained if a strong pair breaking mechanisms in the barrier is taken into account.