D. Terpstra

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.

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.

Transport processes and reduction of I/sub c/R/sub n/ product in YBa/sub 2/Cu/sub 3/O/sub x//PrBa/sub 2/Cu/sub 3/O/sub x//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-type Josephson junctions

The mechanisms of current passage and the causes of I/sub c/R/sub n/ (critical-current normal-resistance) product reduction of YBCO/PBCO/YBCO ramp-type junctions are analyzed. At PBCO (PrBa/sub 2/Cu/sub 3/O/sub x/) barrier thicknesses L=8-20 nm the junction characteristics are determined by the thickness of the PBCO barrier and its nature. The boundary resistance and depression of the YBCO (YBa/sub 2/Cu/sub 3/O/sub x/) superconducting parameters near the interface do not strongly affect the junction parameters. The behaviour of the YBCO/PBCO/YBCO junctions cannot be described by simple SNS (superconductor-normal metal-superconductor) weak-link or SIS (superconductor-insulator-superconductor) tunnel models. A strong pair-breaking effect and a one-center inelastic tunneling process are taken into account to explain the Josephson and normal state characteristics of these junctions.<<ETX>>

Multilayer studies and applications in template bi-epitaxial DC SQUIDS

Multilayer deposition for the creation of a well-defined grain boundary, based on different in-plane orientations of c-axis oriented thin YBa/sub 2/O/sub 2/Cu/sub 3/O/sub 7- delta / layers on a single substrate, has been performed on three different kinds of substrates: (1102)-oriented Al/sub 2/O/sub 3/,

Noise study of a high-Tc Josephson junction under near-millimeter-wave irradiation

Noise studies of both the dc and ac Josephson effects have been performed on a high‐Tc ramp‐type Josephson junction irradiated at 176 GHz. Well‐established analytical results for noise in overdamped RSJs are used to model the measured I‐V characteristics, and their agreement is excellent. Noise‐rounded I‐V curves at the critical current and the first and second Shapiro steps under coherent 176 GHz radiation have been studied in detail at several temperatures and rf power levels. The noise temperatures inferred from these simulations are close to the physical temperatures. An increase of noise temperatures at high radiation power levels is a result of radiation heating, which could be due to a bolometric effect.

Near‐millimeter‐wave response of high Tc ramp‐type Josephson junctions

We have studied the response of a YBCO/PBCO/YBCO ramp‐type junction to coherent radiation at 176 and 270 GHz. The I‐V characteristic of the junction closely resembles the prediction of the RSJ model. The I cR n product of the junction is 0.25 mV at 5 K. The millimeter‐wave radiation is coupled to the junction via a quasioptical structure that focuses the radiation onto the junction through a yttrium‐stabilized ZrO2 substrate. At 176 GHz, we have observed as many as six Shapiro steps at the maximum power level of our Gunn oscillator‐pumped frequency doubler. Shapiro steps are still clearly seen up to 65 K. The amplitudes of the zeroth, first, and second Shapiro steps, as functions of the square root of the radiation power, agree remarkably well with a Bessel function fit, indicating the junction is voltage‐biased at the radiation frequency (rf). At 270 GHz, due to a combination of the heavy rf loss in the ZrO2 substrate and the lack of radiation power, we have observed only the first Shapiro step.

Observation of the AC Josephson effect up to THZ frequencies in YBCO/PBCO/YBCO ramp-type Josephson junctions

We present the response to 100 GHz irradiation of high-Tc Josephson junction devices for mixer/detector applications in the (sub-) mm wave range. These devices consist of a YBCO/PBCO/YBCO ramp-type junction combined with a planar logarithmic periodic antenna. The critical current and the first two Shapiro steps modulate with 100 GHz power according to the resistively shunted junction (RSJ) model. At 10 K clear Shapiro steps have been observed up to a voltage of about 7.5 mV. This corresponds to phase locking of 3.6 THz AC Josephson oscillations by the 36th harmonic of the 100 GHz signal. The number of observed steps is currently limited by the available power, but they are present up to voltages strongly exceeding the IcRn product of the junction at all temperatures.

Fabrication aspects of microwave devices, including ramp-type high-Tc Josephson junctions and log-periodic antennas

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.

Vortex motion and Josephson effect in superconducting microbridges in

Microbridges with dimensions smaller than the effective penetration depth have been prepared in epitaxially grown

Response of YBCO/PBCO/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.