E. Aharoni

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by in situ laser ablation deposition

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.

All YBa2Cu3O7 Josephson edge junctions with YBa2CoCu2O7 as the barrier/insulator

Abstract A systematic investigation of all YBa2Cu3O7 edge junction “shorts”, prepared with YBa2CoCu2O7 as the insulator but without the barrier fabrication step, using our in situ laser ablation deposition method is reported. We find that the coupling across these junctions varies significantly from a strong-link behavior (actual shorts) when the base electrodes are thick (≥0.8 μm), to a typical weak-link response when thinner bases are used (≤0.2 μm). A model for the structure of the junctions is presented where the base and the overlaying insulator layers are tapered at the edge. This leads to a wedge type barrier which is thicker when the base is thinner. Coupling occurs at the tip region of this wedge which implies a “wedge-edge” geometry for the present junctions.

Flux focusing in all high-Tc Josephson edge junctions and SQUIDs

Abstract Josephson edge junctions with adjacent superconducting pads exhibit strong magnetic flux focusing. A model for calculation of the focusing factor near the edge of a superconducting disk was carried out by integrating over the magnetic fields generated by the screening currents inside the disk. The fact that near the disk edge the screening currents are limited by the critical current density was taken into account. Good agreement was found between the measured and calculated magnetic flux focusing in the magnetic response of junctions and SQUIDs.

Infrared emission spectroscopy of YO in the laser ablation deposition process of YBa2Cu3O7−δ thin films

Abstract Plume emission spectra in the infrared are reported under the conditions of laser ablation deposition of YBa 2 Cu 3 O 7 thin films at the location of the substrates. Two prominent emission peaks of YO at 700 and 815 cm -1 were observed which correspond to the P and R branches of the electronically excited Δ states. No observable contribution to the infrared emission from the electronic ground state was found. It is suggested that the massive presence of electronically excited fragments at the surface of the growing film facilitates the in situ film deposition at substrate temperatures of 700 °C which are significantly lower than the temperatures needed in post-annealed films ( 900 °C).

Barriers formed by a plasma discharge process in all YBCO Josephson edge junctions

Abstract All YBCO Josephson edge junctions were prepared with a barrier formed by an rf plasma discharge under different conditions. The structure of the barrier and the transport properties of these junctions, have been investigated. Under typical plasma conditions of 30–60 s plasma of 50–80 W at 100 mTorr of CF 4 gas, and 500–600 °C, we found that the junctions behaved like weak links. The normal resistance was of the order of a few Ω, with a minimum at around 50 K, typical to localization versus temperature. The temperature dependence of the critical current, I c , was found to be I c ∞ ( T c − T ) 1.5 . This reflects the asymmetric structure of these junctions, in which only the base electrode is exposed to the plasma. To learn about the thickness of the barriers a separate experiment was carried out on ultra thin YBCO films of various thicknesses that were exposed to the plasma. It was found that under the above plasma conditions films thinner than 100 A have lost their superconductivity. This determines an upper limit on the thickness of the barrier in our junctions. The composition of the barriers was investigated by Auger spectroscopy performed on thicker films that were also exposed to the same plasma. It was found that the surface layer down to about 50 A has lost its 1-2-3 stoichiometry, and contained a significant amount of carbon. This indicates a barrier thickness of about 50 A, in agreement with the result obtained from the ultra thin films.

Barrier and electrode properties of high Tc Josephson junctions formed by a plasma discharge process

Abstract The influence of rf discharge in CF 4 and O 2 gases on the properties of thin YBCO films and edge junctions was investigated by transport measurements and Auger spectroscopy. It was found that oxygen plasma of short duration can serve as a cleaning step before the formation of the barrier. In contrast, CF 4 plasma tends to substitute oxygen in the superconducting film by fluorine which yield an insulating layer. A CF 4 plasma discharge of more than one minute turns the YBCO film irreversibly into an insulator. At shorter durations, the deterioration of the films is limited to a thickness of the order of few tens of angstroms.

In-Situ YBa 2 Cu 3 O 7 /SrTiO 3 /YBa 2 Cu 3 O 7 a-b Plane Josephson Edge Junctions

YBCO/SrTiO3/YBCO thin film edge junctions were prepared in-situ and characterized. The epitaxial growth of SrTiO3 on YBCO led to a sharp and well defined junction edge with a very high yield. Typical junctions showed critical currents up to 83 K, with Ic ∝ (1 — T/Tc)2 temperature dependence. Sharp Shapiro steps were observed under microwave radiation at temperatures up to 82 K. A typical diffraction pattern was found in the voltage response of the junctions to transverse magnetic field.

Properties of all YBa sub 2 Cu sub 3 O sub 7 Josephson edge junctions prepared by in situ laser ablation deposition

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by insitu laser ablation deposition

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.

Properties of all YBa2Cu3O7Josephson edge junctions prepared byinsitulaser ablation deposition

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.