J. H. Lu

Characteristics of YBa2Cu3Oy step-edge Josephson junctions on MgO substrate

Abstract We fabricated and characterized YBa2Cu3Oy step-edge Josephson junctions and DC SQUIDs on MgO (001) substrates. Sharp steps on MgO (001) were obtained by the techniques of photolithography and Ar-ion milling with step heights varying from 300 to 350 nm. The current-voltage (I-V) characteristic of the junction is consistent with the resistively shunted junction (RSJ) model. We obtained Jc=1.0×105A/cm2 and IcRn=110 μV at 77 K. In addition to the conventional Shapiro steps resulting from the AC Josephson effect, half-integral steps were observed. The width of these half-integral steps was sensitive to the RF power and the magnetic field. DC SQUIDs have been operated below 78 K and the observed voltage modulation agrees with the expected value.

Microwave effects on YBa2Cu3Oy Josephson junctions with step-edge, bi-epitaxial, and sandwich geometries

Fabrication and characterization of YBa2Cu3Oy Josephson junctions with step‐edge, bi‐epitaxial, and sandwich geometries are performed in this study to investigate the effects of microwave irradiation on the I–V characteristics. I–V curves under microwave irradiation reveal Shapiro steps. The width of the constant voltage steps were modulated by the microwave power. The period of the magnetic induction ΔB detected from the V–Φ curves for superconducting quantum interference devices is temperature dependent. Additionally, Tc and Ic of the proximity sandwich junctions are enhanced by the microwave irradiation.

The observation of non-integral Shapiro steps of a high-Tc superconducting de SQUID

Abstract An unusual oscillatory behavior of the magnitude of non-integral Shapiro steps versus applied magnetic fields was observed, on a bi-epitaxial dc SQUID, illuminated by a microwave source. Preliminary results suggest that the expected position of the first Shapiro step was splitted into two steps; one is larger, while the other smaller than the magnitude of the expected step. A proposed reason for the observation of non-integral steps is attributed to the possible existence of a finite resistance, connected in series to one of the single Josephson junctions, in the asymmetric dc SQUID circuitry, causing a splitting effect of the Shapiro steps.

Observation of half-integer Shapiro steps in step-edge YBa2Cu3Oy Josephson junctions☆

Abstract Current-voltage (I–V) characteristics of step-edge YBa 2 Cu 3 O y junctions on MgO(001) substrates were measured over various temperature and frequency ranges to examine the effects of microwave irradiation and magnetic fields on the I–V behaviors of the junctions. In addition to the usual integer Shapiro steps, half-integer Shapiro steps were observed under microwave irradiation. Both the integer and half-Integer Shapiro steps were sensitive to the applied magnetic field and the microwave power. The results are discussed.

Critical Current Characteristics of Step-Edge YBa2Cu3Oy Weak Links

YBa2Cu3Oy (YBCO) junctions were fabricated across the step-edge on MgO (001) substrates. The line of the step-edge made an angle, a, of 90°, 45°, or 22.5°° with the crystal a-axis. The step heights were 120–350 nm. YBCO films were grown onto the MgO (001) (the step-edge substrate) using an off axis rf magnetron sputtering system. The YBCO films were patterned to a 10 jam narrow strip crossing the edges. The critical current densities of YBCO junctions at T/Tc = 0.9 varied from 103 — 105 A/cm2 depending on ratio of the step height to the film thickness. The I-V curves under microwave irradiation showed Shapiro steps, indicating weak-link behavior. Using the step-edge junctions, we fabricated and characterized junctions and SQUIDs.

As grown Bi2Sr2CaCu2Oy films by an off axis RF magnetron

Abstract Bi2Sr2Cu2Oy (BSCCO) films were in intu grown by an off axis rf magnetron sputtering. The sputtering targets were stoichiometric Bi2Sr2CaCu2O8 compounds. The sputtering gas was a maximum of O2 and Ar and the sputtering pressure varied from 50 - 100 mTorr. The films were grown at 570 – 600 °C. After growth the films were quickly cooled down to 300 K in 30 minutes. One found that the distance from the substrate to the target d is an important factor in affecting the deposition rate of the as grown BSCCO films. By reducing the distance d to 3 cm, we have achieved a deposition rate of about 1500 A per hour in a sputtering power of 60 watts and sputtering pressure of 70 mtorr. With optinum growth conditions where the sustrate temperature was kept at 580 – 600 °C and the pressure was kept at 70 – 80 mTorr, superconducting BSCCO films with zero resistan at 75 – 85 K can be reproduced.