Current-voltage characteristics, Josephson radiation spectra, and critical current versus magnetic-field dependences were measured in epitaxial, c-axis YBa2Cu3O7 step-edge Josephson junctions (SEJs) on SrTiO3 and LaAlO3 substrates with various step angles α. The results were correlated with microstructural data to determine the origin of the observed weak-link behavior. It was shown that on steps with α≳45° the SEJ is a series connection of two weak links unambiguously correlated with the occurrence of two 90° tilt grain boundaries. On steep steps, α≥70°, the boundary at the upper step edge has, on average, the (103) symmetry, while the lower one is predominantly of the basal-plane-faced (010)(001) type. Correspondingly, one link is weaker than the other, with the weaker link originating on the (010)(001) boundary. However, others have shown that analogous grain boundaries in planar (103) and biepitaxial a-axis/c-axis films do not exhibit a strong magnetic-field dependence of critical current, which is characteristic of a weak link. Hence, it is proposed that the weak-link behavior of boundaries on step edges originates from their defect structure.
Abstract. We have fabricated and characterized thin film YBa,Cu,O, step-edge microbridges for application in SQUIDS. Epitaxial YBCO films were pulsed-laserdeposited on SrTiO, and had T,'s cf 8 & ? 0 Y. Sharp slaps in SrTIO, we;* obtained by photolithographic techniques and Ar ion milling with step heights (h) between 100 and 250 mm. We investigated step-edge junctions (SEJ) with different film thicknesses (d) from 100 to 250 mm but with a constant raliodlh = 1. The width of the microbridges was 2-2.5 pm. The T,'s 01 the bridges were 6-85 K, depending on film thickness and the duty cycle during ion milling. The I-V curves were RSJ-like and clear Shapiro steps were observed. T h e junction normal resistance was independent of temperature. With applied magnetic field the critical current I , showed a T-dependent modulation. indicating that there was self-shielding and I C non-uniformity in the junctions. Low-frequency noise measured in RFSQUIDS was relatively low. W e showed that it was generated predominantly in the junctions.
Results are presented on the fabrication and characterization of high critical temperature Josephson junctions in thin films of YBa2Cu3O7−δ produced by the process of focused electron-beam irradiation using 350 keV electrons. The junctions so produced have uniform spatial current densities, can be described in terms of the resistive shunted junction model, and their current densities can be tailored for a given operating temperature. The physical properties of the damaged barrier can be described as a superconducting material of either reduced or zero critical temperature (Tc), which has a length of ∼15 nm. The Tc reduction is caused primarily by oxygen Frenkel defects in the Cu–O planes. The large beam currents used in the fabrication of the junctions mean that the extent of the barrier is limited by the incident electron-beam diameter, rather than by scattering within the film. The properties of the barrier can be calculated using a superconductor/normal/superconductor (SNS) junction model with no boundary resistance. From the SNS model, we can predict the scaling of the critical current–resistance (IcRn) product and gain insight into the factors controlling the junction properties, Tc, and reproducibility. From the measured IcRn scaling data, we can predict the IcRn product of a junction at a given operating temperature with a given current density. IcRn products of ∼2 mV can be achieved at 4.2 K. The reproducibility of several junctions in a number of samples can be characterized by the ratio of the maximum-to-minimum critical currents on the same substrate of less than 1.4. Stability over several months has been demonstrated at room and refrigerator temperatures (297 and 281 K) for junctions that have been initially over damaged and then annealed at temperatures ∼380 K. Junctions manufactured using conventional lithography (0.5 μm wide) and which are suitable for digital electronics (Ic=500 μA at 40 K) can achieve IcRn products of 650 μV at 40 K. The production of 100 of these stabilized junctions could be accomplished in ∼4 h of irradiation time. The IcRn scaling also indicates that junctions suitable for high sensitivity superconducting quantum interference devices (Ic∼100 μA) can be made with IcRn products of ∼120 μV at 77 K.
We fabricated and characterized microwave rf SQUIDs integrated into a planar, S-shaped λ/2 microstrip resonator. This 3 GHz resonator was fabricated from a pulsed-laser-deposited YBa2Cu3O7 epitaxial film. The SQUID structures incorporated double step-edge junctions and had a loop inductance of 120 pH. Such unoptimized SQUIDs operated between 4.2 and 85 K with dV/dΦ=18–20 μV/Φ0 at 77 K. At that temperature, the energy resolution of (8±2)×10−29 J/Hz above 0.1 Hz (in the best samples) was limited by the white noise, SΦ1/2=(7±1)×10−5 Φ0/Hz1/2. Optimization may increase dV/dΦ and improve the energy resolution by up to an order of magnitude.
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