Jau-Han Chen

High quality step-edge substrates for high-Tc superconducting devices

Despite the significant progress in fabrication methods of step edge, the lack of reproducibility still hinders their use in more complicated systems. To pursue the high reproducibility and quality of step edge for high-Tc superconducting devices, we have developed the technique to fabricate high quality step-edge substrates with arbitrary step angles. We used two steps to improve the step ramp quality substantially. The surface microscopy of step substrates shows high uniformity with respect to any step angle. There are no needles, waves, trenches, cascades, or other flaws on these surfaces. Serial Josephson junctions and superconducting quantum interference device arrays were fabricated onto step-edge substrates. The step-edge devices exhibit excellent results.

Cross-Coupled YBCO Filters With Spurious Suppression Using Tap-Connection Technique and Skew-Symmetric Feeds

Narrow-band microstrip cross-coupled band-pass filters based on a quadruplet geometry are designed for wireless-communication applications. We have fabricated the high-Tc (HTS) superconducting filters by patterning doubie-sided deposited YBa2Cu3Oy (YBCO) films. The 4-pole 15-mm-square and 20-mm-square filters with skew-symmetric feeds are designed with center frequencies of 2.45 and 1.90 GHz, respectively. The inductive non-adjacent coupling between the resonators produces a pair of transmission zeros near the pass-band edge. Also observed is an additional pair of transmission zeros that enhance the out-of-band rejection characteristics, which is originated from the skew-symmetric-feed structure. Furthermore, a tap-connection technique by tapping the input/output microstrip lines at certain locations is applied to the band-pass filters so as to suppress the second and/or the third passbands, which are spurious and undesirable. In this paper, it is confirmed that tapping somewhere along the lambdainput/output microstrip lines with lambda /4 open-circuited stub or a lambda/2 short-circuited stub, where lambda refers to the guided wavelength at 2fo or 3fo, may effectively suppress harmonics at 2fo and/or 3fo by more than 20 dB. The realized YBCO filters show a low insertion loss with high spurious suppressions. The temperature-dependent microwave properties are discussed.

Simulation of the magnetic field due to defects and verification using high-Tc SQUID

Abstract The defect field due to a flaw in the conducting slab is studied numerically and experimentally in this report. It was found that the magnitude of the defect field exhibits a nearly exponential decrease with the increasing flaw depth, and the phase of the defect field shows a linear dependence on the flaw depth. The calculated defect field was compared with the results measured by using a high- T c SQUID.

Adjustable multilayer HTS filters

An adjustable multilayer high-temperature superconductor (HTS) bandpass filter has been fabricated. Taped comb-line filters with multilayer structures were designed, YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films deposited on LaAlO/sub 3/ were used to construct the packaged multilayer HTS filter. The electrical length of the resonators in the filter was smaller than a quarter-wave length. The frequency responses of the filter were measured at liquid nitrogen temperature 77 K. The insertion loss of the packaged filter was determined to be less than 0.5 dB. By mechanically adjusting the multilayer structure, the center frequencies of the filter changed from 1.78 to 1.92 GHz, and the variations of the bandwidth from 60 to 150 MHz were also obtained.

Nondestructive evaluation of flaws using SQUIDs

Abstract Using high-Tc SQUIDs, we have designed a nondestructive evaluation system to probe buried flaws. The frequency of the excitation magnetic fields was varied from 10 to 800 Hz and the magnetic fields generated from the eddy current was lock-in detected. The SQUIDs output data revealed the most effective excitation frequencies for the flaws with different depths and widths. The signal-to-noise ratio of the deep flaws can be further enhanced either by using higher excitation fields or using the SQUIDs with higher low-frequency sensitivity.

Fabrication and Properties of High-

We have successfully fabricated YBCO Josephson junctions and SQUIDs with variable thickness bridges. The variable-thickness bridges of YBCO thin film were fabricated by photolithography and Focused ion beam mill. The Josephson effects of variable thickness bridges were obtained in a 80 nm-thick-film of high-Tc YBa2Cu3Oy. The Shapiro steps were observed in single junction in the gigahertz range from 3.02 to 14.64 GHz. The voltage-current and voltage-field characteristics were measured in SQUID magnetometer. The SQUID shows a peak-to-peak voltage swing of 1.5 μV at 70 K. The properties of one junction and SQUID magnetometer have been investigated.

T_{\rm c}

We measured I-V curves and noise characteristics of high-T/sub c/ YBa/sub 2/Cu/sub 3/O/sub y/ step-edge Josephson junctions and serial junction arrays under microwave irradiation. The junctions were fabricated on step-edge MgO[100] substrates with low step angles (/spl sim/30/spl deg/). The junction array shows the resistively shunted junction (RSJ) behavior for at least 50 junctions and reveals a coherent phase locking under microwave irradiation. The variation of I/sub c/ was 16% for a 150-junctions array distributed along the step-edge line of 1.5 mm in width. The voltage noise, S/sub v/, of the serial junction array scales as the number of junctions, N. The value of S/sub IR/, (S/sub IR/=|(/spl delta/I/sub c//I/sub c/)||(/spl delta/R/R)|), for a single and 50-junctions obtained from the fluctuation measurement is consistent with the result derived from I/sub c/R/sub n//spl prop/(J/sub c/)/sup q/, with q=0.5.

YBCO Josephson Junction and SQUID With Variable Thickness Bridges by Focused Ion Beam

High-Tc Josephson junctions were successfully fabricated by focused ion beam (FIB) direct milling. The characteristics of the junction barrier were carefully controlled by tuning the thickness of the link region. The optimal remaining thickness for the YBCO is about 70‐80 nm even though the YBCO thicknesses are different. The temperature-dependence of the critical current provides good evidence of the superconductor‐normal‐superconductor weak link of the junctions. Although the flux-flow behavior increasingly dominates at lower temperatures, the voltage‐current shows resistively shunted junction-like characteristics at temperatures close to Tc. The Shapiro steps of a single junction irradiated with microwaves were measured, revealing strong Josephson effects in ion-damaged junctions. This method will be of great benefit to nanoSQUIDs for the detection of spin systems. (Some figures may appear in colour only in the online journal)

Coherent phase locking of high-T/sub c/ YBa/sub 2/Cu/sub 3/O/sub y/ Josephson junctions

A gate-voltage-controlled high transition temperature direct current superconducting quantum interference device (high-Tc dc SQUID) magnetometer was fabricated to study how the electric field effect affects the device’s voltage-flux and voltage-current characteristics. The magnetometer consists of a monolithic YBa2Cu3O7−δ superconducting film and bicrystal junctions covered by a polymethylmethacrylate film as the gate insulator. It was found that the magnetic flux modulated voltage swing of the SQUID magnetometer varies by 48% when the gate voltage is changed from −1.9 to 9.5 V. The variation in voltage swing is attributed to the change of effective capacitance in grain-boundary junctions.

High-Tc Josephson junctions fabricated by focused ion beam direct milling

The effect on effective area and voltage modulation depth of a serial superconducting quantum interference device (SQUID) magnetometer was simulated and observed. The variation in the effective area with the number N of serial SQUIDs was simulated. To reduce the beat phenomenon and optimize the serial SQUID magnetometer, the distance x between two adjacent SQUIDs should exceed 40 μm or even 65 μm, or dummy SQUIDs should be added at the edge of the serial SQUIDs. The optimal layout of a high-Tc serial SQUID array is discussed.