Yunsung Huh

Serial array high T/sub c/ SQUID magnetometer

SQUID magnetometers have been fabricated from series arrays of YBa/sub 2/Cu/sub 3/O/sub 7/ dc SQUIDs by pulsed laser film deposition and ion mill patterning techniques. The array was designed either in linear chain with flux focusers or in meander structure. The arrays consisted of 50-130 step-edge junction SQUIDs and each SQUID had inductance of 20-40 pH with 2-5 /spl mu/m junction width. A linear array of 50 SQUIDs showed a modulation amplitude of 150 /spl mu/V which corresponds to a gain of /spl sim/10. One of 80 SQUID meander arrays had a gain of as high as 50. These results provide feasibility of the series array of SQUIDs in the direct readout SQUID amplifier circuitry with wide bandwidth. Larger number SQUID arrays and processes to increase the gain to the theoretical value need to be studied.

Microwave coupling of the Josephson junction arrays with an external source

We have fabricated a YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) array of five Josephson junctions designed with additional inductive coupling lines (AICLs). The junctions in this array were dc-biased in parallel to oscillate coherently in-phase and integrated with AICLs to improve the impedance matching between this array and external circuits such as a waveguide. The effect of AICLs was investigated by studying its influence on the current-voltage characteristics of the Josephson junction array (JJA) under external microwave irradiations with frequency of 22 GHz. The experimental results show that the behavior of the Shapiro-steps depends upon the effective inductance of AICLs which affects the total impedance and microwave coupling. We were able to obtain an oscillation frequency of about 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions in the array at a temperature of 12 K. By calculating the total impedance of the array, we obtained the suitable parameters of the array for microwave coupling with waveguide for various values of inductance, ac frequencies and number of Josephson junctions.

Growth of NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films and fabrication of directly coupled dc SQUID magnetometer

Thin films of NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// were fabricated by off-axis rf sputtering and pulsed laser deposition (PLD) methods. The deposition parameters for thin film fabrication, such as gas pressure, substrate temperature, annealing conditions, and rf power or laser energy density were studied. Highly c-axis oriented thin films with the maximum T/sub c/ (R=0) of 87 K were obtained at the optimum deposition conditions of rf sputtering. It, however, was more difficult to optimize the PLD conditions. The directly coupled dc SQUIDs were fabricated on SrTiO/sub 3/ bi-crystal substrates using single layer NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// film grown by PLD method.

Surface Properties of a YBa2Cu3O7-δ Thin Film After Photo-Lithographic Treatment

The effect of photolithographic treatments on the surface of a YBa2Cu3O7-δ thin film has been investigated by measuring the electrical contact resistance and performing X-ray photoemission spectroscopy (XPS) experiment. Water plays an important role in increasing the electrical contact resistance of the YBa2Cu3O7-δ thin film, as compared with the developer and photoresist (PR); water removes oxygen from the surface and increases the contact resistance. The chemical interaction with the PR or developer is not a key element in increasing the contact resistance. XPS spectra show that the oxygen concentration of the surface of a YBa2Cu3O7-δ thin film is reduced after the photolithographic process.

Experimental study of quasiparticle tunneling in high Tc junctions

Abstract We have studied tunneling properties of YBa 2 Cu 3 O 7 -insulator-normal metal junctions by measuring the current-voltage curves, the low-voltage junction resistance as a function of temperature, and the dynamic resistance as a function of the junction bias voltage. The insulating barrier was either SrTiO 3 or AlO x and the normal electrode was Au, Ag, or Nb. The current-voltage curves and the temperature dependent junction resistance showed tunneling characteristics with semiconducting background. The dynamic resistance versus bias voltage had fine structures in low voltages which might indicate the superconducting gap.

Self-radiation power and microwave coupling of YBCO Josephson junction arrays on bicrystal MgO substrates

Abstract We have studied the microwave properties of the Josephson junction array by measurement of Josephson self-radiation power and Shapiro-steps. The serial–parallel and parallel Josephson junction arrays were prepared with and without additional inductive coupling lines. The maximum detected power of 22 GHz from the serial–parallel array is about 25 pW and its peak point revealed a good quality of frequency-locking and exactly satisfied the Josephson voltage–frequency relationship. Although the parallel array has similar physical properties of average critical current and normal resistance of Josephson junctions to serial–parallel array, the maximum detected power is only about several tens of fW. The received power of serial–parallel array is higher than that of the parallel array for the same intensity of irradiation power.

Direct-coupled high T/sub c/ DC SQUID gradiometer with single layer YBa/sub 2/Cu/sub 3/O/sub 7/

We have studied first derivative high T/sub c/ dc SQUID (Superconducting QUantum Interference Device) gradiometer from a single layer of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ thin film using pulsed laser deposition (PLD) and argon ion beam etching method. The planar gradiometer consists of two pickup loops coupled directly to dc SQUID with an inductance of about 20 pH in the center of the structure. The gradiometer contains step-edge type junctions of 3 /spl mu/m width and is made on the SrTiO/sub 3/ substrate of 12/spl times/25 mm/sup 2/ in size. The SQUID showed RSJ type junction I-V characteristics with T/sub c/ of 86 K and swing voltage of /spl sim/4 /spl mu/V at 79.3 K. The magnetometer was sensitive only to the gradient of the magnetic field with well balanced null response to uniform fields.

Effects of capacitive shunt on the current-voltage characteristics of high T/sub c/ Josephson junctions

We have studied hysteresis of the grain boundary YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions with Au film shunt capacitors. The capacitive shunt clearly developed hysteresis in the current-voltage curves of the junctions. However, the McCumber parameter /spl beta//sub c/, obtained from data was much smaller than the calculated values. The reason is believed to be due to the fact that the capacitive reactance was dominated by the kinetic inductance of the bridge and the series resistance of the Au film connecting the two capacitors structured on both sides of the electrodes. Using a superconducting capacitor built directly on the bridge will enhance the hysteresis. More study is needed on the inductive shunt in the junction.

Modulation of critical current and Shapiro step by current-injection based on YBCO bridge three terminal junction

Abstract Modulation of the critical current and Shapiro steps in the I-V characteristics of the granular YBCO cross-line type bridge junction by the injection current(Iinj) is presented. The modulation of Ic by Iinj is generally determined by the total current summation effect of the transport current and Iinj at the weak link. For the I-V characteristics in the presence of the external fixed microwave irradiation with Iinj, we found the flip-flop-like mode, integer steps and half integer steps were shifted by the modulation of Iinj.