Yoshihisa Soutome

Dual bias current operation of 2-SQUID directly coupled magnetometers

High-T/sub c/ directly coupled magnetometers made from thin films of single layer YBa/sub 2/Cu/sub 3/O/sub y/ deposited on bicrystal substrates were investigated. A series 2-SQUID structure with a common pickup coil was designed to increase voltage modulation depth (/spl Delta/V). A second bias current was introduced to compensate for the disagreement between the two optimal bias currents and the voltage peak positions. The distribution of the second bias current was controlled by selecting the proper electrodes formed on the pickup coil, thus eliminating the difference in the voltage peak positions. The value of /spl Delta/V was doubled. The white noise in the magnetometer decreased by using this 2-SQUID design, but the low-frequency noise increased slightly. In addition, another type of the 2-SQUID magnetometer was proposed. This magnetometer has SQUIDs with different inductances. The overlapped V-/spl Phi/ curve showed the modulation beat caused by the difference in V-/spl Phi/ periodicity, and a high /spl Delta/V was observed at several /spl Phi//sub 0/ intervals.

Magnetic field modulation of critical currents in YBaCuO co-planar Josephson junctions using focused ion beam

The YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// co-planar Josephson junctions by Focused Ion Beam were fabricated by changing the width and the thickness of a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film. The magnetic properties of the junctions were measured at different temperatures. In the junction with 300 nm thickness, the magnetic field modulation of the critical current at 4.2 K was not observed. Near the critical temperature of the junction, the magnetic modulation curve was similar to that of a dc-SQUID. From the SEM observation, some microshorts were observed on the junction. In the junction with 200 nm thickness, the I-V curve was changed from flux-flow junction type to RSJ type. The magnetic modulation curve of the 7 /spl mu/m wide junction at 4.2 K was similar to a Fraunhofer pattern. Thus it is considered that the coupling region was changed from a microshort to weak coupling region, when the thickness of the YBaCuO film was decreased from 300 nm to 200 nm.

Fabrication of YBaCuO junctions by the irradiation of focused ion beam

We have fabricated Josephson junctions in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBaCuO) thin films by the irradiation of focused ion beam (FIB). We used Be/sup 2+/ ions with the energy of 200 keV. When the fluence of Be/sup 2+/ ions was 1.2/spl times/10/sup 16/ ions/cm/sup 2/, the I-V characteristics of the junctions at 4.2 K showed RSJ-like characteristics with the excess current. The I/sub c/R/sub n/ product of the junctions was 1.1 mV at 6.3 K. Shapiro steps up to 20th step could be observed by the irradiation of 8.47 GHz microwave at 6.3 K. The magnetic modulation curve of the junction at 4.2 K was similar to the Fraunhofer pattern. These characteristics remained unaltered after preservation in a desiccator for 6 months.

Fabrication of YBaCuO/nonsuperconductive-YBaCuO/YBaCuO co-planar Josephson junction by focused ion beam

A thin film of YBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) grown on an MgO substrate damaged by a focused ion beam loses superconductivity due to abnormal growth. We have fabricated a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// co-planar Josephson junction whose coupling region is nonsuperconductive YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grown on an MgO substrate damaged locally by focused ion beam scanning. The I-V characteristic of the junction behaves like a flux flow type. At temperatures from 4.2 K to 60 K, Shapiro steps were observed for the junctions in which the length of the coupling region was from 0.2 /spl mu/m to 1 /spl mu/m. The highest I/sub c/R/sub n/ product of the junction was 5.0 mV. The critical current density decreased exponentially with the junction length. It is considered that nonsuperconductive YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grown on the damaged substrate works as a normal layer in an S-N-S junction. Also the junction responded to the magnetic field and behaved like an asymmetric dc-SQUID.<<ETX>>

Magnetic Field Properties of YBaCuO/nonsuperconductive-YBaCuO/YBaCuO Co-planar Josephson Junction using Focused Ion Beam

YBaCuO/nonsuperconductive-YBaCuO/YBaCuO co-planar Josephson junction using Focused Ion Beam was fabricated by changing the size of the junction, and the magnetic field characteristic of these junctions was measured. The magnetic field modulation of the critical current increased with decreasing junction width. When the thickness of a YBaCuO thin film was reduced from 300nm to 200nm, the magnetic field modulation of the 5μm wide junction changed from dc-SQUID like pattern to Fraunhofer diffraction pattern. It is considered that the some Josephson current paths existed on the surface of the junction using the thin film 300nm thick.

Fabrication and estimation of YBaCuO/nonsuperconductive YBaCuO/YBaCuO coplanar Josephson Junction by Focused Ion Beam

We have fabricated a YBaCuO coplanar Josephson junction whose coupling region was a nonsuperconductive YBaCuO grown on an MgO substrate damaged locally by a scan of Focused Ion Beam (FIB). Shapiro steps could be observed for junctions with 0.4 to 1 μm length of the coupling region. The coupling region was found to be an electrically normal conductor whose characteristic length was ≈ 0.1 μm. We found out that there were microscopic changes of stoichiomertic composition on the coupling region, and that nonsuperconductive YBaCuO thin film was the mixed crystal.