Ushio Kawabe

Observation of Barrier Recrystallization Process and Properties of Ramp-Edge Josephson Junctions with Interface-Modified Barrier

We have studied the recrystallization process and the microstructure of the interface-modified barrier, and the electrical properties of Josephson junctions with the barrier. Electron cyclotron resonance (ECR) ion bombardment was used for making an amorphous layer at the YBa2Cu3O7� � (YBCO) film surfaces. Then, the samples were annealed to form an interfacemodified barrier from the amorphous. A halo pattern showing the existence of an amorphous was changed to three types of patterns by changing the ECR and annealing conditions. The YBCO surface damaged at an ion acceleration voltage of 700 V showed clear recrystallization to YBCO. The surface damaged at lower voltages showed reflection high energy electron diffraction (RHEED) patterns different from that of YBCO. In these cases, regions with a cubic or pseudo-cubic structures were observed at the interface by transmission electron microscopy (TEM). Ramp-edge Josephson junctions with an interfacemodified barrier have been fabricated. The junction properties strongly depend on the distance between the sample and the laser plume when upper layer YBCO is deposited. The junctions fabricated under optimum conditions exhibited resistively and capacitively shunted junction (RCSJ)-like I–V curves with a typical IcRn product at 4.2 K of 1.0–3.2 mV. We show that the recrystallization of an interface-modified barrier from an amorphous layer strongly depends on the kinetic energy from laser plume plasma as well as the thermal energy from substrate heating, and the Ic values of the junctions strongly depend on the actual bombardment energy of ions incident on the ramp surface. The difference of Ic may be explained in terms of the difference of the barrier microstructure which depends on the ion bombardment energy. [DOI: 10.1143/JJAP.41.1998]

Evaluation of fabrication process for interface-modified ramp-edge junctions

Abstract We have examined the parameters in the fabrication process of interface-modified ramp-edge junctions which influence their Ic spread. The distribution of ion-beam energy and the uniformity of ion beam for making an amorphous layer and the base-electrode film quality such as the density of outgrowth or particles and the roughness of the ramp surfaces are important factors determining the Ic spread. We have found another important factor, that is, the recrystallization energy from a laser plume. By adding proper adjustment of the relative location between substrate and the laser plume, Ic spread values of 5.7% and 7.3% have been obtained for arrays of 100 JJ and 1000 JJ, respectively.

Properties of ramp-edge Josephson junctions with a surface-modified barrier fabricated by high-rate PLD

Abstract YBa 2 Cu 3 O 7− δ (YBCO) ramp-edge Josephson junctions have been fabricated by pulsed laser deposition (PLD) with a relatively high deposition rate (∼50 nm/min). 200 nm thick YBCO films deposited on LaSrAlTaO x (LSAT) substrates in a PLD system with laser-beam scanning exhibited a typical T c of 88∼90.5 K. LSAT/YBCO bilayers were fabricated and then patterned to form a ramp structure. Electron cyclotron resonance ion bombardment and subsequent annealing in a various atmosphere were employed to form an amorphous layer and a recrystallized barrier layer on the ramp surface. Junction properties strongly depend on distance between the sample and the laser plume when upper layer YBCO is deposited. The junctions fabricated under optimum conditions exhibited resistively and capacitively shunted junction-like I – V curves with a typical I c R n product at 4.2 K of 1.0∼3.2 mV.

Magnetic imaging of high-Tc thin films and devices by scanning SQUID microscope

Abstract Magnetic field images of high- T c thin film patterns with various size and a single-flux-quantum (SFQ) sampler chip cooled in a weak magnetic field have been taken by a scanning SQUID microscope system in which the sample temperature can be varied between 4 and 100 K. The images of flux quanta trapped in (Hg, Re)-1212 and Nd-123 thin film patterns are clearly observed in a wide temperature range. The critical field as a function of line width for complete flux expulsion is confirmed to agree well with Clems prediction. In the magnetic images of the sampler chip, some features peculiar to its multilayer structure are found.

Observation of barrier recrystallization process and properties of interface-modified ramp-edge Josephson junctions

Abstract The recrystallization process of the interface-modified barrier (IMB) has been successfully observed by reflection high energy electron diffraction (RHEED) and transmission electron microscopy. A halo RHEED pattern showing the existence of an amorphous phase was changed to three types of patterns by changing the ion bombardment conditions. Two cases are observed, one leads to recrystallization to YBa 2 Cu 3 O 7− δ , the other leads to two types of cubic or pseudo-cubic structure regions. Ramp-edge Josephson junctions with an IMB fabricated under optimum conditions exhibited RCSJ-like I – V curves with a typical I c R n product at 4.2 K of 1.0–3.2 mV. It is also shown that the recrystallization of the barrier from an amorphous layer strongly depends on the actual bombardment energy of ions incident on the ramp surface. The difference of I c may be also explained by the difference in the barrier microstructure which depends on the ion bombardment energy.

X-ray pulse-shape analysis on bridge-type microcalorimeters with Ti-Au transition-edge sensors

We have fabricated a bridge-type structure for microcalorimeters with superconducting transition-edge sensors (TES). Instead of open space under a SiNx membrane in conventional TES microcalorimeters, the bridge-type calorimeters have SiNx membrane floating on the Si substrate covered by a SiO2 layer with a small gap of 30 - 50 micrometer. The bridge-type structure ensures that the calorimeters are mechanically tough. In addition, the thermal conductance can easily be controlled by changing the width, length, or thickness of the SiNx bridge. The calorimeters, of which operating temperature is 0.43 K, consist of a Ti/Au bilayer TES and an Au absorber. The x-ray events were read out by a DC-SQUID current amplifier with a 200-series array of SQUIDs placed on a 4.2 K stage. By analyzing the output pulse shapes, it has been found that the pulses are put into two categories. One has a fast rise time of approximately 3 microseconds and two decay components with time constants of approximately 10 microseconds and about approximately 130 microseconds. Another one has a longer rise time of approximately 10 microseconds and a single decay component of approximately 130 microseconds. It is considered that the pulse shapes depend on the x-ray absorption positions and the heat-flow pass.

Post Filtering Technique for High-Resolution Magnetic Image of an HTS Scanning SQUID Microscope

We have developed a post filtering technique by the numerical analysis program in MATLAB to improve the high-resolution magnetic image of an HTS scanning SQUID microscope. The SQUID has a high-permeability flux guide protruded to a sample in air at room temperature. Software filtering has an advantage that an object to analyze can be freely selected in comparison with hardware filters. The magnetic image of such a high-resolution as we can discriminate the magnetic microstructure of a few micron-size magnetic particles was obtained by means of our post filtering processing. It took only 38 seconds in case of our PC with a 2GHz processor.