N. Inoue

Preparation of La-doped Yb-123 thin films for high-Tc devices

Abstract We fabricated superconducting La-doped Yb-123 thin films on LSAT substrates by a pulsed laser deposition technique. Simultaneous partial substitution of La for Yb and Ba is effective to prepare impurity-free Yb-123 thin films with a very smooth surface. Pure 123-type thin films with c -axis orientation were successfully prepared at substrate temperatures of 650–700 °C. These films exhibited T c (zero)s of 74–79 K. The present La-doped Yb-123 film is useful for fabrication of high- T c devices, particularly the upper layers of multilayer structures.

(Hg, Re)Ba2CaCu2Oy thin films with improved surface morphology

Abstract We have succeeded in fabricating smooth and flat epitaxial (Hg,Re)Ba 2 CaCu 2 O y thin films containing no appreciable pinholes and outgrowths. The films, with a thickness of approximately 75 nm, were prepared on (100) SrTiO 3 substrates (STO) by employing a two-step process, i.e., (1) preparation of thin (approximately 100 nm) Re 0.1 Ba 2 CaCu 2 O z precursor films using a pulsed laser deposition technique, and (2) heat treatments in Hg-vapor atmosphere. Homogeneous films with smooth surface have been obtained by simply using thin precursor films rather than thicker ones as previously employed (more than 200 nm). The average roughness R a is 3–4 nm and the maximum peak-to-bottom surface roughness was about 10 nm. As-fabricated films exhibited the highest transport J c value of 1.0×10 7 A/cm 2 at 77 K in a self-field, also indicating a substantial improvement.

A new process to fabricate (Hg,Re)-1212 thin films with smooth surface

Abstract We have succeeded in fabricating (Hg,Re)Ba 2 CaCu 2 O y ((Hg,Re)-1212) thin films with smooth surface by a new process. The general process for the films consists of depositing precursor films and subsequent annealing in a quartz tube. However, when the film thickness was more than 100 nm, submicron-size defects such as outgrowths and pinholes appeared remarkably in the films. To prepare thicker films, we developed a following new method; (1) fabricating rather thin films (less than 100 nm) by above-mentioned process, (2) repeating the same process on the films. 150-nm-thick (Hg,Re)-1212 films were fabricated by repeating the 75-nm-thick process. The surface of the obtained films was quite smooth. Submicronsize defect were not found apparently by a scanning electron microscope (SEM) and a transmission electron microscope (TEM) observation of the surface and cross-section.

Fabrication of (Hg,Re)Ba2CaCu2Oy thin films on LSAT substrates by long heat treatment

Abstract We have succeeded in improving the superconducting properties of (Hg,Re)Ba 2 CaCu 2 O y ((Hg,Re)-1212) thin films on (LaAlO 3 ) 0.3 –(SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates. The 300-nm-thick films were fabricated by repeating the two-step process. It was found that the formation reaction for the 1212 phase depends on the amount of Hg-vapor source or the Hg-vapor pressure. 220-nm-thick films were slowly grown on 80-nm-thick seed films in a low Hg-vapor pressure for a long duration of several tens hours (65 h). The films exhibited a flat surface and no appreciable outgrowths. Their T c values were 117–120 K and the J c values in a self-field were 4–6×10 6 A/cm 2 at 77 K. They were substantially higher than those of the films prepared by the previously reported process, i.e. rather fast growth in a high Hg-vapor pressure for several hours (5 h).

Influences of microstructures on superconducting properties in (Hg,Re)-1212 thin films

Abstract Superconducting (Hg,Re)-1212 thin films were successfully grown on SrTiO 3 (STO), LaAlO 3 (LAO) and (La,Sr) 2 AlTaO 6 (LSAT) substrates by a so-called two-step process. The films on STO, LAO and LSAT exhibited substantially different transport J c values of 9.0×10 6 , 2.2×10 6 and 3.6×10 6 A/cm 2 at 80 K in a self-field, respectively, while similar T c values of 112–116 K were observed. The microstructures of these thin films were examined by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The films were found to have different structural features and different types of defects. Based on the SEM and TEM investigations, the influences of the film microstructures on their superconducting properties are discussed.

Properties of (Hg, Re)-1212 bicrystal junctions with different misorientation angles

The relationship between the properties of bicrystal junctions with the Hg-based superconductor and the misorientation angle has been investigated. Smooth and homogeneous (Hg,Re)Ba 2 CaCu 2 O y ((Hg,Re)-1212) thin films exhibiting T C ( zero)= 114-116 K were fabricated on SrTiO 3 bicrystal substrates with various misorientation angles θ of 5-45°. In the low angle region up to 10°, the junctions exhibited flux-flow (FF) type current-voltage (I-V) characteristics. For the θ ≥ 20°, they exhibited resistively shunted junction (RSJ) like I-V characteristics. The 0 = 15° junctions had both FF and RSJ like characteristics. The critical current density (J C ) value decreased monotonically with an increase in 0 in the range form 0° to 24°. On the other hand, the J C of the junctions with θ ≥ 24° remained unchanged. A large I C R n product of 706 μV was recorded at 77 K in the 0 = 20° junctions.

Preparation and properties of (Hg,Re)-1212 Josephson junctions using STO and LSAT bicrystal substrates

Bicrystal junctions with Hg-based superconducting thin films were fabricated on SrTiO 3 (STO) and (LaAlO 3 ) 0.3 -(SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates. (Hg,Re)Ba 2 CaCu 2 O y ((Hg,Re)-1212) thin films with T c = 109-116 K were prepared on the bicrystal substrates with misorientation angles of 5-45° for STO and 24-36.8° for LSAT, respectively. For angles larger than 20°, the bicrystal junctions exhibited resistively shunted junction like current-voltage characteristics at 4.2 and 77 K. Scanning electron microscope (SEM) observation revealed that precipitates existed along the (Hg,Re)-1212 bicrystal boundary on STO, while no such defects were observed along the boundary on LSAT. The junctions on LSAT behaved as Josephson junctions with a more homogeneous current distribution, being consistent with the SEM results.

Microstructural relationships between 45° [001] tilt bicrystal substrates and epitaxial superconducting films

Abstract Microstructures of superconducting thin films epitaxially grown on both symmetric and asymmetric 45° cubic ceramic bicrystal substrates have been investigated by transmission electron microscopy. The grain boundary (GB) of the asymmetrically tilted SrTiO 3 (STO) substrate is straight with a single (1xa01xa00)/(1xa00xa00) facet, while that of the symmetric 45° bicrystal substrate is wavy and contains many asymmetric step-like (1xa01xa00)/(1xa00xa00) nanofacets. GBs of the superconducting films grown on such symmetric 45° bicrystal STO substrates are composed of only (1xa01xa00)/(1xa00xa00) facets, hence meander. All the results indicate that the interfacial energy of asymmetrical (1xa01xa00)/(1xa00xa00) is lower than the symmetric configuration (2xa05xa00)/(5xa02xa00) in the STO. A combination using asymmetric 45° bicrystal MgO substrates (another kind of cubic ceramic that has the same crystal geometric microstructure with that of STO and larger thermal expansion coefficient than that of YBa 2 Cu 3 O 7− δ (YBCO)) and enhanced lateral growth method, such as liquid phase epitaxy, enables us to obtain long single facets (about 10 μm long) in YBCO films.

Preparation of tetragonal La-Ca-Ba-Cu-O superconducting thin film by pulsed laser deposition method

Abstract (La,Ca)(Ba,La,Ca) 2 Cu 3 O z superconducting thin films were prepared by a pulsed laser deposition method. X-ray and electron diffractions revealed that the obtained thin films had the tetragonal “123” structure. Moreover, c -axis orientation and in-plane alignment were accomplished. The films showed T c (onset)=80 K and T c (zero)=76 K in resistivity measurement. The critical current density was about 2.8×10 6 A/cm 2 at 4.2 K in zero magnetic field. The surface of the films was more stable than that of a commercial YBa 2 Cu 3 O y film in water environment.

Noise properties of (Hg,Re)-1212 dc SQUIDs

Abstract We have fabricated Hg 0.9 Re 0.1 Ba 2 CaCu 2 O y ((Hg,Re)-1212) dc superconducting quantum interference devices (SQUIDs) using grain boundary junctions on SrTiO 3 bicrystal substrates and evaluated their flux noise. The large 1/ f noise observed in a dc bias scheme was dramatically decreased by using an ac bias scheme, and the white flux noise below 20 μ Φ 0 /Hz 1/2 was obtained. This noise is comparable to that for SQUIDs made from high-quality YBa 2 Cu 3 O y thin films. However, some devices showed a substantial residual low-frequency noise, indicating the contribution of the noise from vortices motion in the superconducting films and/or junctions.