Table of contents

It is a pleasure to introduce a first for Superconductor Science and Technology . This issue is devoted entirely to reviews and contributed papers written by workers from a single country - Japan. It was conceived as an opportunity for the Japanese superconductivity community to present to the world, at a scholarly and rigorous level, a collective overview of the state of the science in their country. The invitations to contribute were issued by the Japanese Regional Board of the Journal, under the leadership of the Japanese Regional Editor, Professor Koichi Kitizawa, and on behalf of the editorial staff of the journal, I would like to express thanks to him, to his team, and to all the authors, for the magnificent job they have performed. All the papers were completed on time, refereed, revised and delivered to IoPP precisely to the timetable promised by Professor Kitizawa nearly a year ago. Of course, content is far more important than timetable, and here too I must express my appreciation. Within the leaves of this issue is a balanced and complete view of recent Japanese achievements, and if I am to single out any particular feature, it must be the emphasis on applications. Throughout, this issue gives promises that there is a commercial future ahead for HTS technology: it does indeed, as Professor Kitizawa says `provide readers with valuable hints towards the opening of the bright millennium of superconductivity'.

Gordon B DonaldsonHonorary Editor

It is a great pleasure for the Japanese Editorial Board members to be able to contribute to this Japanese special issue of Superconductor Science and Technologywith the selected papers on latest advances: from materials development of the high-temperature superconductors, their basic superconducting phenomenology to the most recent efforts towards their power and electronics applications. Fifteen important achievements were selected as the best representation of the major recent developments in Japan. We would like to express our deep appreciation to the authors who all willingly accepted our request and put a lot of effort in preparing the articles on time in spite of the tight schedule.

Since the first practical application of the HTS started as power leads in liquid helium-less superconducting magnets two articles are devoted to the so called `dry magnets'. A few hundred magnets of this type have been produced in the last five years on a commercial basis and are now in use for various purposes. A 7 T magnet made of Bi-2223 HTS coil and a 15 T magnet made of LTS coil are included as typical examples of this successful application. Bi-2212 wires have proved to be suitable for coils generating super high fields above 20 T at low temperatures. Three articles describe conductors made from tape wires and developments in round and multi-layer wires. Other major examples of attempts of application of Bi-2223 wires are reviewed in two articles on transformers, SMES and magnets for magnetic separation and crystal growth of silicon.

Recently efforts have been directed towards fabrication of coated tape conductors with rare earth-123 in order to obtain better performances in high magnetic fields at 77 K. Two approaches are included: one using gas phase and the other using liquid phase.

Development of electronic applications has been slower than expected initially due to the difficulty of fabrication of good HTS Josephson junctions. Very recently, however, much better performances with higher reproducibility have been achieved for edge junctions as described in one of the papers. On the other hand, a lot of attention has been paid to the built-in Josephson junction behaviour in the highly anisotropic HTS. Two papers report the attempts to fabricate ultra-high frequency oscillators and single electron transistors (SET). A review describes the current status of passive device development.

Every new technology widely used now once experienced a period when a large technological gap had to be overcome prior to wide application. The Japanese Regional Board sincerely wishes that active exchange of information such as this special issue may provide readers with valuable hints towards the opening of the bright millennium of superconductivity.

Gordon B DonaldsonHonorary Editor Koichi KitazawaUniversity of Tokyo, Chief Regional Editor Norio KobayashiTohoku University Shuichi TaharaNEC Advanced Research Laboratory, Tsukuba Akira ShojiElectrotechnical Laboratory, Tsukuba Kazumasa ToganoNational Research Institute for Metals, Tsukuba Masato MurakamiSRL-ISTEC, Tokyo Izumi HirabayashiSRL-ISTEC, Nagoya Tsutomu HoshinoKyoto University, Kyoto Teruo MatsushitaKyushu Institute of Technology, Izuka

This paper reviews the progressive results of the vortex-matter phase diagram for the high-T_csuperconductor YBa₂ Cu₃ O_yin magnetic fields up to 30 T. Evidence of the first-order vortex-lattice melting transition and the second-order vortex-glass transition is demonstrated by resistivity and magnetization measurements. The feature of the novel second magnetization peak is summarized and the origin of the new line H *(T ), which is defined by the sharp onset of the second peak, is discussed on the basis of the field-driven disordering transition theory. Effects of the weak point disorder on the vortex-matter phase diagram are also studied by the irradiation of 2.5 MeV electrons. It is revealed that three transition lines of the vortex-lattice melting line H_m(T), the vortex-glass transition line H_g(T), and the field-driven disordering transition line H*(T) terminate at the critical point and divide the vortex-matter phase into three different phases: the vortex liquid, the Bragg glass, and the vortex glass. Interesting similarities of the vortex matter phase diagram between YBa₂Cu₃O_yand Bi₂Sr₂CaCu₂O_yare also discussed.

We have successfully demonstrated a 15.1 T liquid helium-free superconducting magnet with a room-temperature bore of 52 mm using a Nb₃ Sn/NbTi hybrid coil, Bi2223 current leads and two Gifford-McMahon cryocoolers. The magnet has 830 mm outside diameter, 1221 mm height and 720 kg weight. The magnet was cooled to 3.6 K in 114 h. A central magnetic field of 15.1 T was achieved in 38 min. The temperature of the coil increased to 5.7 K due to ac losses during the excitation, but it decreased to 4.0 K before reaching 15.1 T. The temperature of the coil remained at a constant value of 3.8 K over the 24 h of operation. The demonstration indicated the usefulness of a liquid helium-free superconducting magnet to generate high magnetic fields up to 15 T.

Refrigerator-cooled all high-T_csuperconducting magnets, i.e. coils and current leads, 4 T and 7 T types, have been developed. The 4 T type successfully produced a central magnetic field of 4.0 T over 30 min and 3.5 T for 48 h. High ramp rate excitation up to 0.35 T s^-1was achieved. The 7 T type magnet stably generated 7.1 T over 24 h and was excited at a rate of 7 T min^-1 . These ramp rates were 20-100 times faster than that of the refrigerator-cooled magnets using metallic superconductors. A new cooling scheme and stability criterion for conduction-cooled magnets are proposed.

High critical density (J_c ) over 500 kA cm^-2(at 4.2 K and 10 T) was obtained in Bi₂ Sr₂ CaCu₂ O_xmultilayer superconducting tapes processed by the pre-annealing and intermediate rolling (PAIR) technique. The PAIR process was effective in creating a dense core with lower carbon content, resulting in homogeneous chemical reactions, small amount of impurity phases and good alignment of superconducting grains. In production of long tapes, reducing the carbon content was the most serious challenge. A J_cvalue of 7.1 ×10⁵A cm^-2at 4.2 K in self-field was obtained in a 100 m long tape when the carbon content could be controlled to be 0.002 wt%. We fabricated a double-pancake coil using a 100 m long tape and tested it at 4.2 K in backup fields. The coil generated 2 T in a backup field of 4 T.

Recent achievements in the newly-developed Bi-2212/Ag round-shaped wire with tape-shaped multifilaments are summarized. The new round-shaped wire includes 126-1320 tape-shaped filaments with triple rotation symmetry, having a good crystal alignment of Bi-2212 in each filament. We refer to the new wire as ROSATwire , (ROtation-Symmetric Arranged Tape-in-tube wire). The ROSATwire fabrication process markedly improved productivity and lowered the cost, and also improved the transport J_cof the wire. The I_cand J_creached values greater than 340 A and 1000 A mm^-2 , respectively, at 28 T and 4 K. Insulated wires of over 400 m length, 2 mm in diameter, are now available and ready for magnet winding. Several solenoid magnets with a 130-160 mm outer diameter and 100-600 mm height are now under fabrication.

Bi-2212 is a very interesting superconductor from the point of view of high-magnetic-field generation because it has an extraordinarily high upper critical field. This paper reviews recent remarkable progress on Bi-2212/Ag superconductors and their applications to high-field magnets. Bi-2212/Ag multifilamentary and multilayer tape conductors with excellent performance in high fields were developed by the application of a powder-in-tube method and a coating method, respectively. High J_cvalues of (1.5-3) × 10⁵A cm^-2in a field of 30 T at 4.2 K were obtained for both multifilamentary and multilayer tapes. Pancake-type magnets were fabricated with these Bi-2212 tapes. The combination of Bi-2212 magnets and an 18 T conventional superconducting magnet generated a world record of 23.4 T at 4.2 K. A Bi-2212 magnet having a practical size of clear bore of 61 mm was also fabricated. This magnet generated 3 T in a backup field of 18 T at 4.2 K with a total field of 21 T. This 21 T magnet system has been operated many times since 28 September 1998, and the magnet system is now being used for experiments such as the J_c -Bmeasurements of superconductors. These results clearly demonstrate that a Bi-2212 conductor is very promising for high-field applications. One of the interesting applications of a high-field superconducting magnet is a high-field NMR spectrometer. The electric field versus current density (E -J ) characteristics and the persistent-current mode of magnet operation are discussed in connection with the NMR.

Various attempts for improving flux pinning properties of Hg- and Bi-based superconductors have been summarized in the present paper. For both materials, it has been shown experimentally that introduction of disorder as well as decreasing electromagnetic anisotropy, which can be achieved by a moderate chemical substitution, are effective for the improvement of their flux pinning properties.

In Japan, four projects for the application of Bi-based superconducting magnets to practical apparatus are currently underway. These projects involve the development of an insert magnet for a 1 GHz nuclear magnetic resonance spectrometer, a magnet for a silicon single-crystal pulling apparatus, a magnet for a magnetic separation system, and a 1 T pulse magnet for a superconducting magnet energy storage system. For example, the magnet for the silicon single-crystal pulling apparatus is of the class with stored energy of 1 MJ to be operated at around 20 K. This review focuses on the present status of the development of these magnets, followed by a discussion of the problems of the present superconducting tapes that need to be overcome for future applications.

Domestic and international cooperative projects for applications of high-T_csuperconducting (HTS) transformers have started in parallel over the world, taking into account the advantageous performances and merits. The applicability of high-T_csuperconductors to ac uses in the region of liquid nitrogen temperature is considered in relation to the practical critical surface for temperature and magnetic field. Recent activities for the applications of HTS transformers in Japan are briefly summarized. The first topics are the developments of a 800 kV A laboratory-test device cooled by subcooled liquid nitrogen and the research and development of a high-voltage-type device to be tested in a power grid and the second topic is a conceptual design study of 20 MV A class practical devices in underground distribution substations. These activities in Japan are compared with other progressing plans in the world. Finally, it is pointed out that the projects to develop the prototypes of HTS transformers are relevant to economic strategy for prior investment in future worldwide HTS industries.

It is now a widespread view that a high-J_csuperconducting wire with a high performance under strong magnetic fields at liquid-nitrogen temperature needs to be realized by the material Y-123. However, because of the weak linking nature of this material, the fabrication of the wire must be based on a coated-conductor process to achieve a highly textured Y-123 thin film on a long starting wire tape. Various attempts to obtain a textured substrate with sizable length have been made by various methods, with or without the aid of buffer layers. Approaches to the fabrication of an epitaxial thin film of Y-123 have also been made from vapour, liquid, or solid phases by different methods. Therefore, combinations of these proposed processes in each category have led to several promising on-going worldwide approaches to achieve the realization of practical coated superconducting wires in the near future. This review focuses on the most outstanding achievements in Japan.

The study of processing by liquid-phase epitaxy (LPE) is promising because the LPE process can provide rapid and thick film formation of RBa₂ Cu₃ O_7-(RBCO). The growth rate can reach 10 µm min^-1and films of more than 5 µm in thickness formed by the LPE process show a high critical current density of about 1 MA cm^-2 . To apply this process widely, a decrease of the processing temperature is required because it gives rise to a reaction between the solution and many of the substrate materials. Film formation on yttria-stabilized ZrO₂and silver substrates, which are more reactive than other materials such as MgO or SrTiO₃ , has been achieved by decreasing the temperature to below 920 °C, with the addition of BaF₂in the solution. This paper describes processing by LPE for forming RBCO films and efforts for practical application towards coated conductors and RF electronic devices.

This paper summarizes our recent investigations on the fabrication process, microstructure, and characteristics of high-temperature superconducting edge-type Josephson junctions with modified interface barriers. The junctions were fabricated by an in situedge fabrication process using an epitaxial insulator layer as an etching mask. The barriers were formed not by intentional deposition but by interface modification. The modified interface barrier material was identified as a Ba-based perovskite: Ba(Y_1-x Cu_x )O_y . The junctions showed resistively and capacitively shunted junction-like current-voltage characteristics and excellent uniformity. 100 junctions showed a spread in the critical current as low as 1 = 8% at 4.2 K and the junction characteristics remained the same after a high-temperature process at about 700 °C.

Recent observations on novel electromagnetic emissions from the microwave to THz range due to tunnel injection of quasiparticles into the c -axis direction of high-T_csuperconductors are briefly reviewed. The results for YBa₂ Cu₃ O_7-y , ErBa₂ Cu₃ O_7-yand Bi₂ Sr₂ CaCu₂ O_y(BSCCO) thin-film tunnel samples and BSCCO single crystals are reported. The observed emission phenomena are qualitatively different between the high-voltage injection regime (V >> /e , where is the gap or gap-sum parameter) and the low-voltage injection regime (V /e ). For V >> /e , to which regime most thin-film samples might belong, the nonequilibrium broadband emissions extending to the THz range due to the incoherent plasmon scattering are observable. For V /e , as realized in the case of a BSCCO crystal, in addition to the broadband emissions, a sharp microwave emission peak corresponding to the coherent Josephson plasma peak is observable.

We fabricated submicron-sized intrinsic Josephson junctions of Bi-2212 by the focused-ion-beam (FIB) etching method. The main result was a reduction of the in-plane junction area to 0.3 µm²by direct FIB etching with no degradation in the critical transition temperature (T_c ). At the current (I )-voltage (V ) characteristic of these stacks, the gap structure and the normal-state resistance are clearly observed by a reduction of the Joule heating. The critical current density is considerably suppressed, the hysteresis and multibranch structure are eliminated and a periodic structure of current peaks appears reproducibly on the I -Vcurves. The period Vof the structure is consistent with the Coulomb charging energy of a single pair, V= e /C , where Cis the effective capacitance of the stack. It is considered that this behaviour originates from the Coulomb blockade of intrinsic Josephson tunnelling in submicron Bi-2212 stacks. We demonstrated the junction operating up to 10 K for the stack with an in-plane size of 0.3 µm²and an elementary junction number of 50.

High-temperature superconducting (HTS) passive microwave devices, such as filters and antennas, are promising devices. In particular, HTS filters may be successfully marketed in the near future. Cross-coupled filters, ring filters, and coplanar waveguide filters are good options to reduce filter size. On the other hand, HTS patch antennas which can be cooled by a cryo-cooler are also promising devices as well, since they show higher efficiency than normal antennas. This paper examines the design process and filter properties of HTS filters as well as the gains, directivity, and cooling system of HTS patch antennas.