Manabu Hirai

Stereoselective synthesis of optically active perhydrofuro[2,3-b]furan derivatives

(1R,5S)-2,8-Dioxabicyclo[3.3.0]octan-3-one and its derivatives, important subunits in various biologically active natural products, have been synthesized based on a new approach using the asymmetric oxyselenenylation of 2,3-dihydrofuran as the key step.

Superconducting properties of the heavy-ions and neutron irradiated (Cu,C)Ba2Can−1CunO2n+4−δ (n=3, 4 and 5)

Abstract For further enhancement of critical currents density Jc and irreversibility field Hirr of (Cu1−xCx) Ba2Can−1CunO2n+4−δ ((Cu,C)-(n−1)n, n=3, 4 and 5) family, pinning centers were introduced by ion and neutron irradiation in these compounds. The polycrystalline samples were irradiated with 3.6×1017 neutrons/cm2 or with Au15+ ions (240 MeV energy) at a fluence of 1×1011 ions/cm2. Jc and Hirr were determined for the irradiated samples as well as unirradiated samples. The intragrain Jc was evaluated from M–H curves using Beans critical state model. In (Cu,C)-12(n−1)n, Jc (77 K, 1 T) shows a remarkable increase from 3.9×104 to 9.1×105 A/cm2 for neutron irradiated (Cu,C)-1234 and from 3.9×104 to 4.1×106 A/cm2 for heavy-ion irradiated (Cu,C)-1234. These results indicate the possibility of further enhancement of Jc and of achieving a very high Hirr of (Cu,C)-1234 in (Cu,C)-12(n−1)n family.

TlBa2Ca2Cu3Oy superconducting films on MgO with different morphologies

Abstract TlBa 2 Ca 2 Cu 3 O y (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 °C. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J c . Critical current density J c determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J c is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 °C. In these samples, the substrate’s flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 °C and showing rough surface and Ca-segregates, J c was lower. However, J c was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended.

TlSr2CaCu2Oy template for the growth of superconducting Tl(Ba, Sr)2Ca2Cu3Oy thin films on CeO2 buffered sapphire

Thin films of Tl(Ba, Sr)2Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Ce present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840–890 °C. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm−2 at 77.3 K and 0 T.

Electron-doped superconductivity in Sr1−xCaxCuO2−δ infinite-layer thin films

Abstract Thin films of infinite-layer compound Sr1−xCaxCuO2−δ have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed ( T c onset =42 and 50 K, T c (ρ=0) =11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity.

AC susceptibility and higher harmonics studies of heavy-ion irradiated CuBa2Ca3Cu4Oy bulk superconductor with highest irreversibility field above liquid-nitrogen temperature

Abstract A 50 μm thick slice of (Cu,C)Ba 2 Ca 3 Cu 4 O y (Cu:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au 15+ ions (10 11 cm −2 ). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample’s thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature.

Superconducting properties from AC susceptibility and harmonic generation in CuBa2Ca3Cu4Oy bulk superconductors

Abstract We measured for the first time the fundamental and higher harmonic components of AC susceptibility of CuBa 2 Ca 3 Cu 4 O y superconductor produced by the high-pressure technique, in various experimental conditions (temperature, DC magnetic field, AC field amplitude and frequency). The results are discussed qualitatively in the frame of the critical state and other models in the literature. By using a very simple frequency-dependent critical state model, we estimated the parameters of practical interest, critical current density and pinning potential at various DC fields and temperatures and the irreversibility field. We also extrapolated our estimations to both practical temperature of 77 K and longer time scale as of DC magnetization and transport measurements. The results appeared to be quite self-consistent.

Unconventional Variation of Tc in the Multilayered Cuprate Superconductor (Cu,C)Ba2Ca4Cu5Oy

We report the anomalous T c behavior in a five-layered compound of the cuprate superconductor (Cu,C)Ba 2 Ca 4 Cu 5 O y . The T c decreases once from 94.7 K (as-synthesized) to 93.3 K (annealed at 460 °C under N 2 flow), and then increases to 99.0 K (annealed at 530 °C under N 2 flow). We argue that this T c change reflects the CuO 2 planes governing the bulk T c , shifting from the three inner CuO 2 planes to the two outer CuO 2 planes. A model to explain this behavior is proposed, drawing two bell-shaped curves against the oxygen content (doping state), and the T c values change along the curves from that of the inner CuO 2 plane to that of outer one.

Intra- and inter-grain critical current density in (Cu,C):1234 superconductors

Abstract From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra- and inter-grain critical current density J c of (Cu,C):1234 high- T c superconductors, in the frame of critical state models. The inter-grain J c was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J c resulted from AC susceptibility measurements and of the impressive increase of intra-grain J c due to heavy-ion and neutron irradiation are presented.

Growth of TlBa2Ca2Cu3Oy superconducting thin film on CeO2 buffered sapphire substrate

We report the growth of TlBa 2 Ca 2 Cu 3 O y superconducting thin film on CeO 2 buffered r-cut sapphire substrate by amorphous phase epitaxy method, wherein an amorphous phase of a composition, TlBa 2 Ca 3 Cu 4 O y was deposited by rf magnetron sputtering and subsequently crystallized by annealing at high temperatures in a closed system. There exists an optimum thickness of about 200 A of CeO 2 buffer layer that gives a smooth buffer layer surface. At the temperature around 840 °C we get only Tl-2212 phase and for a longer annealing time it results in Tl-1212 phase. At higher temperatures Ba reacts with Ce to form BaCeO 3 . To avoid this reaction process we introduced another amorphous buffer layer, TlSr 2 CaCu 2 O y onto CeO 2 layer. By this method we could prepare a good quality Tl-1223 thin film which showed a T c of 104 K and a J c = 0.3 MA/cm 2 at 77 K and 0.1 T field.