Takato Machi

High critical current density of MgB2 bulk superconductor doped with Ti and sintered at ambient pressure

Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and Jc of the samples change significantly with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2×106 A/cm2 in the self-field and 5×104 A/cm2 in 5 T. At 20 K, the Jc is still as high as 1.3×106 A/cm2 in the self-field and 9.4×104 A/cm2 in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB2 matrix, and this may play an important role in flux pinning enhancement.

63Cu(2) nuclear spin-spin relaxation in YBa2Cu3O6.98, YBa2(63Cu)3O7-δ, and YBa2Cu4O8

We have measured the temperature dependence of the transverse relaxation time of 63 Cu(2) nuclear spin in YBa 2 Cu 3 O 6.98 , YBa 2 ( 63 Cu) 3 O 7 , and YBa 2 Cu 4 O 8 . After subtraction of the T 1 process, the decay curve of the spin-echo envelope for YBa 2 Cu 4 O 8 in a strong H 1 followed a single Gaussian form from 280 K to 4.2 K, while for YBa 2 Cu 3 O 6.98 it follows only above T c . The absolute value and the temperature dependence of the decay rate 1/ 63 T G could be understood by the combined contributions of the direct dipole-dipole and the temperature-dependent indirect nuclear spin-spin coupling via χ( q ). From the temperature dependence of 1/ 63 T G for YBa 2 Cu 4 O 8 below T c , we considered that χ( Q ) even below T c remains enhanced. In a viewpoint of the 63 Cu(2) nuclear spin-spin relaxation experiments, so far as YBa 2 Cu 4 O 8 is concerned, the high- T c superconductivity seems to exhibit the d -wave pairing.

Nanoparticle structure of MgB2 with ultrathin TiB2 grain boundaries

The microstructure of the Ti-doped MgB2 which shows a significantly improved critical current density, Jc [Appl. Phys. Lett. 79, 1154 (2001)], is investigated. It is found that Ti does not occupy the atomic site in the MgB2 crystal structure, but forms a thin TiB2 layer (with a thickness about one unit cell of TiB2) in the grain boundaries of MgB2. Besides, MgB2 grains are greatly refined by Ti doping, forming a strongly coupled nanoparticle structure. It is argued that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take responsibility for the enhancement of Jc in the Ti-doped MgB2 bulk superconductor.

Nuclear spin-lattice relaxation and Knight shift in YBa2Cu4O8

Abstract The temperature dependences of the 63Cu nuclear spin-lattice relaxation rate 1/T1 and Knight shift K in YBa2Cu4O8 have been measured by nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) techniques. In the normal state, both the 1 T 1 for the Cu(2) sites and the in-plane component of the Knight shift K2⊥ increase rapidly with temperature from Tc up to about 250 K. Approximate T-linear temperature dependence of 1 T 1 was observed in a wide temperature range between 250 and 680 K, that is in general accord with previous analyses.

Doping effect of Zr and Ti on the critical current density of MgB2 bulk superconductors prepared under ambient pressure

Abstract The doping effect of Ti and Zr on the superconducting properties and the microstructure of sintered MgB 2 bulk materials has been studied. The transition metal is found to work as a sintering assistant, which significantly enhances the grain connection of the MgB 2 bulk materials during sintering processing. With this technique, we have fabricated Ti-doped MgB 2 bulk materials consisting of fine 10 nm scale particles and achieved a high J c over 1 MA/cm 2 in 0 T at 20 K. The large enhancement of J c is explained by the excellent connection between grains and a high density of pinning centers served by grain boundaries and MgO nanoparticles.

Pseudo spin-gap in single-layer High-Tc Cu oxide HgBa2CuO4+δ

We observed the pseudo spin-gap behavior of spin excitations in lightly doped single- CuO 2 -layer HgBa 2 CuO 4+δ ( T c =50, 72 and 80 K) using the Cu NMR spin-echo technique. The results indicate that lightly doped high- T c Cu oxide inherently possesses the pseudo spin-gap.

Pseudo Spin-Gap Spectrum in the Monolayer HgBa2CuO4+δ

We report the doping dependence of the planar 63 Cu Knight shift ( 63 K ), the nuclear spin-lattice relaxation rate 63 (1/ T 1 T ) and the Gaussian spin-echo decay rate 63 (1/ T G ) for the high- T c monolayer HgBa 2 CuO 4+δ over a wide doping range [ T c =50, 72, 82 K (underdoping), 96 K (optimal doping) and 30 K (overdoping)], measured by the Cu NMR spin-echo technique. We found the following new facts; (1) both the static and the dynamical spin susceptibilities exhibit the pseudo spin-gap behavior over a wide q region around the staggered mode Q , (2) the dynamical pseudo spin-gap in 1/ T 1 T can mainly be ascribed to the static one in 1/ T G , and (3) a spin fluctuation amplitude probed by the empirical ratio T 1 T /( T G ) 2 is a monotonic decreasing function of the doping level but not proportional to T c . From comparison with La 2- x Sr x CuO 4 , we discussed the role of the pseudo spin-gap in the high- T c superconductivity.

Zn-neighbor Cu NQR in Zn-substituted YBa 2 Cu 3 O 7 − δ and YBa 2 Cu 4 O 8

We studied local electronic states near Zn in optimally doped YBa 2 (Cu 1 - x Zn x ) 3 O 7 - δ and underdoped YBa 2 (Cu 1 - x Zn x ) 4 O 8 via satellite signals of plane-site Cu(2) nuclear quadrupole resonance (NQR) spectra. Fromthe relative intensity of Cu NQR spectra, the satellite signals are assigned to Zn-neighbor Cu NQR lines. The Cu nuclear spin-lattice relaxation time of the satellite signal is shorter than that of the main signal, which indicates that the magnetic correlation is locally enhanced near Zn both for the underdoped and the optimally doped systems. The pure YBa 2 Cu 4 O 8 is a stoichiometric, homogenous, underdoped electronic system; nevertheless, the Zn-induced inhomogeneous magnetic response in the CuO 2 plane is more marked than that of the optimally doped YBa 2 Cu 3 O 7 - δ .

Influence of Bending and Torsion Strains on Critical Currents in YBCO Coated Conductors

A YBCO coated conductor is expected to be a next generation high performance superconductor. To design superconducting machines using YBCO conductors, evaluation of not only superconducting properties but also mechanical properties is very important. Hence, we applied bending and torsion to the YBCO conductor and measured degradation of the conductor against the strain. To compare with the YBCOs data, a Bi2223 conductor was also tested under the same experimental condition. From the bending test, degradation of the Bi conductor was faster than that of the YBCO conductor and the critical current gradually decreased with increasing bending strain. The YBCO conductor kept better performance than the Bi conductor, and degradation suddenly occurred. And when the superconducting layer in the YBCO conductor was subjected to compressive strain in the bending test, the critical current hardly decreased at approximately one percent of the bending strain.

Irreversibility line for a Pb‐doped Hg‐Ba‐Ca‐Cu‐O superconductor

By measuring the magnetization hysteresis, we studied the critical current density, Jc, and irreversibility fields, Hirr, of a c‐axis aligned Hg‐Ba‐Ca‐Cu‐O sample doped with Pb containing the Hg‐1223 phase as the main phase. Anisotropy in ΔM was clearly observed in two different magnetic field directions. Even though the Hg‐1223 phase contains three CuO2 sheets, the Hirr vs [1‐(T/Tc)] curve was located at roughly the same positions as those reported for the Hg‐1201 and Hg‐1212 phases, indicating that the higher Tc phase, i.e., Hg‐1223, is advantageous to the other phases in practical uses.