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Search Results: 1 - 10 of 138836 matches for " K. Kishio "
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Permanent magnet with MgB2 bulk superconductor
A. Yamamoto,A. Ishihara,M. Tomita,K. Kishio
Physics , 2014, DOI: 10.1063/1.4890724
Abstract: Superconductors with persistent zero-resistance currents serve as permanent magnets for high-field applications requiring a strong and stable magnetic field, such as magnetic resonance imaging (MRI). The recent global helium shortage has quickened research into high-temperature superconductors (HTSs) materials that can be used without conventional liquid-helium cooling to 4.2 K. Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2) makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in this magnet is uniformly distributed, as for single-crystalline neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk magnet was detrmined. Because MgB2 is a simple-binary-line compound that does not contain rare-earth metals, polycrystalline bulk material can be industrially fabricated at low cost and with high yield to serve as strong magnets that are compatible with conventional compact cryocoolers, making MgB2 bulks promising for the next generation of Tesla-class permanent-magnet applications.
MgB2 bulk superconducting magnet
A. Yamamoto,A. Ishihara,M. Tomita,J. Shimoyama,K. Kishio
Physics , 2012,
Abstract: We have developed disk-shaped MgB2 bulk superconducting magnets (20, 30 mm in diameter, 10 mm in thickness) using the in-situ process from Mg and B powders and evaluated the temperature dependence of trapped magnetic field. A pair of two disc-shaped bulks of 30 mm in diameter and 10 mm in thickness magnetized by field-cooling condition showed trapped fields of 1.2, 2.8 and 3.1 T at 30, 20 and 17.5 K, respectively. High trapped field over 3 T was recorded for the first time.
Contrasting Pressure Effects in Sr2VFeAsO3 and Sr2ScFePO3
H. Kotegawa,T. Kawazoe,H. Tou,K. Murata,H. Ogino,K. Kishio,J. Shimoyama
Physics , 2009, DOI: 10.1143/JPSJ.78.123707
Abstract: We report the resistivity measurements under pressure of two Fe-based superconductors with a thick perovskite oxide layer, Sr2VFeAsO3 and Sr2ScFePO3. The superconducting transition temperature Tc of Sr2VFeAsO3 markedly increases with increasing pressure. Its onset value, which was Tc{onset}=36.4 K at ambient pressure, increases to Tc{onset}=46.0 K at ~4 GPa, ensuring the potential of the "21113" system as a high-Tc material. However, the superconductivity of Sr2ScFePO3 is strongly suppressed under pressure. The Tc{onset} of ~16 K decreases to ~5 K at ~4 GPa, and the zero-resistance state is almost lost. We discuss the factor that induces this contrasting pressure effect.
Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)
J. G. Naeini,X. K. Chen,J. C. Irwin,M. Okuya,T. Kimura,K. Kishio
Physics , 1998, DOI: 10.1103/PhysRevB.59.9642
Abstract: We report the results of Raman scattering experiments on single crystals of La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped (x =0.22). The spectra are consistent with the existence of a strong anisotropic quasiparticle interaction that results in a normal state depletion of spectral weight from regions of the Fermi surface located near the zone axes. The strength of the interaction decreases rapidly with increasing hole concentration and the spectral evidence for the pseudogap vanishes when the optimum doping level is reached. The results suggest that the pseudogap and superconducting gap arise from different mechanisms.
The Josephson Plasma Resonance in Bi_2Sr_2CaCu_2O_8 in a Tilted Field
Sibel P. Bayrakci,Ophelia K. C. Tsui,N. P. Ong,K. Kishio,S. Watauchi
Physics , 1999, DOI: 10.1209/epl/i1999-00564-y
Abstract: The dependence of the Josephson plasma frequency omega_p in Bi_2Sr_2CaCu_2O_8 on a tilted field H is reported. Measurements over a large range of B and tilt angle theta allow a detailed comparison with a recent calculation by Koshelev. With a slight modification of the model, close agreement is obtained. From the fits, we find values for the in-plane correlation length and the zero-field critical current density J_{c0} (4,600 A/cm^2 at 30 K). An analogy to Bragg diffraction is described, as well as a picture for the fractional-exponent behavior of omega_p versus H.
Oxygen isotope effect on the in-plane penetration depth in underdoped La_{2-x}Sr_{x}CuO_{4} single crystals
J. Hofer,K. Conder,T. Sasagawa,Guo-meng Zhao,M. Willemin,H. Keller,K. Kishio
Physics , 1999, DOI: 10.1103/PhysRevLett.84.4192
Abstract: We report measurements of the oxygen isotope effect (OIE) on the in-plane penetration depth \lambda_{ab}(0) in underdoped La_{2-x}Sr_{x}CuO_{4} single crystals. A highly sensitive magnetic torque sensor with a resolution of \Delta \tau ~ 10^{-12} Nm was used for the magnetic measurements on microcrystals with a mass of ~ 10 microg. The OIE on \lambda_{ab}^{-2}(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen mass dependence of the in-plane effective mass m_{ab}*. The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.
Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting layered pnictide oxide with a thick perovskite oxide layer
H. Ogino,Y. Matsumura,Y. Katsura,K. Ushiyama,S. Horii,K. Kishio,J. Shimoyama
Physics , 2009, DOI: 10.1088/0953-2048/22/7/075008
Abstract: A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by solid-state reaction. This material has an alternating layer stacking structure of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group of the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543 A, respectively. The interlayer Fe-Fe distance corresponding to the c-axis length is the longest ever reported in the iron-based oxypnictide systems. In both magnetization and resistivity measurements, the present compound exhibited superconductivity below 17 K, which is much higher than that of LaFePO and the highest in arsenic-free iron-based oxypnictide systems under ambient pressure.
Switching current distributions and subgap structures of underdoped (Hg,Re)Ba2Ca2Cu3O8+d intrinsic Josephson junctions
S. Ueda,T. Yamaguchi,Y. Kubo,S. Tsuda,J. Shimoyama,K. Kishio,Y. Takano
Physics , 2009, DOI: 10.1063/1.3236666
Abstract: We have investigated the intrinsic Josephson properties in slightly underdoped (Hg,Re)Ba2Ca2Cu3Oy [Hg(Re)1223] intrinsic Josephson junctions (IJJs) with a dimension of 1.0x1.5x0.11 um^3. The current-voltage characteristics of the IJJs exhibit clear multiple branches with subgap structures similar to those of other cuprate superconductors. The switching current distributions P(I) from the zero-voltage to the nonzero-voltage state in the current-biased IJJs agree well with the theoretical curves of the thermally assisted escape model at temperatures above ~5 K. The plasma frequency fp of the IJJs is estimated to be 1.3 THz from the fluctuation-free critical current density of 2.0x10^5 A/cm2, which is one of the highest among cuprate superconductors, reflecting the high Tc and a relatively low anisotropy of the Re doped Hg system. The P(I) gradually becomes independent of temperature below ~5 K, which suggests a crossover of the escape process from thermal activation to quantum tunneling at such a high temperature.
Universal relationship between crystallinity and irreversibility field of MgB2
A. Yamamoto,J. Shimoyama,S. Ueda,Y. Katsura,I. Iwayama,S. Horii,K. Kishio
Physics , 2005, DOI: 10.1063/1.1937991
Abstract: The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including carbon substituted samples was studied. The Hirr was found to increase with an increase of FWHM of MgB2 (110) peak, which corresponds to distortion of honeycomb boron sheet, and their universal correlation was discovered even including carbon substituted samples. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr can explain the large variation of Hirr for MgB2 bulks, tapes, single crystals and thin films.
Successive transition from superconducting to antiferromagnetic phase in (Ca_6(Al, Ti)_4O_y)Fe_2As_2 studied via ^{75}As and ^{27}Al NMR
T. Nakano,N. Fujiwara,S. Tsutsumi,H. Ogino,K. Kishio,J. Shimoyama
Physics , 2011, DOI: 10.1103/PhysRevB.84.060502
Abstract: An unusual successive phase transition from superconducting (SC) to antiferromagnetic (AF) phases was discovered via ^{75}As and ^{27}Al nuclear magnetic resonance (NMR) in (Fe_2As_2)(Ca_6(Al, Ti)_4O_y) with four (Al, Ti)O layers intercalated between FeAs planes. Although the spatially-uniform AF ordering is clearly visible from ^{27}Al spectra, the ordered moments are very small and the low-frequency fluctuation is much suppressed, contrary to existing pnictides with localized magnetic elements. Furthermore, the temperature (T) dependence of the fluctuation at both nuclei is very similar throughout the entire temperature range. These facts suggest that some hybridization between Ti and Fe orbitals induces a uniform electronic state within FeAs and (Al, Ti)O layers accompanied by the SC and AF transitions. The iron-based pnictide with Ti-doped blocking layers is the first high-T_c compound having metallic blocking layers.
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