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ABSTRACT: $^{75}$As and $^{45}$Sc NMR measurements unravel the electronic state for
Fe-based superconductors with perovskite-type blocking layers
Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ ($T_c^{onset}=47$ K) and
Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$ ($T_c^{onset}=41$ K). In
Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$, the nuclear spin relaxation rate
$1/T_1$ shows pseudogap behavior below $\sim80$ K, suggesting that the
electronic state is similar to that of LaFeAs(O,F) system with moderate
electron doping. The presence of the pseudogap behavior gives an interpretation
that the hole-like band (so-called $\gamma$ pocket) is located just below the
Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the
$\gamma$ pocket yields the suppression of the low-energy spin fluctuations. On
the other hand, in Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ satisfying the
structural optimal condition for higher $T_c$ among the perovskite systems, the
extrinsic contribution, which presumably originates in the Ti moment, is
observed in $1/T_1T$; however, the moderate temperature dependence of $1/T_1T$
appears by its suppression under high magnetic field. In both systems, the high
$T_c$ of $\sim40$ K is realized in the absence of the strong development of the
low-energy spin fluctuations. The present results reveal that the structural
optimization does not induce the strong development of the low-energy spin
fluctuations. If we consider that superconductivity is mediated by spin
fluctuations, the structural optimization is conjectured to provide a benefit
to the development of the high-energy spin fluctuations irrespective to the
low-energy part.
10/2012;
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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.
08/2012;
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ABSTRACT: The two most common types of MgB2 conductor fabrication technique - in-situ
and ex-situ - show increasing conflicts concerning the connectivity, an
effective current-carrying cross-sectional area. An in-situ reaction yields a
strong intergrain coupling with a low packing factor, while an ex-situ process
using pre-reacted MgB2 yields tightly packed grains, however, their coupling is
much weaker. We studied the normal-state resistivity and microstructure of
ex-situ MgB2 bulks synthesized with varied heating conditions under ambient
pressure. The samples heated at moderately high temperatures of ~900{\deg}C for
a long period showed an increased packing factor, a larger intergrain contact
area and a significantly decreased resistivity, all of which indicate the
solid-state self-sintering of MgB2. Consequently the connectivity of the
sintered ex-situ samples exceeded the typical connectivity range 5-15% of the
in-situ samples. Our results show self-sintering develops the superior
connectivity potential of ex-situ MgB2, though its intergrain coupling is not
yet fulfilled, to provide a strong possibility of twice or even much higher
connectivity in optimally sintered ex-situ MgB2 than in in-situ MgB2.
11/2011;
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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.
07/2011;
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ABSTRACT: To clarify cation nonstoichiometry in Y123, Y123 sintered bulks and Y123 thin films starting from various yttrium compositions were synthesized. These samples were annealed under moderately high P <sub>O2</sub> to promote yttrium substitution for barium-site. Y123 sintered bulks showed decreases in T <sub>c</sub> accompanying increases in a -axis length and decreases in c -axis length when they were synthesized from Y-excess composition or annealed at ~ 900°C under high P <sub>O2</sub> . Y123 thin films with various yttrium compositions showed different T <sub>c</sub> behavior against oxygen annealing conditions, which might originate from the cation nonstoichiometry. These results suggest the existence of Y-excess composition even in Y123. Therefore, more attention must be paid for the cation nonstoichiometry in Y123 materials.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: New two methods to fabricate Y123 melt-solidified bulks with excellent superconducting properties have been developed. One is the new crystal growth method from the seed crystal which was put its c -axis parallel to the top surface and the other is addition of high-temperature annealing process in moderately reducing atmosphere after the crystal growth. The former method increased high-J<sub>c</sub> regions and the latter enhanced J<sub>c</sub> particularly at positions far from the seed crystals and at low temperatures through suppression of yttrium substitution for barium in the present study. The field trapping property was also improved by introduction of the reductive annealing process prior to the oxygen annealing at low temperatures.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: Tri-axial orientation of twin-free powder of Y<sub>2</sub>Ba<sub>4</sub>Cu<sub>7</sub>O<sub>15-δ</sub> (Y247) and uni-axial orientation of various FeAs-based compounds were attempted using modulated rotating and rotating magnetic fields, respectively. Tri-axial orientation with high degrees of inplane and c -axis orientation of Y247 was achieved under magnetic fields above 5 T, while magnetic fields at which reduction of the degrees began were different. We found from these crossover fields that quantitative magnetic anisotropy could be obtained. This determination method is particularly useful in that single crystalline samples are not required. From the magnetic orientation using the rotating field, all the obtained FeAs-based compounds showed a hard axis of magnetization parallel to the c-axis direction. On the other hand, the degrees of c -axis orientation were dependant on the types of ions in the blocking layers, and almost complete c -axis orientation was achieved for the compounds with transition-metal ions containing 3d electrons. Magnetic orientation using powder samples is a strong tool for the clarification of physical properties of Cooper pairs and critical current properties in FeAs-based superconductors.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: Enhancement of J <sub>c</sub> has been always expected for Bi(Pb)2223 tapes for their extensive applications. Among several strategies to improve critical current properties, enhancement of T <sub>c</sub> by control of nonstoichiometric cation composition is one of the most promising ways. In the present study, we have attempted to enhance T <sub>c</sub> of silver-sheathed Bi(Pb)2223 tapes by control of starting compositions and heat-treatment conditions. The T <sub>c</sub><sup>onset</sup> of the Bi(Pb)2223 tapes starting from nearly stoichiometric cation composition was enhanced up to 115.2 K by post-annealing at 700 <sup>°</sup>C in moderately reducing atmospheres and generation of Pb3221 phase was suppressed during the annealing. Moreover, introduction of the pre-annealing process further improved T <sub>c</sub> after post-annealing. T <sub>c</sub><sup>onset</sup> up to 115.2 K with sharp superconducting transition was achieved in a multi-filamentary tape with nearly stoichiometric cation composition.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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06/2011: pages 57 - 65; , ISBN: 9781118144121
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ABSTRACT: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7 T at 2 K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter, framework which would be consistent with the recent theoretical model where the height of the P in the Fe–P–Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
Superconductor Science and Technology 12/2009; 23(2):022001. · 2.66 Impact Factor
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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. Comment: 6 pages, 4 figures
11/2009;
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ABSTRACT: We have investigated the intrinsic Josephson properties in slightly underdoped ( Hg , Re ) Ba <sub>2</sub> Ca <sub>2</sub> Cu <sub>3</sub> O <sub>y</sub> [Hg(Re)1223] intrinsic Josephson junctions (IJJs) with a dimension of 1.0×1.5×0.11 μ m <sup>3</sup> . 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 f<sub>p</sub> of the IJJs is estimated to be 1.3 THz from the fluctuation-free critical current density of 2.0×10<sup>5</sup> A / cm <sup>2</sup> , which is one of the highest among cuprate superconductors, reflecting the high T<sub>c</sub> 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.
Journal of Applied Physics 11/2009; · 2.17 Impact Factor
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ABSTRACT: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains. Comment: Published version
08/2009;
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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. Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jpn. No.12 (2009)
08/2009;
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ABSTRACT: We have fabricated Ag-free and Ag-added Dy123 cylindrical melt-solidified bulks with large c -growth region. Samples cut from c -growth region of each bulk exhibited high J <sub>c</sub> up to high magnetic fields and excellent field trapping properties. On the contrary, J <sub>c</sub> and field trapping properties in the a -growth region were suppressed by subgrain boundaries. Magnetic flux penetration along the subgrain boundaries was confirmed in magneto-optical images of the a -growth region. Both J <sub>c</sub> and field trapping properties of Dy123 bulks were improved by Ag-addition due to low level Ag substitution for Cu site in the CuO chain.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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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. Comment: 6pages, 4 figures
03/2009;
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S. Horii,
H. Kai,
M. Mukaida,
K. Yamada,
R. Teranishi,
A. Ichinose,
K. Matsumoto,
Y. Yoshida,
R. Kita, J. Shimoyama,
K. Kishio
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ABSTRACT: Anomalous changes in irreversibility lines (ILs) and critical current properties depending on growth conditions were demonstrated in ErBa <sub>2</sub> Cu <sub>3</sub> O <sub>y</sub> films with Ba–(Nb,Er)–O nanorods. Despite the formation of nanorods in the matrix at all growth temperatures (T<sub>s</sub>) , Bose-glass-like vortex pinning was strengthened with decreasing T<sub>s</sub> . In particular, the behavior of ILs can be understood in terms of the morphology of introduced nanorods, and T<sub>s</sub> strongly affects the vortex states. Our present study suggests the importance of optimization of growth conditions in the production process of rare-earth-based coated conductors containing nanorods as vortex-pinning centers.
Applied Physics Letters 11/2008; · 3.84 Impact Factor
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ABSTRACT: Effect of Ga substitution in the (Lu,Pr)<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (Pr :LuAG) scintillator was examined at the crystals grown by the micropulling down (mu-PD) method. Host luminescence due to an exciton localized around the Lu-Al antisite defect was suppressed by Ga admixture. Concentration dependence was investigated and suppression was observed even at the lowest Ga concentration of 5 mol%. Faster decay time as well as less intense slower components were obtained upon increasing the amount of Ga. In the thermostimulated luminescence measurement, the glow curve peaks are gradually shifted to lower temperatures with the gallium admixture, and noticeable intensity decrease was observed only for the (Lu,Pr)<sub>3</sub>(Ga<sub>0.2</sub>Al<sub>0.8</sub>)<sub>5</sub>O<sub>12</sub>. The undesired quenching effect of Ga on the Pr<sup>3+</sup> luminescence seems to be reasonably low up to (Lu,Pr)<sub>3</sub>(Ga<sub>0.2</sub>Al<sub>0.8</sub>)<sub>5</sub>O<sub>12</sub> sample. It is estimated to introduce nonradiative losses of less than 10% with respect to Ga-free Pr:LuAG. Bulk crystals were also grown by the Czochralski (Cz) method and these effects were confirmed.
IEEE Transactions on Nuclear Science 07/2008; · 1.45 Impact Factor
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S. Horii,
M. Takamura,
M. Mukaida,
A. Ichinose,
K. Yamada,
R. Teranishi,
K. Matsumoto,
R. Kita,
Y. Yoshida, J. Shimoyama,
K. Kishio
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ABSTRACT: Anomalous superconducting properties were demonstrated in anisotropic high-critical-temperature superconductors (HTSCs) by simultaneously introducing two different planar types of vortex-pinning centers. The nanostructure, magnetic field, and field-angle dependency of critical current properties for an a-axis-grown multilayered film consisting of superconducting and semiconducting layers were characterized. Semiconductive layers running perpendicular to CuO2 superconducting layers behave as novel two-dimensional vortex-pinning centers that reverse the typical magnetic response in the superconducting state observed for conventional HTSCs. The results indicate that the vortex behavior in nanostructured HTSCs can be controlled; therefore, significant potential exists for further improvement of the critical current properties.
Applied Physics Letters 03/2008; 92(13):132502-132502-3. · 3.84 Impact Factor
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ABSTRACT: The intrinsic effects of Ag-addition on crystal structure and superconducting properties of Y123 melt-solidified bulks have been investigated. Increased c-axis length and decreases of maximum Tc and carrier density caused by Ag-addition suggested the incorporation of Ag into the Y123 matrix, in which Ag+ was considered to substitute for Cu ion of Cu-O chain. It was also found that Ag-addition enhanced Jc properties all applied fields at 77 K. The distinct second peak of Jc observed for the Ag-added bulk suggested that the Ag-incorporated regions acted as effective pinning centers under disordered vortex lattice.
Journal of Physics Conference Series 03/2008; 97(1):012007.
