N. Kameda

The University of Tokyo, Tokyo, Tokyo-to, Japan

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Publications (44)53.54 Total impact

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    ABSTRACT: The sample size effect on the flux pinning and vortex dynamics has been studied on micron-sized Bi2Sr2CaCu2O8+y crystals. The second magnetization peak (SMP) and the magnetic relaxation rate change with the sample size. A crossing point is found in field dependence of the normalized magnetic relaxation rate S for samples with different sizes. There is a sudden increase of S at the SMP field, which possibly originates from the propagation of phase boundary of vortex matter at the SMP field.
    08/2006; 850:789-790.
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    ABSTRACT: Nature of vortices in a highly anisotropic superconductor Bi2Sr2CaCu2O8+y is investigated in the presence of largely tilted magnetic fields. Direct visualizations of vortex distributions were performed under large in-plane fields up to 4000Oe by magneto-optical imaging technique. Our visualization clearly revealed inhomogeneous evolutions of the vortex penetration forming one-dimensional stripes. Comparative studies on the local magnetization revealed the existence of novel multiple peak structures in hysteresis loops. These peaks appear only in the increasing branch of the hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. The origin of the novel anomalies is discussed on the basis of inhomogeneous vortex distribution.
    Physical Review B 01/2005; 71(22). · 3.66 Impact Factor
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y (BSCCO) with spatially dependent interlayer phase coherence (IPC). The half-irradiated BSCCO (HI-BSCCO), in which columnar defects are introduced only in a half of the sample, shows several resonance peaks, which are not simple superposition of the peaks in irradiated and pristine parts. JPR in HI-BSCCO changes its character from irradiated to pristine type at a crossover frequency (ωcr). We demonstrate that the one-dimensional linearized sine-Gordon equation, which takes into account the spatial dependence of IPC, can reproduce most of the experimental findings including the presence of ωcr.
    Physical Review B 04/2004; 69(18). · 3.66 Impact Factor
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    ABSTRACT: In the highly anisotropic superconductor Bi2Sr2CaCu2O8+y, two kinds of vortices, pancake vortices (PVs) and Josephson vortices (JVs), coexist under tilted magnetic fields and form a novel ground state of vortex matter. Such a state is called the crossing-lattices state, where attractive interactions between PVs and JVs lead to various arrangements of vortices. We demonstrate the complicated phase diagram of the crossing-lattices state by means of micro-Hall probe magnetometry, magneto-optical observations, and Bitter decoration technique. The attractive interactions between PVs and JVs is also utilized to control the motion of PVs using JVs.
    Superconductor Science and Technology 01/2004; 17(2):S88. · 2.76 Impact Factor
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    ABSTRACT: Local magnetization hysteresis is studied in a highly anisotropic superconductor Bi2Sr2CaCu2O8+y under tilted magnetic fields using a micro Hall probe. At moderately high temperatures, we found novel multiple peak structures in the local magnetization hysteresis curves. These peaks appear only in the increasing branch of hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. Possible origins of the novel peak structures are discussed on the basis of the evolution of the peaks with temperature.
    Physica C Superconductivity 01/2004; s 412–414:440–443. · 0.72 Impact Factor
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    ABSTRACT: Magneto-optical observations of distribution of vortices and vortex-lattice melting transition fields are used to characterize the inhomogeneities in high-quality Bi2Sr2CaCu2O8+y single crystals grown by the floating-zone method. Two kinds of characteristic features are found; one with expansion of vortex liquid phase forming arclike regions, and another with linear structures which are observed only in the vortex solid phase. The former is found to be related to the compositional inhomogeneities of the crystal, while the latter is related to the structural defects, most probably small angle grain boundaries. The origin of different evolutions of the vortex liquid region upon increase in the applied field is explained based on the sample-size dependent field profile in the crystal.
    Physical Review B 03/2003; 67(10). · 3.66 Impact Factor
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y (BSCCO) with inhomogeneous phase coherence caused by partial introduction of columnar defects. In these BSCCO, we observed several resonances, which cannot be regarded as a superposition of those from constituent parts. By calculating linearized sine-Gordon equation, we can reproduce the above results as well as the existence of a crossover frequency (ωcr) at which the character of JPR changes from irradiated- to pristine-type in partially irradiated BSCCO.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2003; 388:423-424.
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    ABSTRACT: Phase diagram of a novel vortex state realized under tilted fields, crossing-lattices state, is extensively studied using Hall probe magnetometry. Precise measurements of angular dependence of second magnetization peak and the vortex penetration fields clarify the transition from the tilted lattice state to the crossing-lattices state. Under large tilted magnetic fields, irreversible magnetization and AC local permeability show several anomalies. With all these observations we construct a phase diagram identifying three boundaries with nearly constant c-axis fields and one with nearly constant in-plane field.
    Journal of Low Temperature Physics 01/2003; 131. · 1.18 Impact Factor
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y (BSCCO) with inhomogeneous phase coherence caused by partial introduction of columnar defects. We calculate the linearized sine-Gordon equation with characteristic length scale of λc in various electromagnetic configurations. The calculated results reproduce electromagnetic configuration and size dependence of JPR in inhomogeneous BSCCO.
    Physica C Superconductivity 01/2003; 392:378-381. · 0.72 Impact Factor
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    ABSTRACT: We visualize the vortex chains in Bi2Sr2CaCu2O8+y by Bitter decorations and differential magneto-optical observations. Bitter decorations on a sample with intentionally introduced field gradient enable us to determine the transition field from the vortex chain state to chain + lattice state. We also demonstrate using magneto-optical technique that we can control the density and the direction of vortex chains by the strength and the direction of the in-plane field. Precise determination of the spacing between vortex chains offers a unique way to quantitatively estimate the anisotropy parameter in highly anisotropic superconductors.
    International Journal of Modern Physics B 01/2003; 17:3401-3406. · 0.46 Impact Factor
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    ABSTRACT: Phase diagram of a novel vortex state realized under tilted fields, crossing-lattices state, is extensively studied using Hall probe magnetometry. Precise measurements of angular dependence of second magnetization peak and the vortex penetration fields clarify the transition from the tilted lattice state to the crossing-lattices state. Under large tilted magnetic fields, irreversible magnetization and AC local permeability show several anomalies. With all these observations we construct a phase diagram identifying three boundaries with nearly constant c-axis fields and one with nearly constant in-plane field.
    Journal of Low Temperature Physics 01/2003; 131(5):1003-1007. · 1.18 Impact Factor
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    ABSTRACT: The magnetic penetration depth $\lambda(T,H,j)$ was measured in the presence of a slowly relaxing supercurrent, $j$. In single crystal $\mathrm{Bi_2Sr_2CaCu_2O_8}$ below approximately 25 K, $\lambda(T,H,j)$ is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. The Campbell length depends upon the ratio $j/j_{c}$ where $j_{c}$ is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.
    Physical Review B 11/2002; · 3.66 Impact Factor
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    ABSTRACT: Vortex states under tilted fields in Bi2Sr2CaCu2O8+y are studied using differential magneto-optical technique. Vortex-lattice melting transition is found to be essentially the same in the two distinct vortex states, tilted lattice state and crossing-lattices state at small and large field angles from the c-axis, respectively. At relatively large in-plane field, stripe-like penetration of pancake vortices (PVs) is observed. We interpret this phenomenon as a result of easy channel for PVs on Josephson vortices in the crossing-lattices state.
    Physica C Superconductivity 10/2002; · 0.72 Impact Factor
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    ABSTRACT: We studied vortex states of Bi2Sr2CaCu2O8+y (BSCCO) in tilted-fields with utilizing a rotatable micro-Hall probe. Local magnetization measurements on highly-overdoped BSCCO clarify the existence of characteristic peak effects in tilted fields. On increasing tilting angle (θ), the center peak splits into two branches. Systematic studies on the angular dependence revealed that the additional center peak (H*cp) shows square root dependence as H*cpcosθ∝(H*cpsinθ)1/2. Assuming one-dimensional arrangement of pancake vortices (PVs) on Josephson vortices (JVs) for small Hcosθ, this square root dependence means that the H*cp takes place when the PV spacing on a JV approaches to the penetration length. Namely, this result shows the existence of the second lower critical field characteristic of vortex states in tilted fields.
    Physica C Superconductivity 10/2002; · 0.72 Impact Factor
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y with inhomogeneous phase coherence caused by partial introduction of columnar defects. We find that the resonance in the longitudinal configuration occurs at the same field as that in the transverse configuration, which excite plasma mode perpendicular to the boundary. Through systematic studies on sample size dependence of JPR, we find the existence of a characteristic length, Lcr at which the character of JPR changes from inhomogeneous to homogeneous type.
    Physica C Superconductivity 10/2002; · 0.72 Impact Factor
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    ABSTRACT: Recent advances in the energy resolution and cryogenic techniques used in photoemission spectroscopy have enabled us to directly observe the superconducting electronic structure of superconductors with a relatively lower transition temperature (low-T c). In this paper, we report photoemission results on low-T c superconductors measured with an ultrahigh-resolution low-temperature photoemission spectrometer. The result of Nb 3 Al exhibits a superconducting gap and a phonon-induced peak–dip–hump structure characteristic of the strong-coupling superconducting spectral func-tion. From measurements of Ba 0:67 K 0:33 BiO 3 , we observe isotropic s-wave superconducting gap opening below T c as well as depletion of intensity near the Fermi level above T c , which can be attributed to a phonon-induced pseudogap. We will compare these photoemission results with those of high-T c superconductors.
    Physica C Superconductivity 10/2002; 2570(74). · 0.72 Impact Factor
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    ABSTRACT: The evolution of vortex states and vortex-lattice melting processes in Bi2Sr2CaCu2O8+y under tilted fields are observed a by differential magnetooptical technique. The vortex-lattice melting transitions in the tilted vortex-lattice state and in the crossing-lattice state are found to be similar to each other. When the field is applied at large angles from the c axis, the nucleation position of the vortex-liquid paddle is shifted toward the direction of field inclination. The shift of the shielding current leading to the shift of the vortex dome naturally explains the observed phenomena. When a large in-plane field is applied, pancake vortices penetrate through one-dimensional channels starting from a fixed position at the sample edge. We can find a natural explanation for such a feature based on the crossing-lattice ground state.
    Physical Review B 05/2002; 65(21). · 3.66 Impact Factor
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y with inhomogeneous phase coherence caused by partial introduction of columnar defects. We find the presence of a crossover frequency (ωcr) at which the character of JPR changes from irradiated to pristine type. Sample size and electromagnetic field configuration dependence of JPR indicate that the current flow across the boundary between irradiated and pristine parts affects the resonance.
    Physica C Superconductivity 02/2002; 367:400-403. · 0.72 Impact Factor
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    ABSTRACT: We studied vortex states of Bi2Sr2CaCu2O8+y (BSCCO) in tilted-fields with utilizing a rotatable micro-Hall probe. Local magnetization measurements on highly-overdoped BSCCO clarify the existence of characteristic peak effects in tilted fields. On increasing tilting angle (θ), the center peak splits into two branches. Systematic studies on the angular dependence revealed that the additional center peak (H*cp) shows square root dependence as H*cpcosθ∝(H*cpsinθ)1/2. Assuming one-dimensional arrangement of pancake vortices (PVs) on Josephson vortices (JVs) for small Hcosθ, this square root dependence means that the H*cp takes place when the PV spacing on a JV approaches to the penetration length. Namely, this result shows the existence of the second lower critical field characteristic of vortex states in tilted fields.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2002; 378:475-478.
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    ABSTRACT: We study the Josephson plasma resonance (JPR) in Bi2Sr2CaCu2O8+y with inhomogeneous phase coherence caused by partial introduction of columnar defects. We find that the resonance in the longitudinal configuration occurs at the same field as that in the transverse configuration, which excite plasma mode perpendicular to the boundary. Through systematic studies on sample size dependence of JPR, we find the existence of a characteristic length, Lcr at which the character of JPR changes from inhomogeneous to homogeneous type.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2002; 378:546-549.