Masahiro Akamatsu

Phase diagram of a high-temperature superconductor in the Bi2Sr2CuOy system

Abstract The resistive transition of the superconductivity was measured for Bi2Sr2CuOy (Bi-2201) single crystals with maximal Tc in the configuration of H//c-axis. We observed the breaking of bulk superconductivity due to the external magnetic field at 1.6 K. The superconducting phase boundary was obtained from the irreversibility line defined by the zero-resistance temperature, and the full diagram of the superconductivity was determined in the H-T plane. We estimated the fluctuation superconductivity region whose onset boundary was located far from the bulk superconductivity.

Zn substitution effect for Cu on the anisotropic transport properties in Bi2Sr2CaCu2O8+y single crystals

Abstract Anisotropic transport properties have been investigated for Bi2Sr2Ca(Cu1−xZnx)2O8+y single crystals with Zn substitution for Cu (0 ≦ x ≦ 0.01). We found that an abrupt decrease of the out-of-plane resistivity took place with Zn doping, whereas the in-plane resistivity indicated a moderate increase. The anisotropic parameter, γ, estimated from the scaling plot of the angular dependence of the in-plane resistivity, exhibited a reduction by about one order of the magnitude with 1% Zn doping. The results provide us with information about the inter-plane charge dynamics of the high-Tc superconductors.

Site-selective substitution effect on Tc in the Bi2(Sr2−xCax) (Ca0.8R0.2)Cu2O8+y system (R=rare-earth)

Abstract In order to investigate the factors governing the superconducting transition temperature, T c , other than the carrier concentration, we examined the double doping effects in the 2212-phase Bi-cuprate with maintaining the optimum carrier concentration corresponding to maximum T c . The variable parameters are the SrCa solution rate x at the Sr site and the kind of rare-earth, R(=Y, La, Nd, Eu, Gd, Dy, Yb or Lu), at the Ca site. T c decreased systematically with increasing x as well as with increasing the ionic radius of the doped rare-earth. We estimated the individual dependence of T c on the SrCa solution rate and on the distance of the CuO 2 double layers.

The effect of Ln substitution for Ca in Bi2Sr2CaCu2O8+y

The dependence of Tc on the substitution of lanthanide (Ln) for 20% Ca was investigated for the 2212-phase BSCCO with maintaining the maximum Tc condition. The Tc increases with decreasing the ionic radii of the Ln atoms.

Superconductivity of (La1−xSrx)2CuO4−γ (0≦χ≦0.03)

Abstract The dependence of the fractional volume of an apparent superconductor, its transition temperature and paramagnetic susceptibility on the strontium concentration χ is studied in (La 1−x Sr x ) 2 CuO 4−θ for the small region of χ (0 ≦χ≦0.03). Superconductivity observed in undoped La 2 CuO 4−γ compound disappears at χ = 0.01 together with the disappearance of the antiferromagnetic phase. The origin of this superconductivity is discussed taking account of the anomalous property, i.e., the Meissner effect is larger than the shielding effect. The phase diagram of (La 1−x Sr x ) 2 CuO 4−γ is interpreted from a phenomenological crude model based on weakly localized holes around oxygen sites in the CuO 2 plane.

Magnetic properties of the optimally doped Bi-2201 single crystals

Abstract In the optimally doped Bi 2 (Sr,La) 2 CuO y single crystal, we studied magnetic properties of the superconducting mixed state. The mean-field superconducting parameters, λ ab (0)≈438 nm, H c 2 (0)≈20.2 T, ξ ab (0)≈4.0 nm and κ ≈110, were estimated from DC magnetization data.

Magnetic Properties of Bi-2212 Single Crystals

We measured the magnetic properties of the mixed states focusing particularly on the peak effect for the pure and Zn-doped Bi2Sr2CaCu2Oy (Bi-2212 phase) single crystals in the configuration of H//c axis. The shielding effect in zero-field cooling process indicated step-like behavior with increasing temperature, and the step occurred around 30K. We found that the behavior was closely related to the second maximum observed in M-H space, i.e., so-called anomalous peak effect. As the Zn was substituted for Cu, the doping effect on the peak effect and the irreversibility line were investigated.

The Behavior of The Hall Resistivity in The Mixed State of Single Crystalline Bi-2212

The temperature dependence of Hall resistivity ρxy and the longitudinal resistivity ρxx of single crystalline Bi2Sr2CaCu2O8 in low magnetic field (H≤1T) were measured in the mixed state of the superconductivity. We found that the curves of ρxy have a different behavior with that in high field (H>1T). Our key finding is that all of the curves of ρxy cross at one point near Tc in the low field and ρxy approach zero at this point. We believe that the vortices disappear at this temperature due to the decrease of the superconducting electronic densities.

Superconducting Properties in Bi-2201 Single Crystals

In Bi2(Sr,La)2CuOy single crystal, we studied magnetic properties of the superconducting state for the samples with various carrier concentrations in the underdope region. Irreversibility line was shifted to a higher magnetic field side with increasing the carrier concentration. An in-plane penetration depth was reduced with increasing the carrier concentration.

Dissipative flux motion of the dilute vortex lines in Bi2Sr2CaCu2O8 crystals

Abstract We found a new dissipation in resistive transition below Tc for BSCCO (2212) single crystals in the low magnetic field regime: A fluctuation drop is followed by the moderate decrease of the excess dissipation as lowering temperature from Tc.