Michikazu Kosuge

Transport properties of the Pb-based superconducting cuprate (Pb1+x/2Cu1-x)Sr2(Y1-xCax)Cu2Oy

Abstract The resistivity, Hall effect and Seebeck effect of (Pb (1+ x )/2)Sr 2 (Y 1- x Ca x )Cu 2 O y ( x ⩾0.35) were determined. Superconductivity appeared only for x =0.30 and 0.35 and only when the sample was quenched from high temperature. Transport data showed that hole carriers increase with x and with decreasing oxygen content, y , from 7.1 to 7.0 by quenching. This indicates that, unlike the case of “Tl-1212”, the “Pb-1212” system cannot be explained by over-doping arguments. Positive magnetoconductance was observed for the slowly cooled sample with x =0.35. Substitution and quenching effects on the transport properties are discussed from the viewpoints of self-doping, charge transfer and localization.

Correlation between Hall Numbers and Tc in Y1-xCaxBa2Cu3O7-δ ( 0≤x≤0.5) Thin Films Prepared by RF Thermal Plasma Deposition Method

The temperature dependence of the resistivity and the Hall coefficient R H was measured for Y1-x Cax Ba2Cu3O7-δ thin films prepared on SrTiO3 substrates using the RF thermal plasma deposition method. Transition temperature T c and resistivity of the as-grown film decrease with increasing Ca concentration. However, oxygen treatment affects the transport properties of the films and a slight increase in oxygen deficiency δ increases T c in heavily Ca-doped samples through changes in the hole density. There is a maximum T c of about 90 K with variation of the carrier density and further reduction of the oxygen content lowers T c again. The value of the maximum T c is independent of Ca concentration and a parabolic relationship between Hall number and T c exists in the Y1-x Cax Ba2Cu3O7-δ system.

p-type Carrier doping in ACuO2 infinite-layer thin film

Abstract Thin films of ACuO 2 (A:alkaline earth) with “infinite-layer” structure were prepared by the RF thermal plasma evaporation method on SrTiO 3 (100) substrates. The lattice constant c increases systematically with the increase of A ion size. Electric-resistivity measurements showed that all the as-grown samples were semiconducting. In order to dope holes into this system, two methods were tried. One is substitution of Na for the alkaline earth. The other one is exposing the samples to O 2 plasma. O 2 plasma annealing caused a structural change in the samples. The physical properties of the plasma-annealed samples were measured and a possible structural model is proposed. Neither the Na doped sample nor the O 2 plasma-annealed sample showed superconductivity. The crystalline condition for the absence of superconductivity is discussed.

Pressure effect on critical temperature of HgBa2CaCu2O6+δ

Abstract The superconducting critical temperature, T c , of a HgBa 2 CaCu 2 O 6+δ (Hg-1212) ceramic sample was determined by four-probe resistivity measurement under hydrostatic pressures up to 8 GPa. T c increased to 144 K at the onset and 140 K at the midpoint under a pressure of 8 GPa. The initial gradient of the T c -pressure curve, d T c /d p , was 3 K/GPa for T c onset and 2K/GPa for T c midpoint , respectively. We discuss the large enhancement in T c based on a pressure-induced delocalization effect and compare it to the case of the Pb-1212 phase which is isomorphous to the Hg-1212 phase.

High-Pressure transport phenomena in (Pb(1+x)2Cu(1−x)2)Sr2(Y1−xCax)Cu2O7+δ

Abstract Pressure effect on Tc for “Pb-1212” phase is investigated up to 2GPa using a piston- cylinder apparatus. Unusually large pressure coefficient of Tc is observed ( dln T c d p =0.65 GPa −1 , Tc=16.8K at latm.). Pressure induced-delocalization of holes is discussed.

Hall effect and superconducting properties of (Nd0.8−xCexSr0.2)2CuO4−δ with T∗ structure

Abstract Hall effect and superconducting properties of polycrystalline (Nd 0.8− x Ce x Sr 0.2 ) 2 CuO 4−δ compounds with the T ∗ structure have been studied. The temperature dependence of the positive Hall coefficient has a broad maximum around 150 K. This indicates that simple single carrier models for normal metals do not apply. Three possibilities for the temperature dependence of the Hall coefficient are discussed, and a van Hove singularity near the Fermi level is the most plausible explanation for the temperature dependence. The upper, lower and thermodynamic critical fields obtained for (Nd 0.7 Ce 0.1 Sr .2 ) 2 CuO 4−δ are 15, 0.04 and 0.11 T, respectively. A comparison with the results of T and T′ phase materials suggests that the coherence length is closely associated with the structures of the CuO n polyhedra in cuprate superconductors.

NQR study in YBa 2Cu4O8 under high pressure

Abstract The temperature dependence of the 63 Cu nuclear spin-lattice relaxation rate (T 1 T) −1 and spin-spin relaxation rate T −1 2G at the Cu(2) planar site in the normal state of YBa 2 Cu 4 O 8 have been measured under high pressure by the nuclear quadrupole resonance (NQR) technique. T −1 1 and T −1 2G could be fitted by single exponential and Gaussian at all temperatures, respectively. It seems that the pseudo-spin gap temperature at 1 GPa which was determined by the T -dependence of (T 1 T) −1 slightly decreased compared with at ambient pressure.

Characterization of heavily Ca doped Y1-xCaxBa2Cu3O7-d thin films

Temperature dependence of in-plane resistivity ρab, Hall coefficient Rab H, and thermoelectric power have been measured for c-axis oriented Y1-xCaxBa2Cu3O7-δ thin films with a wide range of x=0–0.5 and δ=0.08–1.0. We have controlled carrier concentration in the sample from under- to over-doped region by changing x and 8, and have shown that Tc solely depends on the carrier concentration in Y1-xCaxBa2Cu3O7-δ system. Hall mobility μH has been calculated from ρab and Rab H, and μH -1 satisfies the relation of μH -1=αT2+C in all the samples. The systematic investigation of μH -1 has revealed that the slope a is essentially independent of carrier doping level and disorder. Moreover, the doping independent α is universal in various high-Tc superconducting cuprates.