Yoshitami Saito

Composition Dependence of Superconductivity in Y–Ba–(Ag, Co)–O System

High Tc superconductors, YBa2(AgxCu1-x)3O9-y with composition of 0x0.6 were prepared by a ceramic technique, and the compositional change of Tc was examined. Samples with nominal compositions of x<0.4, expecially YBa2(Ag0.1Cu0.9)3O9-y and YBa2(Ag0.2Cu0.8)3O9-y, show sharp superconducting transitions with widths as narrow as about 1 K or less. The superconductivity is discussed in relation to the results of X-ray diffraction study.

Anomalous x dependence of Tc and possibility of low-temperature structural phase transition in La2−xSrxCu0.99M0.01O4 (M Ni, Zn, Ga)

Abstract Effects of the 1% substitution of Ni, Zn and Ga for Cu on the x dependence of T c have been investigated in La 2−x Sr x CuO 4 . The T c increases by the Cu-site substitution in all compositions of x. The local minimum of T c as a function of x, appearing slightly near x = 0.115 for La 2−x Sr x CuO 4 , becomes conspicuous with decreasing T c . However, structural phase transition from the orthorhombic mid-temperature phase to the tetragonal low-temperature phase has not been detected from powder X-ray diffraction measurements at low temperatures down to 12 K. The local minimum of T c is attributable to the fluctuation of the undiscovered structural phase transition, which will occur at a low temperature below 12 K as long as samples are in the normal-conducting state. In the 1% Ga-substituted system for Cu, the local minimum clearly shifts from x = 0.115 to 0.125. This suggests that the undiscovered low-temperature structural phase transition is due to the optimum hole concentration of 0.115 per Cu atom, analogously to La 2−x Ba x CuO 4 .

Trial of intercalation of Br and Li into Bi2Sr2Can−1CunO2n+4 (n = 1, 2, 3)

Abstract Intercalation of Br and Li into Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 ( n = 1, 2, 3) has been tried. For heavily Br-intercalated samples of n = 2 and 3 with x ≈ 1 in Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 Br x , enlargement of the c-axis is detected, suggesting that intercalation compounds Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 Br with Br atoms between double BiO layers are synthesized. For Li-intercalated samples, on the other hand, clear evidence to show the formation of intercalation compounds is not obtained. For lightly Br- and Li- intercalated samples of n = 1, 2, 3, changes of the electrical resistivity, the Hall coefficient and T c by the intercalation are observed, which are attributable to charge transfer between intercalants and CuO 2 planes in the host materials Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 . That is, the intercalation of Br and Li is effective for hole and electron doping into CuO 2 planes, respectively. For n = 3, T c decreases by the light intercalation of both Br and Li, suggesting that Bi 2 Sr 2 Ca 2 Cu 3 O 10 has almost optimum hole concentration to attain the maximum of T c .

Superconductivity and low-temperature structural phase transition in La1.98-xCe0.02BaxCuO4

Abstract Effects of the partial substitution of Ce for La on the superconductivity and the low-temperature structural phase transition have been investigated in La2-xBaxCuO4. The local minimum of Tc as a function of Ba concentration x, appearing near x = 0.125 for La2-xBaxCuO4, clearly shifts to x = 0.145 for La1.98-xCe0.02BaxCuO4. The range of x, where a resistance minimum relating to the low-temperature structural phase transition appears, shifts to that of larger x values, too. These suggest that the low-temperature structural phase transition around x = 0.125 in La2-xBaxCuO4 is due to the optimum hole concentration of 0.125 per Cu atom rather than plausible atomic ordering of La and Ba.

Normal and superconducting properties of Pb2Sr2Y1−xCaxCu3O8+δ: II. Dependence on the oxygen concentration

The dependence on the oxygen concentration of normal and superconducting properties of ceramic samples of Pb2Sr2Y1−xCaxCu3O8+δ has been investigated in the composition range of 0⩽x⩽0.5 and 0⩽δ<2. The superconductivity in each sample with different x is suppressed with increasing δ, which is in good correspondence to the increase in resistivity and Hall coefficient with increasing δ, but is contradictory to the speculation by Ohta and Maekawa [Phys. Rev. B41 (1990) 6524]. It is found from chemical analysis that holes introduced by the extra oxygen first enter the CuOδ layer and then the PbO layer rather than the pyramidal CuO2 layer. With increasing δ, a structural phase transition from the orthorhombic phase to the tetragonal phase is observed at δ∼1 in every sample with different x. The dependence on the oxygen concentration of the magnetic susceptibility is also reported.

Two effects of iodine intercalation on Tc in Bi2Sr2Ca1−xYxCu2O8: Two-dimensionality and charge transfer☆

Abstract Effects of the stage-1 iodine-intercalation on T c have been investigated in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 . The iodine intercalation produces two large effects; one is enhancement of the two-dimensionality due to expansion of the crystal along the c -axis, and the other is increase of conductive hole carriers due to charge transfer between intercalated iodine atoms and CuO 2 sheets in the host material. For the stage-1 iodine-intercalation compounds, it has been found that the former effect decreases T c by about 10 K and that the latter effect shifts the x dependence of T c to larger x values by about 0.05 because of the increase of conductive hole carriers by about 0.025 per CuO 2 unit. It has been concluded that the decrease in T c through the I intercalation in Bi 2 Sr 2 CaCu 2 O 8 is due to both effects.

High-Tc superconducting properties in (Y1−xTlx)Ba2Cu3O7−y, Y(Ba1−xKx)2Cu3O7−y and YBa2(Cu1−xMgx)3O7−y

Abstract The concentration dependence of the transition temperature Tc has been studied in the high-Tc compounds (Y1-xTlx)Ba2Cu3O7-y, Y(Ba1-xKx)2Cu3O7-y and YBa2(Cu1-xMgx)3O7-y. In the TI-compound, the results show littl e effect on Tc. In the K-compound, Tc gradually drops with increasing K concentration. In the Mg-compound, Tc abruptly drops with increasing Mg concentration.

Dependence of Tc on the excess oxygen content δ in Bi2Sr2CaCu2O8+δ annealed under high pressures of oxygen

Abstract The dependence of the superconducting transition temperature T c on the excess oxygen content δ has been investigated for Bi 2 >Sr 2 >CaCu 2 >O 8+δ annealed under high pressures of oxygen. With increasing δ, T c decreases for δ≲0.3 and then becomes constant for δ≳0.3. It has been found by chemical titration that this strange δ dependence of T c is related to the δ dependence of the average valency of Cu ions. That is, the average valency increases with increasing δ for δ≲0.3 and then is saturated for δ≳0.3. For δ≳0.3, the Curie-Weiss component in the DC magnetic susceptability has been found to increase with increasing δ. Therefore, the saturation is ascribed to the fact that most of the excess oxygen atoms exceeding δ≈0.3 are introduced in the form of O 2 instead of O 2- .

Synthesis and superconductivity of IBr-intercalated Bi2Sr2CaCu2O8

Abstract We have succeeded in synthesizing a new intercalation compound of Bi 2 Sr 2 CaCu 2 O 8 (IBr) 1 2 with the stage-1 structure. The increase in spacing between the BiO-BiO double layers through IBr intercalation is 2.1 A, which is a little larger than the 2.0 A in the case of Br intercalation alone and is smaller than the 3.5 A in the case of only iodine intercalation. The superconducting transition temperature T c of Bi 2 Sr 2 CaCu 2 O 8 (IBr) 1 2 decreases with increasing x for 0 ≦×⪅0.4 and is almost constant, 55 K, for x⪆0.4. This x dependence of T c is very similar to those of Bi 2 Sr 2 CaCu 2 O 8 Br x and Bi 2 Sr 2 CaCu 2 O 8 I x . This is discussed in relation to the changes of the two-dimensionality and hole concentration in the CuO 2 sheets through intercalation. A trial of ICl intercalation into Bi 2 Sr 2 CaCu 2 O 8 is also described.

Effects of the Cu-site substitution on the anomalous x dependence of Tc in La2−xBaxCuO4

Abstract Effects of the partial substitution of Ni, Zn and Ga for Cu on the anomalous x dependence of Tc and the resistance anomaly around 60 K have been investigated in La2−xBaxCuO4. Tc decreases by the substitution in comparison with non-subsituted samples in almost all compositions of x except compositions near x = 0.125. For 1% Ga-substituted samples for Cu, the local minimum of Tc as a function of x, appearing near x = 0.125 for non-substituted samples, clearly shifts to x ≈ 0.135. This suggests that the low-temperature structural phase transition around x = 0.125 in La2−xBaxCuO4 is primarily due to the optimum hole concentration of 0.125 per Cu atom rather than the atomic ordering of La and Ba. The resistance anomaly near 60 K does not change clearly by the substitution.