Y. Eckstein

Transport measurements in the 1-2-3 system CLBLCO in both the oxygen-underdoped and -overdoped regions

Abstract “CLBLCO” is a family of 1-2-3 superconductors, represented by the chemical formula (Ca x La 1− x )(Ba c − x La 2− c + x )Cu 3 O y . By annealing in oxygen at various pressures, up to 4300 bar, at a temperature of 300°C and above, we have prepared specimens whose oxygen content covers a wide range, from y =6.4 (a totally “underdoped” insulating phase) to a highly overdoped phase with y ≳7.354. In contrast to YBCO, the specimens remain tetragonal for all values of x , c , and y . Measurements of the superconducting transition temperature, the resistivity, and the thermoelectric power, over a range of y values, 6.84≤ y ≤7.30 are presented, for samples with calcium content from x =0.1 to 0.4, and with c =1.75. We also report some transport data for CLBLCO with nickel impurities. Some significant differences were found between the properties of the underdoped and overdoped regimes. In contrast to YBCO, the thermoelectric power characteristics behave like those of most other high- T c cuprates. We found that a proposed universal relation between the thermoelectric power and the ratio between T c and its maximum value T c,max is not truly universal; it holds in some cases, but fails in others. The temperature-dependence of the resistivity shows a departure from linearity in the overdoped region, which is perhaps the signature of the appearance of a normal-state gap. The temperature at which the non-linearity sets in appears to increase as the doping level falls.

Structural properties of CaBaLaCu3Oy studied by NMR/NQR method

Abstract The first NMR/NQR study of the high- T c superconductor CaBaLaCu 3 O y has been carried out on a sample enriched with 43 Ca and 135 Ba which allowed one to observe signals from all cations. Direct evidence was obtained that the superconducting compound contains less Ca than the starting composition and a large part of Ca and Cu forms a spurious dielectric phase. The composition of the superconducting phase is close to (Ca 0.5 La 0.5 )(Ba 1.25 La 0.75 )Cu 3 O y as was proposed for the first time by Keller-Belest et al. A highly asymmetric electric-field gradient observed at the Ba and La sites in the Ba/La plane indicates that the local environment of these ions is not tetragonal in spite of the overall tetragonal crystal symmetry. A symmetric field gradient, however, is found for the cations in the central Ca/La plane. No evidence for cation ordering or a special superstructure of the oxygen could be detected.

Determination of oxygen stoichiometry in a small mass of cuprate superconductor by iodometric microtitration

The authors have developed a method of determining oxygen stoichiometry (y) of small masses of cuprate oxides, based on iodometric microtitration. It comprises double microtitration. The first microtitration was carried out on a Cu2+ solution of accurately known concentration. A small mass ( approximately 500 mu g) was accurately determined by an absolute calibration of a microbalance to within +or- approximately 0.8 mu g. They obtained y=6.960+or-0.009 from ten microtitrations on small masses extracted from a large ceramic sample of YBa2Cu3Oy. This method can be used to determine oxygen stoichiometry in single crystals and in thin films of cuprate superconductors.

Nonlinear resistivity in fully oxygenated YBa2Cu3O7−δ Frenkel disorder of chain oxygens

Abstract Fully oxygenated ceramic YBa 2 Cu 3 O 7−δ has a non linear resistivity [ϱ( T )] reminiscent of the resistivity ϱ b (along chain s) in untwinned single crystals. The excess resistivity above 200 K, measured with respect to the linear portion of ϱ( T ) is activated. We suggest a Frenkel-disorder process, in which a chain oxygen jumps from its O(1) chain site to the nearby O(5) site, to account for the excess electron scattering. The activation energy, which represents the difference in energy between both sites, is ≈ 120 meV for ceramics, thin films and single crystals, in agreement with the activation of the orthorhombic to tetragonal transition. The characteristic defect resistivity is ≈ mΩ cm, close to the minimum metallic conductivity, perhaps implying strong disorder Our model supports the view that chains and planes make separate contributions to normal-state conductivity.

Internal charge transfer in tetragonal superconductor (CaxLa1−x)(Ba1.75−xLa0.25+x) Cu3Oy

Abstract We report (Ca x La 1− x ) (Ba 1.75− x La 0.25+ x ) Cu 3 O y , that is a tetragonal superconductor family within the 1:2:3 system existing for 0≤ x ≤0.5. With constant Q (= 7.25, Q denoting the charge on the non copper cations) and constant y ( y ≅7.1) the average and mobile hole densities do not change with x . Nevertheless, there is a big increase in T c with x . Besides, T c is very sensitive to small changes in Q or in y . In addition, the resistivity of low x compositions increases by an order of magnitude and changes from metallic to semiconductorlike behaviour upon small decrease in y . We attribute this unusual behaviour to La that substitutes for Ba. It attracts more oxygens in the plane of the “chains” that induce the orthorhombic to tetragonal phase transition. These oxygens then promote additional internal electron transfer from the planes to the “chains”.

Dependence of Tc on hydrostatic pressure in a 123 superconductor

Abstract:Hydrostatic pressure studies on the tetragonal 123 superconductor (La1-xCax) (Ba1.75-xLa0.25+x)Cu3Oy for x=0.1 and variable oxygen content y show that Tc increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in Tc is observed only for the most underdoped sample.

Bond valence sums for YBCO and CLBLCO

Abstract We present bond-valence sum (BVS) calculations for YBa 2 Cu 3 O y (“YBCO”), using the data of Cava et al., and for (Ca x La 1− x )(Ba 1.75− x La 0.25+ x )Cu 3 O y (“CLBLCO”), using the refined neutron diffraction data of Chmaissen et al. It has been conjectured that from the BVS results one can estimate the concentration of mobile charge carriers in the CuO 2 plane. We have used both the method of Brown et al. and that of Tallon et al. for estimating this number. The difference between the two methods will be discussed, and the conjecture will be critically reviewed.

Characterization of tetragonal 1:2:3 (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy thin films prepared by laser ablation deposition

Abstract Tetragonal c -axis oriented films of 1:2:3 (Ca x La 1− x ) (Ba 1.75− x La 0.25+ x ) Cu 3 O y (CLBCO) were prepared from a target with x =0.45 and T c onset ≈ 81.5 K. The cation composition was controlled by changing the distance D ts between the target and the substrate. T c onset and T c ( R = 0) of the films were 78 K and 75 K on (110) LaGaO 3 (LGO) substrates, 72 K and 69 K on (100) MgO, and 72 K and 65 K on (100) SrTiO 3 (STO), respectively. The width of the rocking curves of the films on LGO, MgO and STO were 1.8°, 1.5°, and 0.3°, respectively, indicating a correlation between a high T c (0) and increased mosaicity. This could be due to a shorter diffusion path of oxygen and improved uptake at high degree of mosaicity. The large ϱ ab (300 K) ( ≈ 550μΩ cm ) and the small ν ab (300 K)/ ϱ ab (100 K) ratio (⩽ 1.8) in our films compared to YBCO, suggests that the CuO 2 planes are the main conduction element in CLBCO, consistent with total chain disorder in the basal plane of CLBCO. The relatively low J c (40 K) ≈ 10 6 A/cm 2 in our films, may result from the absence of twins (pinning centers).