D. Goldschmidt

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 the absorption edge of a thin film from transmission measurements

An accurate formula that calculates the transmission of a thin absorbing film on a thick nonabsorbing substrate in terms of the interfacial intensity reflection coefficients is derived. Some special cases of the transmission, namely, its value at the maximum and the minimum and at midpoints of the interference pattern, are shown to yield simple formulas to determine the absorption coefficient as well as an internal check procedure that checks the quality of the spectrum. These methods are shown to yield results for the absorption coefficient that are consistent with one another in films of evaporated amorphous germanium, i.e., they give rise to a smoothly varying absorption edge. The transmission of a nonabsorbing, thin oxide layer on top of the film is also calculated and shown to make a negligible contribution to the nonoxidized film transmission.

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”.

Amorphous germanium as a medium temperature solar selective absorber

Abstract The optical absorption coefficient of thin amorphous germanium films was measured in the temperature range 25–400 °C and in the spectral range 0.5–2 eV. With increasing temperature the spectrum shifts to lower energy, increasing the overlap with the solar spectrum and thus the solar absorptance. In an antireflected film (of thickness 0.3 μm) the solar absorptance was estimated to be about 90%. No increase in the thermal emissivity with increasing temperature was observed. Thus the thermal emittance is limited by the value for the underlying metal which was estimated to be 5%. Using a tenfold solar concentrator a maximum solar-to-thermal conversion efficiency of 75% at 350 °C was calculated. A generator converting this heat to electricity with an efficiency of approximately one-third converts solar energy to electricity with more than 20% efficiency. A decrease in selectivity due to substantial crystallization may occur after 20 years of operation.

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).