Electronic structure of high-temperature superconductors

Leistungen

Phase forming processes have been studied in the HgO-BaO-CaO-CuO systems for temperatures of 700-950 C and O2 + Hg pressures of 6-8 MPa in the range of reacting mixture compositions Hg:Ba:Ca:Cu = (3.0-3.5):2.0:(0-1.5):(3.5-6-0). The two following variants of synthesis technique of single crystals in the HgO-BaO-CaO-CuO system from flux have been developed on this base: 1. with HgO + CuO as a solvent; 2. with KCl or RbCl as a solvent. A special apparatus with a working volume of 600 cm³ providing means for synthesis of crystals at pressures up to 12 MPa and temperatures up to 1000 C has been engineered, manufactured and tested. Hg-1223 crystals (c = 15.7 Å) up to 0.1 x 0.1 x 0.02 mm³ in size have been obtained using KCl as a solvent at cooling rates higher than 15 C/h. The onset temperatures for the superconducting transition temperature have been determined to Tc(on) = 70-80 K for as-grown Hg-1223 crystals. The formation of Hg-1223 crystals at low cooling rates is hampered by the Hg loss in the solution. Using a HgO + CuO mixture as a solvent, it turned out that Hg-based HTSC phases could not be grown from solution in the pressure range provided by the equipment available because of the low solubility of mercury in the oxide melt. in this case growth of Hg-free single crystals in two different HTSC phases takes place. The temperature dependence of microwave absorption showed that the as-grown crystals of the perovskite-like phase CaCuO2 display a superconducting transition at Tc(on) = 64-80 K. The structure of the new orthorhombic non-perovskite-like phase Ba2Cu3O6-x belongs to a new structural type of superconducting oxides. This phase showed a superconducting transition at Tc(on) = 5-13 K. The structure of Ba2Cu3O6-x contains CuO2-chains which are parallel to [110] and are formed by CuO4-squares sharing edges. This structure has the following main features: 1. Absence of ladders 2. Existence of two non-interacting sets of infinite CuO2-chains 3. CuO2-chains do not form common planes. The discovery of superconductivity in these crystals may be an indication against the conjecture that superconductivity in [(Sr,Ca)2Cu2O3]7[CuO2]10 is linked to holes in the ladder sites since there the same type of CuO2-chains exist as in orthorhombic Ba2Cu3O6-x. The synthesis of orthorhombic Ba2Cu3O6-x crystals is of great importance for the study of the superconducting mechanism since they belong to a new class of one-dimensional cuprate superconductors. The study of the influence of postgrowth treatment conditions on Ba2Cu3O6-x crystal structure and superconducting properties are in progress.

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