Single-crystal growth and superconductivity of (La 1−xSr x) 2CaCu 2O 6+δ

Abstract

The effects of growth conditions and composition on the crystal growth of La-2126-phase superconducting materials have been studied by the floating-zone method. The composition of La-2126-phase single crystals was (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 – 0.2 5 ). The crystals were grown under oxygen pressures from 1 bar to 11 bar. A smooth planar solid–liquid growth interface of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h, or the Sr concentration x > 0.0 7 5 . When the planar solid–liquid growth interface broke down into a cellular interface, the single-crystal size in an as-grown rod decreased abruptly and the as-grown rod was not of single phase. Various small sizes of CaO second phases were found inside the as-grown single crystals of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x ⩾0.125). Based on the occurrence of these CaO second phases inside the as-grown crystals, the estimated solubility of as-grown single crystals is x = 0.0 7 5 for (La 1− x Sr x ) 2CaCu 2O 6+ δ . The melting point of the La-2126 materials increases with increasing oxygen pressure. Cubic-centimeter-size single crystals were successfully grown for (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ). The superconducting transition temperature of the as-grown (La 1− x Sr x ) 2CaCu 2O 6+ δ single crystals increases from T c = 2 8 K to 46 K with increasing the Sr concentration x from 0.075 to 0.25 at 11 bar oxygen pressure.