The Occurrence of Superconductivity in the TlBa2CuO5-δ-Type (1021) System

Abstract
Stable and reproducible superconductivity in the Tl(Ba2-x La x )CuO5-δ (0.0≤x≤0.6) system with the tetragonal TlBa2CuO5-δ-type (1021) structure was reported. A Prototype compound TlBa2CuO5-δ had shown a metastable superconducting onset around 25 K, with zero resistivity at 10 K. With partial substitution of La for Ba ions, T c (50% resistivity drop) increases to 45 K, T c0 (zero resistivity) to 42 K and onset around 50 K. A diamagnetic signal was observed with onset as high as 57 K. Tetragonal lattice parameters decrease with the increasing La concentration due to the partial replacement of larger Ba2+ ions by smaller La3+ ions. The Pairing field energy of 170 K and electron-elementary excitation coupling constant λ of 0.76 were derived from the BCS-like T c formula through comparison with other single Tl-O layer systems TlCa n-1Ba2Cu n O2n+3-δ.