The Occurrence of Superconductivity in the TlBa₂CuO_5-δ-Type (1021) System

Abstract

Stable and reproducible superconductivity in the Tl(Ba 2- x La x )CuO 5-δ (0.0≤ x ≤0.6) system with the tetragonal TlBa 2 CuO 5-δ -type (1021) structure was reported. A Prototype compound TlBa 2 CuO 5-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 Ba 2+ ions by smaller La 3+ 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 -1 Ba 2 Cu n O 2 n +3-δ .