Thallium‐based superconductors—A review

Stable bulk superconductors in the Tl‐Ca‐Ba‐Cu‐O system with zero resistance up to 125 K are discussed. Structural, magnetic, and electronic transport properties, including thermoelectric power data, are presented on a variety of phases in this system. The anisotropic resistivity of Tl2Ba2CaCu2O8 single crystals is discussed. The in‐plane and c‐axis resistivities are ∼7×10−4 Ω‐cm and 2×10−1 Ω‐cm, respectively, at room temperature. The c‐axis resistivity decreases with decreasing temperature in the temperature range from room temperature down to the superconducting transition temperature, and the second derivative of ρc is negative, unlike that of YBa2Cu3O7 and Bi2Sr2.2Ca0.8Cu2O8 single crystals. The anisotropic thermopower of Tl2Ba2CaCu2O8 single crystals is also described. The room temperature value of Sab is about 14 μV/K. Sab increases linearly with decreasing temperature at a rate of 0.06–0.065 μV/K2 down to about 160 K, below which Sab flattens off and drops to zero at Tc. Sc at room temperature is a...