Effect of carrier concentration on the normal transport properties and the superconducting transition temperature in the Tl2Ba2Ca1-xYxCu2O8+y system

We report measurements of electrical resistivity and Hall voltage on Tl2Ba2Ca1-xYxCu2O8+y samples as a function of temperature for different values of x (0≤x≤0.6). A metal-insulator (M-I) transition is observed in the system at x=0.48, which corresponds to a hole concentration p of 0.03/Cu-ion. In the insulating samples (x>0.48) at low temperatures, the conduction is governed by a two-dimensional variable-range-hopping mechanism and a crossover to three-dimensional variable-range hopping occurs as the M-I transition is approached. Superconductivity appears in the vicinity of the M-I transition (p≥0.03). The Tc versus p curve is linear in the range (0.03--0.12)/Cu ion. Maximum Tc (85 K) is observed for p=0.12/Cu ion