VORTEX-LATTICE MELTING IN TL(SR1.5R0.5)CACU2O7-Y (R = ND, SM, AND EU)SYSTEM

The partial substitution Sr of by a rare earth element R (except Pm) i n TlSr2 CaCu2O7-y oxide has been successfully carried out. Measurement s of X-ray diffraction and de magnetic susceptibility show that all th ey exhibit well-defined superconductivity with the TlSr2CaCu2O7-y stru cture and T-c similar to 90 K for all rare earths except Pm. In this s tudy, the irreversibility lines H-irr (T) of three polycrystalline hig h-T-c superconductors Tl(Sr1.5R0.5)CaCu2O7-y With R = Nd, Sm and Eu we re studied in a field range H less than or equal to 10 kOe. The experi mental data of the H-irr(T) line for each sample is well fitted to a t hree-dimensional-like power-law relation: H-irr(T) = H-irr(0)[1 - T/T- mo](2). Based on the nonlocal elasticity theory using Lindemann criter ion, the power function with the exponent approximating to 2 is interp reted as the occurrence of the thermally activated vortex-lattice melt ing in sample.