ELECTRICAL STUDY OF 123 HTSC (77 TO 400 K) WITH VARYING FRACTION OF Y- EXPLANATION OF THE NORMAL-STATE BEHAVIOR

By varying the fraction n of yttrium in copper oxide based ceramic sup erconductors (nRE)BaCuO (RE = Y/Dy) it is found that below a certain n the de percolation path through superconducting grains is lost and th e material behaves insulator-like with variable range hopping mode of electrical transport (In rho proportional to T--1/4). Experiments also show that the variation of resistivity rho with temperature T in the normal state for samples (n greater than or equal to 0.25) exhibiting a superconducting transition is far from linear as the temperature is raised. By an extension of Mott's original ideas to polaron hopping wi th decreased localisation it is found that in rho should vary as T in satisfactory agreement with experiment. Thus, the so-called ''anomalou s temperature variation of resistivity'' in the metal-like normal stat e as well as in the insulator-like state with increased disorder is ex plained within a single framework of hopping of localised carriers.