Superconducting fluctuating diamagnetism versus precursor diamagnetism in heterogeneous superconductors

The fluctuation-induced diamagnetism (FD) above the transition temperature T, is studied through the isothermal field-dependence of the diamagnetic magnetization M-d iota a. While for T approximate to T-c and H -> 0 one has-M-d iota a proportional to (H)(1/2),on increasing field an upturn occurs and vertical bar M-dia vertical bar begins to decrease, in correspondence to field-induced quenching of the fluctuating pairs. For BCS superconductors (SC) as MgB2 the upturn field H-up is in the range 100-800 Oe, while in optimally doped high-temperature SC(HTcSC) Hup is expected in the range of several Tesla, because of the small coherence length. At variance, in non-homogeneous HTcSC (under- or overdoped) or in the presence of impurities causing diffuse transition, strong increase Of M-d iota a and Hup reduced by order of magnitudes are observed. The reasons of these behaviors are discussed in the report. In particular the difference between the FD in heterogeneous HTcSC (related to SF) and the precursor diamagnetism for diffuse transitions such as in Al-doped MgB2 (unrelated to SF) is emphasized and it is shown how the temperature dependence of H-up discriminates between the two effects.