Superconducting phase fluctuations in SmFeAsO0.8F0.2 from diamagnetism at a low magnetic field above Tc

Superconducting fluctuations (SFs) in SmFeAsO0.8F0.2 are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response above Tc to magnetic fields up to 1 T is similar to that previously reported for underdoped cuprate superconductors and justified in terms of metastable superconducting islands of nonzero order parameter lacking long-range coherence because of strong phase fluctuations. In the high-field regime (H≳1.5 T) scaling arguments predicted on the basis of the Ginzburg-Landau theory for conventional SFs are confirmed, at variance with what is observed in the low-field regime. This fact shows that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SFs of novel character and the conventional SFs. High magnetic fields (1.5 T ≲H≪Hc2) are found to suppress the former while leaving unaltered the latter.