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

Abstract

Superconducting fluctuations (SFs) in SmFeAsO${}_{0.8}$F${}_{0.2}$ (characterized by superconducting transition temperature ${T}_{c}\ensuremath{\simeq}52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response above ${T}_{c}$ 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 ($H\ensuremath{\gtrsim}1.5$ 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 $\ensuremath{\lesssim}H\ensuremath{\ll}{H}_{c2}$) are found to suppress the former while leaving unaltered the latter.