Superconducting phase fluctuations in SmFeAsO$_{0.8}$F$_{0.2}$ from diamagnetism at low magnetic field above $T_{c}$

Abstract

Superconducting fluctuations (SF) in SmFeAsO$_{0.8}$F$_{0.2}$ (characterized by superconducting transition temperature $T_{c} \simeq 52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response to magnetic fields up to 1 T above $T_{c}$ is similar to what previously reported for underdoped cuprate superconductors and it can be justified in terms of metastable superconducting islands at non-zero order parameter lacking of long-range coherence because of strong phase fluctuations. In the high-field regime ($H \gtrsim 1.5$ T) scaling arguments predicted on the basis of the Ginzburg-Landau theory of conventional SF are found to be applicable, at variance with what observed in the low-field regime. This fact enlightens that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SF of novel character and the conventional SF. High magnetic fields (1.5 T $\lesssim H \ll H_{c2}$) are found to suppress the former while leaving unaltered the latter one.