Anisotropy dependence of the fluctuation spectroscopy in the critical and Gaussian regimes in superconducting NaFe_1-x Co _x As single crystals

We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe_1-x Co _x As single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x = 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x ≥ 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around T _c (H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above T _c may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level.