Anisotropy dependence of the fluctuation spectroscopy in the critical and gaussian regimes in superconducting NaFe1\u2212xCoxAs single crystals

D Ahmad,Tuson Park,D Sóñora,Yong Seung Kwon,Yoon Seok Oh,J Mosqueira,W J Choi

doi:10.1038/s41598-018-26939-8

Abstract

We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1−xCoxAs 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 Tc(H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above Tc may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level.

Highlights

The phenomenological description of preformed Cooper pairs above Tc as a result of fluctuations of the superconducting order parameter has remained as one of the most important topics in the field of superconductivity
We investigate the superconducting fluctuation effects in the magnetization and electrical conductivity of NaFe1−xCoxAs single crystals with x = 0.03, 0.05, 0.07, 0.073, which cover from the optimal doping to the highly overdoped regime
Ullah and Dorsey (UD) used a self-consistent Hartree approximation to treat the quadratic terms in the Ginzburg-Landau free energy, and obtained an expression for different fluctuation-induced observables

Summary

Introduction

The phenomenological description of preformed Cooper pairs above Tc as a result of fluctuations of the superconducting order parameter has remained as one of the most important topics in the field of superconductivity. In addition to their intrinsic interest, the analysis of fluctuation effects in the vicinity of the transition temperature. Tc allow to obtain superconducting parameters such as the upper critical field, the coherence length, the anisotropy and the dimensionality. Some works reported a two-dimensional (2D) behavior in compounds from the 1111 family like SmFeAsO7, and from the 111 family like LiFeAs23,24.

Methods

Results

Conclusion