Evidence of diamagnetic fluctuations above Tc in BaFe2-xCoxAs2 single crystals

TL;DR

This study reveals the coexistence of magnetic fluctuations and superconductivity above Tc in BaFe2-xCoxAs2, highlighting the interplay between magnetic inhomogeneity and superconducting fluctuations in doped pnictide crystals.

Contribution

It provides new evidence of diamagnetic fluctuations above Tc and constructs a doping-dependent H-T phase diagram linking magnetic and superconducting fluctuations.

Findings

Positive magnetization regions coexist with superconductivity below Tc.

Normal state is inhomogeneous with diamagnetic regions embedded in positive magnetization background.

Superconducting fraction peaks at optimal doping.

Abstract

Increasing doping concentration (x) in Pnictide superconductors results in the suppression of long range magnetic order and the emergence of superconductivity. While doping destroys long range magnetic order however the effect if any of the surviving magnetic fluctuations on superconductivity in Pnictides is currently not well understood. In optimally doped BaFe2-xCoxAs2 single crystals, below Tc we observe local regions with positive magnetization coexisting along with superconductivity at low applied magnetic fields (H). With increasing H the positive magnetization response gets weaker and the robust superconducting diamagnetic response appears. The normal state is found to be inhomogeneous with regions having local diamagnetic response embedded in a positive magnetization background. Estimates of the superconducting fraction in the normal state shows it maximizes for the optimally doped crystal. We construct a doping dependent H - T diagram identifying different fluctuation regimes and our data suggests a close correlation between magnetic and superconducting fluctuations in BaFe2-xCoxAs2.