$^{75}$As NMR of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ in High Magnetic Field

TL;DR

This study uses $^{75}$As NMR in high magnetic fields to investigate the superconducting state of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, revealing insights into its gap structure and vortex dynamics.

Contribution

It provides detailed high-field NMR measurements of the superconducting and vortex properties of an optimally doped iron-based superconductor, highlighting the gap symmetry and vortex phase diagram.

Findings

Knight shift shows minimal vortex current effects at high fields

Evidence supports extended s-wave gap symmetry

Vortex freezing transition observed in $T_2$ measurements

Abstract

The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin relaxation time, $T_2$, indicate a strong indirect exchange interaction at all temperatures. Below the superconducting transition temperature vortex dynamics lead to an anomalous dip in $T_2$ at the vortex freezing transition from which we obtain the vortex phase diagram up to $H = 28$ T.