Muon Spin Rotation Measurement of the Magnetic Field Penetration Depth in Ba(Fe0.93 Co0.07)2 As2 : Evidence for Multiple Superconducting Gaps

TL;DR

This study uses muon spin rotation to measure the magnetic field penetration depth in Ba(Fe0.93Co0.07)2As2, providing evidence for multiple superconducting gaps and revealing unconventional temperature and field dependencies.

Contribution

It presents the first muon spin rotation measurements of this compound, demonstrating the existence of multiple superconducting gaps and their distinct field and temperature behaviors.

Findings

Identification of two superconducting gaps with sizes 6 meV and 3 meV.

Observation of a penetration depth varying more rapidly than BCS predictions.

Evidence for anisotropic vortex lattice structure.

Abstract

We have performed transverse field muon spin rotation measurements of single crystals of Ba(Fe$_{0.93}$Co$_{0.07})_2$As$_2$ with the applied magnetic field along the $\hat{c}$ direction. Fourier transforms of the measured spectra reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice. We have fit the $\mu$SRSR spectra to a microscopic model in terms of the penetration depth $\lambda$ and the Ginzburg-Landau parameter $\kappa$. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of $2\Delta_1=3.7k_BT_c$ and $2\Delta_2=1.6k_BT_c$, corresponding to roughly 6 meV and 3 meV respectively.