Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide Superconductor BaFe${}_{1.8}$Co${}_{0.2}$As${}_2$

Abstract

We present an atomic resolution scanning tunneling spectroscopy study of superconducting BaFe${}_{1.8}$Co${}_{0.2}$As${}_2$ single crystals in magnetic fields up to $9\text{Tesla}$. At zero field, a single gap with coherence peaks at $\overline{\hat{I}}”=6.25\text{meV}$ is observed in the density of states. At $9\text{T}$ and $6\text{T}$, we image a disordered vortex lattice, consistent with isotropic, single flux quantum vortices. Vortex locations are uncorrelated with strong scattering surface impurities, demonstrating bulk pinning. The vortex-induced sub-gap density of states fits an exponential decay from the vortex center, from which we extract a coherence length $\hat{I}¾=27.6\pm 2.9\text{A˜}$, corresponding to an upper critical field $H_{c2}=43\text{T}$.

4 pages, 4 figures