Vortex Imaging in Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ using Small Angle Neutron Scattering
ORAL
Abstract
We report extended studies of the field dependence of the vortex configuration in superconducting Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$ at $T \sim $ 0.1$T_{c}$ ($T_{c}$ = 21 K) using Small Angle Neutron Scattering (SANS). SANS measurements show a ring of scattering indicating a highly disordered vortex lattice mainly due to strong pinning in this material and a very broad rocking curve width. At lower applied fields ($<$ 1 T) the measured scattering vectors ($q)$ are close to the value expected for a hexagonal vortex lattice (VL) [M.R. Eskildsen \textit{et al.}, Phys. Rev. B 79, 100501(R) (2009)] but with increasing fields it falls between $q_{hex}$ and $q_{square}$. A deviation from the hexagonal VL can be related to a Fermi-surface or gap anisotropy in the basal plane. Moreover an exponential fit to the scattered intensity as a function of field for H $>$ 1 T yields a significantly smaller slope than what was found for smaller fields. This work was supported by NSF grants DMR-0804887, PHY-0552843 and DMR-0454672, and DOE BES contract No. DE-AC02-07CH11358.
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