Correlated vortex pinning in slightly orthorhombic twinned Ba(Fe1−xCox)2As2single crystals: Possible shift of the vortex-glass/liquid transition

The interest in twin-boundary (TB) planes as a source of vortex pinning has been recently renewed with the discovery of the new iron-arsenide pnictide superconductors. In the family of compounds Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ a structural transition from a tetragonal to orthorhombic lattice takes place for compounds with $xl{x}_{\mathrm{cr}}\ensuremath{\sim}0.065$. Approaching the critical doping, domain structure shrinks with sizes ultimately becoming comparable to vortex cores. In this work we investigate the changes in anisotropy produced by subtle differences in the Co doping level, in the neighborhood of the structural transition, in good-quality single crystals. Using a scaling approach we are able to determine the angular regions where correlated or uncorrelated disorder prevails. In the tetragonal samples ($xg{x}_{\mathrm{cr}}$) there is no twinning and we find good agreement with the expected scaling function under uncorrelated disorder, with small anisotropy values similar to those reported in the literature. We show that in the orthorhombic samples ($xl{x}_{\mathrm{cr}}$), TBs act as correlated disorder in a broad angular range. We propose that the observed angular dependence could be due to an increase in the vortex liquid-glass transition temperature.