High superconducting anisotropy and weak vortex pinning in Co-doped LaFeAsO

Here, we present an electrical transport study in single crystals of LaFe0.92Co0.08AsO (T-c similar or equal to 9.1 K) under high magnetic fields. In contrast to most of the previously reported Fe based superconductors, and despite its relatively low T-c, LaFe0.92Co0.08AsO shows a superconducting anisotropy which is comparable to those seen, for instance, in the cuprates or gamma(H) = H-c2(ab)/H-c2(c) = m(c)/m(ab) similar or equal to 9, where m(c)/m(ab) is the effective-mass anisotropy, although, in the present case and as in all Fe based superconductors, gamma -> 1 as T -> 0. Under the application of an external field, we also observe a remarkable broadening of the superconducting transition particularly for fields applied along the interplanar direction. Both observations indicate that the low dimensionality of LaFe1-xCoxAsO is likely to lead to a more complex vortex phase diagram when compared to the other Fe arsenides and consequently, to a pronounced dissipation associated with the movement of vortices in a possible vortex liquid phase. When compared to, for instance, F-doped compounds pertaining to the same family, we obtain rather small activation energies for the motion of vortices. This suggests that the disorder introduced by doping LaFeAsO with F is more effective in pinning the vortices than alloying it with Co.