Highly Mobile In-plane Vortex Arrangement in SmFeAs(O,F)*

We observed a strong enhancement of flux flow dissipation for current flowing along the inter-planar direction in single crystals of the iron pnictide high-T$_c$ superconductor SmFeAs(O,F) (T$_c \sim$ 50K) in high magnetic fields precisely aligned with the FeAs planes. The dissipation reaches significant fractions of the resistance in the normal state at all temperatures and fields, far below H$_{c2}||ab$, estimated to be well above 100T at low temperatures. Even slightest field misalignments from the FeAs planes ($<$0.1deg) restore the dissipation free state characterized by very high critical current densities ($\sim10^6 A/cm^2$) at low temperatures. We attribute this feature to vortices arranging themselves between the FeAs layers, accompanied by a reduced effectiveness of pinning. The qualitative features are reminiscent of the well-known lock-in effect in the cuprates, yet there are clear differences evident: The angular range of enhanced dissipation is reduced upon cooling in SmFeAs(O,F), whereas in the cuprates it significantly broadens as H$_{c1}$ increases at lower temperatures. Furthermore, the lock-in effect is most pronounced in strongly anisotropic materials, while SmFeAs(O,F) is moderately anisotropic ($\gamma \sim$ 6-8), becoming more isotropic at low temperature.