Vortex lock-in transition and evidence for transitions among commensurate kinked vortex configurations in single-layered Fe arsenides

Abstract

We report an angle-dependent study of the magnetic torque $\ddot{I}„(\hat{I}¸)$ within the vortex state of single-crystalline LaO${}_{0.9}$F${}_{0.1}$FeAs and SmO${}_{0.9}$F${}_{0.1}$FeAs as a function of both temperature $T$ and magnetic field $H$. Sharp peaks are observed at a critical angle $\hat{I}{¸}_c$ at either side of $\hat{I}¸=90^{\circ }$, where $\hat{I}¸$ is the angle between $H$ and the inter-planar \emph{c}-axis. $\hat{I}{¸}_c$ is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of $\hat{I}¸$ and as $\hat{I}¸$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the inter-planar distance leading to additional kinked vortex configurations.

5 pages, 5 figures