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

TL;DR

This study investigates vortex behavior in layered Fe arsenide superconductors, revealing lock-in transitions and multiple vortex configurations due to commensurability effects, advancing understanding of vortex dynamics in these materials.

Contribution

It provides experimental evidence for vortex lock-in transitions and multiple kinked vortex configurations in single-layered Fe arsenides, highlighting the role of intrinsic planar structures.

Findings

Identification of a critical depinning angle for vortex lock-in.

Observation of replica peaks indicating commensurability effects.

Evidence for transitions among different kinked vortex states.

Abstract

We report an angle-dependent study of the magnetic torque $\tau(\theta)$ 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 $\theta_c$ at either side of $\theta=90^{\circ}$, where $\theta$ is the angle between $H$ and the inter-planar \emph{c}-axis. $\theta_c$ 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 $\theta$ and as $\theta$ 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.