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

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

G. Li, G. Grissonnanche, B. S. Conner, F. Wolff-Fabris, C. Putzke, N. D. Zhigadlo, S. Katrych, Z. Bukowski, J. Karpinski, and L. Balicas

Phys. Rev. B 87, 100503(R) – Published 11 March 2013

Abstract

We report an angle-dependent study of the magnetic torque $τ (θ)$ 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 $θ_{c}$ at either side of $θ = 90^{\circ}$ , where $θ$ is the angle between $H$ and the interplanar $c$ axis. $θ_{c}$ 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 $θ$ and as $θ$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the interplanar distance leading to additional kinked vortex configurations.

Received 6 April 2011

DOI:https://doi.org/10.1103/PhysRevB.87.100503

Authors & Affiliations

G. Li¹, G. Grissonnanche¹, B. S. Conner¹, F. Wolff-Fabris², C. Putzke², N. D. Zhigadlo³, S. Katrych^3,4, Z. Bukowski³, J. Karpinski^3,4, and L. Balicas^1,*

¹National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
²Dresden High Magnetic Field Laboratory, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
³Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland
⁴Institute of Condensed Matter Physics, EPFL CH-1015 Lausanne, Switzerland

^*balicas@magnet.fsu.edu

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Issue

Vol. 87, Iss. 10 — 1 March 2013

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Images

Figure 1
Magnetic torque $τ$ for a LaO $_{0.9}$ F $_{0.1}$ FeAs single crystal for increasing (blue line) and decreasing (orange line) angle ( $θ$ ) sweeps, at $H = 5$ T and $T = 1.5$ K. $θ = 0^{\circ}$ corresponds to fields ( $H$ ) along the interplanar direction. Sharp spikes, indicated by red arrows, are observed at either side of $θ = 90^{\circ}$ .Reuse & Permissions
Figure 2
From the traces in Fig. 1, the subtraction of a ninth-degree polynomial leads to a residual term, $τ_{R} (θ)$ containing sharp pronounced features as well as smaller nearly oscillatory structures. Each pair of colored arrows indicate a pair of corresponding features such as sharp peaks or dips, which are seen on each trace, but which are slightly displaced in angle. The separation between the red arrows yields $2 θ_{c} ≃ 2 . 36^{\circ}$ . Square indicates the angular region chosen to analyze the fine structure displayed below in Fig. 3a.Reuse & Permissions
Figure 3
(a) Oscillatory component $τ_{R} (θ)$ superimposed onto the $τ (θ)$ traces shown in Fig. 1a. $τ_{R} (θ)$ was obtained by fitting $τ (θ)$ to a polynomial within a limited angular range. As in Fig. 1a, blue and orange lines depict traces for field-increasing and- decreasing sweeps, respectively. Notice (i) the reproducibility of the oscillatory structure and (ii) how their amplitude decreases as $H$ is rotated towards the $c$ axis ( $θ = 0^{\circ}$ ). Each oscillation in a given trace is slightly displaced in angle relative to its counterpart on the other trace. (b) Field dependence of twice the critical lock-in angle 2 $θ_{c}$ for a second LaFeAsO $_{0.9}$ F $_{0.1}$ single-crystal at $T = 8$ K.Reuse & Permissions
Figure 4
(a) Magnetic torque $τ (θ)$ measured under a field $H = 2$ T for a SmFeAsO $_{0.9}$ F $_{0.1}$ single crystal as a function of the angle $θ$ and for several temperatures. Included are traces for increasing and decreasing angle sweeps, respectively. Sharp peaks are observed in $τ (θ)$ at an angle $θ_{c}$ located at either side of $θ = 90^{\circ}$ . (b) Difference in angle $2 θ_{c}$ between both peaks as a function of temperature. Red line is a guide to the eyes.Reuse & Permissions
Figure 5
(a) Sketch depicting a single flux line in a layered superconductor for fields slightly tilted away with respect to the planar direction. The vortex line can be understood as an array of Abrikosov vortices (pink pancakes) piercing single superconducting layers that are interconnected by Josephson strings (red lines). At a small angle, a tilted vortex line is composed of the two-dimensional core of the Abrikosov vortex and the phase core of the Josephson string. (b) By rotating the field towards the $c$ axis, one shortens the Josephson strings, increases their interlayer separation, and increases the density of Abrikosov vortices.Reuse & Permissions

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