Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning

TL;DR

This study reveals that in overdoped BaFe_{2-x}Co_xAs_2 superconductors, vortex pinning remains strong and single-vortex dominated even at high magnetic fields, with a disordered vortex glass phase exhibiting universal scaling behavior.

Contribution

It demonstrates that vortex pinning in overdoped BaFe_{2-x}Co_xAs_2 is dominated by strong single-vortex pinning, contradicting collective pinning predictions, and shows a universal scaling of the vortex correlation length.

Findings

Vortex lattice remains disordered with short-range hexagonal order.

The radial correlation length decreases as H^(-1/2) over four decades of field.

Vortex pinning remains strong and single-vortex dominated up to 9 T.

Abstract

The disordered flux line lattice in single crystals of the slightly overdoped aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by magnetization measurements, small-angle neutron scattering (SANS), and magnetic force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex pinning precludes the formation of an ordered Abrikosov lattice. Instead, a vitreous vortex phase (vortex glass) with a short-range hexagonal order is observed. Statistical processing of MFM datasets lets us directly measure its radial and angular distribution functions and extract the radial correlation length \zeta. In contrast to predictions of the collective pinning model, no increase in the correlated volume with the applied field is observed. Instead, we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the applied magnetic field, where R1 is the radius of the first coordination shell of the vortex lattice. Such universal scaling of \zeta implies that the vortex pinning in iron arsenides remains strong even in the absence of static magnetism. This result is consistent with all the real- and reciprocal-space vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published to date and is thus sample-independent. The failure of the collective pinning model suggests that the vortices remain in the single-vortex pinning limit even in high magnetic fields up to 9 T.