High superconducting anisotropy and weak vortex pinning in Co doped LaFeAsO

TL;DR

This study investigates the anisotropic superconducting properties and vortex dynamics in Co-doped LaFeAsO single crystals, revealing high anisotropy and weak vortex pinning, which suggest complex vortex phases and low vortex activation energies.

Contribution

It provides the first detailed analysis of superconducting anisotropy and vortex pinning in LaFe$_{0.92}$Co$_{0.08}$AsO, highlighting differences from F-doped counterparts.

Findings

Superconducting anisotropy comparable to cuprates with γ_H ≈ 9.

Broadening of transition under magnetic fields indicates complex vortex phases.

Lower vortex pinning energy compared to F-doped compounds.

Abstract

Here, we present an electrical transport study in single crystals of LaFe$_{0.92}$Co$_{0.08}$AsO ($T_c \simeq 9.1$ K) under high magnetic fields. In contrast to most of the previously reported Fe based superconductors, and despite its relatively low $T_c$, LaFe$_{1-x}$Co$_x$AsO shows a superconducting anisotropy which is comparable to those seen for instance in the cuprates or $\gamma_H = H_{c2}^{ab}/H_{c2}^{c} = m_c/m_{ab} \simeq 9$, where $m_c/m_{ab}$ is the effective mass anisotropy. Although, in the present case and as in all Fe based superconductors, $\gamma \rightarrow 1$ as $T \rightarrow 0$. Under the application of an external field, we also observe a remarkable broadening of the superconducting transition particularly for fields applied along the inter-planar direction. Both observations indicate that the low dimensionality of LaFe$_{1-x}$Co$_x$AsO is likely to lead to a more complex vortex phase-diagram when compared to the other Fe arsenides and consequently, to a pronounced dissipation associated with the movement of vortices in a possible vortex liquid phase. When compared to, for instance, F-doped compounds pertaining to same family, we obtain rather small activation energies for the motion of vortices. This suggests that the disorder introduced by doping LaFeAsO with F is more effective in pinning the vortices than alloying it with Co.