Intrinsic pinning on structural domains in underdoped single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study investigates how intrinsic structural domain pinning enhances the critical current density in underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals, revealing the role of domain walls as pinning centers.

Contribution

It demonstrates the intrinsic pinning effect on structural domains in iron-based superconductors and its impact on critical current density, especially in underdoped samples.

Findings

Peak critical current density at x=0.058 doping level.

Domain walls act as extended pinning centers.

Increased domain intertwining reduces orthorhombic distortion.

Abstract

Critical current density was studied in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for the values of $x$ spanning the entire doping phase diagram. A noticeable enhancement was found for slightly underdoped crystals with the peak at $x = 0.058$. Using a combination of polarized-light imaging, x-ray diffraction and magnetic measurements we associate this behavior with the intrinsic pinning on structural domains in the orthorhombic phase. Domain walls extend throughout the sample thickness in the direction of vortices and act as extended pinning centers. With the increasing $x$ domain structure becomes more intertwined and fine due to a decrease of the orthorhombic distortion. This results in the energy landscape with maze-like spatial modulations favorable for pinning. This finding shows that iron-based pnictide superconductors, characterized by high values of the transition temperature, high upper critical fields, and low anisotropy may intrinsically have relatively high critical current densities.