Upper critical field, anisotropy, and superconducting properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals

TL;DR

This study investigates the superconducting properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals, revealing high upper critical fields, low anisotropy, and successful scaling of resistivity data with anisotropic Ginzburg-Landau theory.

Contribution

It provides detailed measurements of upper critical fields and anisotropy in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ crystals, demonstrating the applicability of anisotropic Ginzburg-Landau scaling.

Findings

High upper critical fields increase with transition temperature

Superconducting anisotropy ratio is approximately 2

Resistivity data collapse onto a single curve under GL scaling

Abstract

The temperature dependent resistivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals were measured in magnetic fields up to 9 T. The measurements of temperature dependent resistivity for samples with different doping levels revealed very high upper critical fields which increase with the transition temperature monotonously, and a very low superconducting anisotropy ratio $\Gamma=H_{c2}^{ab}/H_{c2}^c \approx$ 2. By scaling the resistivity in the frame of the anisotropic Ginzburg-Landau theory, the angle dependent resistivity of the Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy $\Gamma$ was determined alternatively for each temperature and was found to be between two and three.