Doping evolution of the anisotropic upper critical fields in iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$

TL;DR

This study investigates how the upper critical magnetic field in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ varies with doping, revealing asymmetries and effects related to paramagnetic limiting and multi-band superconductivity.

Contribution

It provides detailed measurements of $H_{c2}$ across a wide doping range, highlighting the doping-dependent anisotropic behaviors and underlying physical mechanisms.

Findings

Doping asymmetry in $H_{c2}(T)$ shapes.

Paramagnetic limiting effects prominent in overdoped samples.

Multi-band effects significant in underdoped samples.

Abstract

In-plane resistivity measurements as a function of temperature and magnetic field up to 35~T with precise orientation within the crystallographic $ac-$plane were used to study the upper critical field, $H_{c2}$, of the hole-doped iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Compositions of the samples studied were spanning from underdoped $x=$0.17 ($T_c$=12~K) and $x$=0.22 ($T_c$=20~K), both in the coexistence range of stripe magnetism and superconductivity, though optimal doping $x$=0.39 ($T_c$=38.4~K), $x$=0.47 ($T_c$=37.2~K), to overdoped $x$=0.65 ($T_c$=22~K), $x$=0.83 ($T_c$=10~K). We find notable doping asymmetry of the shapes of the anisotropic $H_{c2}(T)$ suggesting important role of paramagnetic limiting effects in $H \parallel a$ configuration in overdoped compositions and multi-band effects in underdoped compositions.