Effects of disorder on the intrinsically hole-doped iron-based superconductor KCa2Fe4As4F2 by cobalt substitution

Abstract

In this paper, the effects of cobalt substitution on the transport and electronic properties of the recently discovered iron-based superconductor $\mathrm{KC}{\mathrm{a}}_{2}\mathrm{F}{\mathrm{e}}_{4}\mathrm{A}{\mathrm{s}}_{4}{\mathrm{F}}_{2}$, with ${T}_{\mathrm{c}}=33\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, are reported. This material is an unusual superconductor showing intrinsic hole conduction ($0.25\phantom{\rule{0.16em}{0ex}}\mathrm{holes}/\mathrm{F}{\mathrm{e}}^{2+}$). Upon doping of Co, the ${T}_{\mathrm{c}}$ of $\mathrm{KC}{\mathrm{a}}_{2}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{4}\mathrm{A}{\mathrm{s}}_{4}{\mathrm{F}}_{2}$ gradually decreased, and bulk superconductivity disappeared when $x\ensuremath{\ge}0.25$. Conversion of the primary carrier from $p$ type to $n$ type upon Co-doping was clearly confirmed by Hall measurements, and our results are consistent with the change in the calculated Fermi surface. Nevertheless, neither spin density wave (SDW) nor an orthorhombic phase, which are commonly observed for nondoped iron-based superconductors, was observed in the nondoped or electron-doped samples. The electron count in the $3d$ orbitals and structural parameters were compared with those of other iron-based superconductors to show that the physical properties can be primarily ascribed to the effects of disorder.