Competition between orthorhombic and re-entrant tetragonal phases in underdoped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ probed by the response to controlled disorder

Abstract

Low-temperature (22~K) irradiation with 2.5~MeV electrons was used to study the competition between stripe ${\mathrm{C}}_2$ and tetragonal ${\mathrm{C}}_4$ antiferromagnetic phases which exist in a narrow doping range around $x=$0.25 in hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$. In nearby compositions outside of this range, at $x=$0.22 and $x=$0.19, the temperatures of both the concomitant orthorhombic/stripe antiferromagnetic transition $T_{C2}$ and the superconducting transition $T_{\mathrm{c}}$ are monotonically suppressed by added disorder at similar rates of about 0.1~K/$\mu \Omega$cm, as revealed through using resistivity variation as an intrinsic measure of scattering rate. In a stark contrast, a rapid suppression of the ${\mathrm{C}}_4$ phase at the rate of 0.24 K/$\mu \Omega \cdot$cm is found at $x=$0.25. Moreover, this suppression of the ${\mathrm{C}}_4$ phase is accompanied by unusual disorder-induced stabilization of the ${\mathrm{C}}_2$ phase, determined by resistivity and specific heat measurements. The rate of the ${\mathrm{C}}_4$ phase suppression is notably higher than the suppression rate of the spin-vortex phase in the Ni-doped CaKFe${}_4$As${}_4$ (0.16 K/$\mu \Omega$cm).