Anisotropic resistivity in underdoped single crystals (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, $0 \leq x<0.35$

Abstract

Temperature-dependent in-plane, $\rho_a(T)$, and inter-plane, $\rho_c(T)$, resistivities were measured for the iron-arsenide superconductor (Ba$_{1-x}$K$_x$)Fe $_2$As$_2$ over a broad doping range from parent compound to optimal doping $T_c\approx 38 K$, $0\leq x \leq 0.35$. The coupled magnetic/structural transition at $T_{SM}$ is clearly observed for samples with $T_c <$26 K ($x <0.25$), however its effect on resistivity is much weaker than in the electron-doped Ba(Fe$_{1-x}$Co$_x$)Fe $_2$As$_2$, and the transition leads only to a decrease of resistivity. In addition to the feature at $T_{SM}$, the inter-plane resistivity shows a maximum at $T^*\sim$200 K, which moves slightly to higher temperature with doping, revealing a trend opposite to the electron-doped materials. A smeared feature at about the same temperature is seen in $\rho_a(T)$. For $T<T^*$, the temperature dependence of resistivity shows systematic evolution and is close to linear at optimal doping. This feature, being most pronounced for $\rho_c(T)$, suggests the existence of a quantum critical point close to optimal doping.