Anisotropic electrical resistivity of LaFeAsO: Evidence for electronic nematicity

Abstract

Single crystals of LaFeAsO were successfully grown out of KI flux. Temperature-dependent electrical resistivity was measured with current flow along the basal plane, ${\ensuremath{\rho}}_{\ensuremath{\perp}}(T)$, as well as with current flow along the crystallographic $c$ axis, ${\ensuremath{\rho}}_{\ensuremath{\parallel}}(T)$, the latter utilizing electron-beam lithography and argon-ion-beam milling. The anisotropy ratio was found to lie between ${\ensuremath{\rho}}_{\ensuremath{\parallel}}/{\ensuremath{\rho}}_{\ensuremath{\perp}}=20--200$. The measurement of ${\ensuremath{\rho}}_{\ensuremath{\perp}}(T)$ was performed with current flow along the tetragonal [1 0 0] direction and along the [1 1 0] direction and revealed a clear in-plane anisotropy already at $T\ensuremath{\le}175$ K. This is significantly above the orthorhombic distortion at ${T}_{0}=147$ K and indicates the formation of an electron nematic phase. Magnetic susceptibility and electrical resistivity give evidence for a change of the magnetic structure of the iron atoms from antiferromagnetic to ferromagnetic arrangement along the $c$ axis at ${T}^{*}=11$ K.