Anisotropic electrical resistivity of LaFeAsO: Evidence for electronic nematicity

Single crystals of LaFeAsO were successfully grown out of KI flux. Temperature-dependent electrical resistivity was measured with current flow along the basal plane, rho(perpendicular to)(T), as well as with current flow along the crystallographic c axis, rho(parallel to) (T), the latter utilizing electron-beam lithography and argon-ion-beam milling. The anisotropy ratio was found to lie between rho(parallel to)/rho(perpendicular to) = 20-200. The measurement of rho(perpendicular to)(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 <= 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.