Nematicity in LaFeAsO1−xFx

authoren Orbital ordering has recently emerged as another important state in iron-based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO1−x Fx with a focus on resistivity, Nernst effect, thermal expansion, and As nuclear magnetic resonance (NMR) data. The transport properties at the underdoped level exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e., at about 160 K. Furthermore, the As-NMR spin-lattice relaxation rate reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping which suggests that in LaFeAsO1−x Fx the nematic phase actually competes with superconductivity. Schematic electronic phase diagram of LaFeAsO1−x Fx.