Nematicity in LaFeAsO$_{1-x}$F$_x$

TL;DR

This study investigates nematicity in LaFeAsO$_{1-x}$F$_x$ superconductors, revealing short-range orbital-nematic order near 160 K that competes with superconductivity, based on transport, NMR, and lattice measurements.

Contribution

It provides evidence for short-range nematic order in LaFeAsO$_{1-x}$F$_x$ and its competition with superconductivity, using comprehensive experimental data.

Findings

Anomalies in transport properties at 160 K in underdoped samples.

Absence of long-range magnetic order and orthorhombic distortion.

Disappearance of nematic signatures near optimal doping.

Abstract

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 LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ 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 $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.