NMR Search for the Spin Nematic State in LaFeAsO Single Crystal

The mechanism underlying high-$T_{c}$ superconductivity in iron-pnictides remains a major puzzle in condensed matter. Earlier NMR measurements provide evidence for a correlation between $T_{c}$ and the enhancement of low frequency spin fluctuations\footnote{F. L. Ning, T. Imai. et al., Phys. Rev. Lett. 104, 037001 (2010).}. However, slowing of spin fluctuations is accompanied by lattice softening, which is a major complication in this scenario. The intermediate temperature range between the tetragonal-orthorhombic structural phase transition at $T_{TO}$ and SDW transition at $T_{SDW}$ may be a realization of spin nematic state\footnote{C. Fang et al., Phys. Rev. B 77, 224509 (2008)}. We report $^{75}$As single crystal NMR study of LaFeAsO \footnote{M. Fu et al., arXiv:1208.5652, to appear in Phys. Rev. Lett.}. We have found that the low frequency spin dynamics exhibits a strong two-fold anisotropy within each orthorhombic domain below $T_{TO}$ This intermediate state then freezes progressively into a static SDW below $T_{SDW}$. Our results reveal the presence of an exotic intermediate spin state below $T_{TO}$ with the signature of spin nematicity.