Observation of orbital ordering and origin of the nematic order in FeSe

Q L Wang,X S Ye,Guoqing Wu,Jian Hu,R X Cao,D A Chareev,Jun Dong,A N Vasiliev,J B Zhang,Bing Wu,Y F Xu,X H Zeng

doi:10.1088/1367-2630/ab4927

Abstract

In iron-based superconductors the interactions driving the nematic order that breaks the lattice four-fold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent 77Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B0 up to 16 T. The 77Se-NMR spectra and Knight shift split when the direction of B0 is away from the direction perpendicular to the iron planes (i.e. B0 ∥ c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77Se nucleus, but these do not happen when B0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature Tnem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.

Highlights

The interactions between structure, magnetism and superconductivity in Fe-based superconductors have been of wide interests [1,2,3]
It is generally believed that the nematic order is electronic and the structural phase transition is the consequence of the nematic order, since the lattice distortion is much smaller than the observed anisotropy of the in-plane resistivity in the nematic phase [9, 14]
Undoubtedly the spectrum split is the result of a structure symmetry break in the ab-plane due to the tetragonal-to-orthorhombic structure phase transition, which is known as the consequence of the electronic nematic order in the Fe-planes [23, 24]

Summary

15 October 2019

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Introduction

Experimental section

Results and discussion

Conclusions