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

TL;DR

This study uses NMR measurements to reveal that orbital ordering causes nematic order in FeSe, with antiferromagnetic spin fluctuations likely driving superconductivity within this nematic phase.

Contribution

It provides direct evidence linking orbital ordering to nematic order and clarifies the role of spin fluctuations in FeSe's superconductivity.

Findings

Orbital ordering observed via NMR spectral splitting and Knight shift suppression.

Orbital order suppressed by magnetic field up to 16 T.

Antiferromagnetic spin fluctuations develop below 40 K, below the nematic transition.

Abstract

To elucidate the origin of nematic order in FeSe, we performed field-dependent 77Se-NMR measurements on single crystals of FeSe. We observed orbital ordering from the splitting of the NMR spectra and Knight shift and a suppression of it with magnetic field B0 up to 16 T applied parallel to the Fe-planes. There is a significant change in the distribution and magnitude of the internal magnetic field across the orbital ordering temperature Torb while stripe-type antiferromagnetism is absent. Giant antiferromagnetic (AFM) spin fluctuations measured by the NMR spin-lattice relaxation are gradually developed starting at ~ 40 K, which is far below the nematic ordering temperature Tnem. These results demonstrate that orbital ordering is the origin of the nematic order, and the AFM spin fluctuation is the driving mechanism of superconductivity in FeSe under the presence of the nematic order.