Spontaneous orbital polarization in the nematic phase of FeSe

TL;DR

This study demonstrates that spontaneous orbital polarization is the primary order parameter driving nematicity in FeSe, revealed through X-ray linear dichroism measurements showing lattice-independent orbital anisotropy and diverging polarizability near the transition.

Contribution

It provides direct experimental evidence that orbital polarization, independent of lattice distortions, underpins the nematic phase in FeSe, advancing understanding of its electronic origin.

Findings

Orbital polarization exists independently of lattice distortion.

Orbital polarizability diverges approaching the nematic transition.

Spontaneous orbital polarization is identified as the primary nematic order parameter.

Abstract

The origin of nematicity in FeSe remains a critical outstanding question towards understanding unconventional superconductivity in proximity to nematic order. To understand what drives the nematicity, it is essential to determine which electronic degree of freedom admits a spontaneous order parameter independent from the structural distortion. Here, we use X-ray linear dichroism at the Fe K pre-edge to measure the anisotropy of the 3d orbital occupation as a function of in situ applied stress and temperature across the nematic transition. Along with X-ray diffraction to precisely quantify the strain state, we reveal a lattice-independent, spontaneously-ordered orbital polarization within the nematic phase, as well as an orbital polarizability that diverges as the transition is approached from above. These results provide strong evidence that spontaneous orbital polarization serves as the primary order parameter of the nematic phase.