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 using 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.
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We thank R. Fernandes, M. Norman, J. Analytis, M. Le Tacon, J. Pelliciari, E. Ergeçen, Q. Jiang, P. Malinowski, M. Bachmann, M. Yi and H. Pfau for discussions. We thank L. G. Pimenta Martins and Y. Lee for assistance with preliminary X-ray experiments. Work at MIT (C.A.O., J.J.S., Q.S. and R.C.) is supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0019126 (resonant X-ray spectroscopy and diffraction measurements, and sample growth), by the Air Force Office of Scientific Research Young Investigator Program under grant FA9550-19-1-0063 (strain set-up development and transport measurements), and by the National Science Foundation under grant no. 1751739 (optical dichroism). The work performed at the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-06CH11357. Sample preparation was performed in part at HPCAT (Sector 16) at the Advanced Photon Source, Argonne National Laboratory. J.J.S. acknowledges the support of the National Science Foundation MPS-Ascend Postdoctoral Research Fellowship under award no. 2138167. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
The authors declare no competing interests.
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Occhialini, C.A., Sanchez, J.J., Song, Q. et al. Spontaneous orbital polarization in the nematic phase of FeSe. Nat. Mater. 22, 985–991 (2023). https://doi.org/10.1038/s41563-023-01585-2