In iron-based superconductors, FeSe has the simplest crystal structure which consists solely of the basic building block, the FeSe layers, that dictates the superconductivity. So, FeSe is an ideal system to investigate the nematicity and superconducting mechanism in iron-based superconductor.

Determination of the electronic structure is essential to understanding the physical properties and superconductivity mechanism in bulk FeSe and its many derivatives. Both theoretical and experimental studies so far have provided a picture that FeSe consists of only one hole-like Fermi surface around the Brillouin zone center in its nematic state. All the experimental and theoretical understandings of FeSe have been based on the single Fermi surface picture.

Recently, Dr. LI Cong and Prof. ZHAO Lin in Prof. ZHOU Xingjiang's group from the institute of physics (IOP) of the Chinese Academy of Sciences, together with collaborators at IOP, using the latest generation of energy analyzer based on time of flight laser-ARPES, combining with the theoretical calculation, found a new Fermi surface structure in FeSe superconductors, and revealed that the FeSe exist an extra hidden order except the nematic order.

By performing latest-generation high resolution laser-based ARPES measurements on FeSe, the researchers have discovered for the first time that there are two hole-like Fermi surface sheets around the Brillouin zone center in single domain bulk FeSe.

The inner Fermi surface is consistent of d_xz orbital while the outer Fermi surface is consistent of d_xz and d_yz orbitals. The measurement of the band structure indicates that three bands around the Brillouin zone center exhibit band splitting.

Further theoretical analysis shows that it is impossible to induce the observed double Fermi surface structures and corresponding band splitting if only considering nematic order and orbital hybridization. An extra new order except nematic order, hidden order, should be present that breaks inversion symmetry or time reversal symmetry to cause the spin degeneracy and induce double Fermi surfaces and band splitting.

The new discovered Fermi surface structure and band splitting in the nematic state of FeSe asks for reexamination of the previous theoretical and experimental understanding of FeSe and will stimulate further efforts to identify the hidden order in the nematic state of FeSe.

This study, entitled "Spectroscopic Evidence for an Additional Symmetry Breaking in the Nematic State of FeSe Superconductor", was published on Physical Review X.

The study was supported by the National Science Foundation of China, the Ministry of Science and Technology and the Academy of Sciences Pilot B project.