Spectroscopic Evidence for an Additional Symmetry Breaking in the Nematic State of FeSe Superconductor
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. It undergoes a structural phase transition at around 90 K to enter a nematic state but without a long-range magnetic order. So, FeSe is an ideal system to investigate the nematicity and superconducting mechanism in iron-based superconductor. It represents a unique system that superconductivity and nematicity coexist in the superconducting state, giving rise to its distinct electronic structure and superconducting gap structure. Many new superconductors have been derived from FeSe with enhanced superconductivity, including KxFe2？ySe2, (Li,Fe)OHFeSe, single-layer FeSe/SrTiO3 films and so on. 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. Cong Li and Prof. Lin Zhao in Prof. Xingjiang Zhou's group from national lab for superconductivity, institute of physics, Chinese Academy of Sciences, cooperate with Dr. Xianxin Wu, Dr. Le Wang, Prof. Youguo Shi, Prof. Jiangping Hu and Prof. Tao Xiang from institute of physics, Chinese Academy of Sciences, 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 reveals the FeSe exist an extra hidden order except the nematic order.
By performing latest-generation high resolution laser-based ARPES measurements on FeSe, they 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 dxz orbital and while the outer Fermi surface is consistent of dxz and dyz orbitals (Fig. 1). The measurement of the band structure indicates that three bands around the Brillouin zone center exhibit band splitting (Fig. 2). The 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 (Fig. 3).
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, the Ministry of Science and Technology and the Academy of Sciences Pilot B project.
Fig. 1. The discovery of double holelike Fermi surface structure around the Brillouin zone center in FeSe.
Fig. 2. Band structure of single-domain FeSe along high-symmetry cuts and their orbital nature.
Fig. 3. Calculated Fermi surface and band structure of FeSe and their comparison with the measured results
Contact: Institute of Physics Zhou Xingjiang Email：XJZhou@iphy.ac.cn
Key word: FeSe, ARPES, Double Fermi surface, Inversion symmetry breaking, Time reversal symmetry breaking
Abstract: The iron-based superconductor FeSe has attracted much recent attention because of its simple crystal structure, distinct electronic structure, and rich physics exhibited by itself and its derivatives. Both theoretical and experimental studies so far have provided a picture that FeSe consists of one holelike Fermi surface around the Brillouin zone center in its nematic state. Here we report direct observation of two holelike Fermi surface sheets around the Brillouin zone center, and the splitting of the associated bands, in the nematic state of FeSe by taking high-resolution laser-based angle-resolved photoemission measurements. These results indicate that, in addition to nematic order and spin-orbit coupling, there is an additional order in FeSe that breaks either inversion or time-reversal symmetries.