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. Determination of its intrinsic electronic structure is crucial to understanding its physical properties and superconductivity mechanism. 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. The new Fermi surface topology asks for reexamination of the existing theoretical and experimental understanding of FeSe and stimulates further efforts to identify the origin of the hidden order in its nematic state.

中文翻译：

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FeSe超导体向列态中另外对称性破裂的光谱证据

铁基超导体FeSe由于其简单的晶体结构，独特的电子结构以及其自身及其衍生物所表现出的丰富物理特性而备受关注。确定其固有的电子结构对于理解其物理性质和超导机理至关重要。到目前为止，理论和实验研究都提供了这样的图像：FeSe由处于向列状态的布里渊区中心周围的一个孔状费米表面组成。在这里，我们通过对高分辨率的基于激光的角度分辨光发射测量进行测量，从而直接观察到布里渊区中心周围两个孔状费米表面片以及相关带的分裂，处于FeSe的向列状态。这些结果表明，除了向列顺序和自旋轨道耦合外，FeSe中还有一个附加的顺序可以打破反演或时间反演的对称性。新的费米表面拓扑结构要求对现有的对硒化铁的理论和实验理解进行重新检验，并刺激人们做出进一步的努力来确定其向列状态下的隐蔽秩序。