Checkerboard Antiferromagnetic Order in FeSe Monolayer

Although the paramagnetic state has been confirmed in FeSe bulk via inelastic neutron-scattering (INS), the magnetic states in FeSe monolayer and thin films remain mysterious due to the difficulties in both experimental and theoretical approaches. Experimentally, current magnetic measurement techniques are extremely difficult to be applied to thin film structures. Theoretically, first principle calculation can not deal with paramagnetic system with strong spin fluctuations. Here we report a systematic approach based on surface defects and in-plane strain, and utilizing scanning tunneling measurements (STM) combining with first-principles calculations, to identify the magnetic states in FeSe monolayer and thin films. We provide a strong evidence of the existence of checkerboard antiferromagnetic order in FeSe monolayer by measuring and computing scanning tunneling microscopy images of two typical surface defects. The checkerboard antiferromagnetic order in FeSe monolayer is further verified by estimating the magnetic property of surface defect experimentally and theoretically. Moreover, we confirm the checkerboard antiferromagnetic order exists also in FeSe thin films by studying the electronic band structure evolution with in-plane strain.