A single monolayer (ML) FeSe film grown on a SrTiO3(001) shows a superconducting transition at 109 K. The mechanism of such a high Tc is still an open question. The AFM spin fluctuations were proposed to play an important role in the pairing of iron pnictides. Unlike other iron pnictides, bulk FeSe crystals do not show AFM order. First-principles calculations have shown that the FeSe/STO interface could enhance the AFM interaction, which helps maintain large spin fluctuations under heavy electron doping. Therefore, it is very interesting to study the magnetic ground state of the 1-ML film before electron doping. Prof. Qian Dong from Shanghai Jiao Tong University and Prof. Wu Di from Nanjing University present direct evidence indicating that the magnetic ground state of the parent 1-ML FeSe/STO film is AFM by using exchange bias effect measurements and ARPES. The magnetic blocking temperature is > 140 K for a 1-ML film. The antiferromagnetic order disappears after electron doping. Their findings provided very important information for the comprehensive understanding of the novel properties in FeSe/STO films. (Phys. Rev. Lett.2018, 120, 097001)

 (a)ARPES of undoped single ML FeSe and biased magnetic hysteresis loop. (b) ARPES of heavily electron doped single ML FeSe and non-biased magnetic hysteresis loop.