The discovery of high-temperature superconductivity in single layer FeSe epitaxially grown on SrTiO₃(001) substrates has instigated extensive debate over whether its pairing symmetry is conventional sign-preserving s-wave or unconventional sign-changing. Here, we probe the pairing state for single layer FeSe/SrTiO₃ grown by molecular beam epitaxy using scanning tunneling microscopy/spectroscopy. We observe robust in-gap bound states induced by non-magnetic Fe-vacancy defects within the Fe-plane, which exhibit strong spatial electron-hole asymmetry with two-fold symmetry in hole states and four-fold in electron states. The bound states exhibit no energy shift or splitting under an applied magnetic field, consistent with a sign-changing order parameter. This is further confirmed by defect bound state quasiparticle interference that shows a sign-changing behavior with a pair of corresponding peaks at the positive and negative energies near the impurity bound states. Our findings provide unambiguous evidence for a sign-changing pairing symmetry for single layer FeSe/SrTiO₃.

在SrTiO ₃（001）衬底上外延生长的单层FeSe中高温超导的发现引发了关于其配对对称性是传统的保持符号s还是非常规的符号改变的广泛争论。在这里，我们探讨了单层FeSe / SrTiO ₃的配对状态使用扫描隧道显微镜/光谱学通过分子束外延生长。我们观察到由Fe平面内的非磁性Fe空位缺陷引起的稳健的间隙内束缚态，这些缺陷表现出强的空间电子-空穴不对称性，其中空穴态具有2倍对称性，电子态具有4倍对称性。束缚态在施加的磁场下不表现出能量移位或分裂，这与符号改变阶数参数一致。缺陷结合态准粒子干涉进一步证实了这一点，该缺陷显示出符号改变行为，在杂质结合态附近在正和负能量处具有一对相应的峰。我们的发现为单层FeSe / SrTiO ₃的符号改变配对对称性提供了明确的证据。