Yuan Yonghao, Fan Xuemin, Wang Xintong, He Ke, Zhang Yan, Xue Qi-Kun, Li Wei
State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing, China.
Frontier Science Center for Quantum Information, Beijing, China.
Nat Commun. 2021 Apr 13;12(1):2196. doi: 10.1038/s41467-021-22516-2.
Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO, but not for multilayer films, in which large strength of nematicity develops. However, the link between the high-transition temperature superconductivity in monolayer and the correlation related nematicity in multilayer FeSe films is not well understood. Here, we use low-temperature scanning tunneling microscopy to study few-layer FeSe thin films grown by molecular beam epitaxy. We observe an incommensurate long-range smectic phase, which solely appears in bilayer FeSe films. The smectic order still locally exists and gradually fades away with increasing film thickness, while it suddenly vanishes in monolayer FeSe, indicative of an abrupt smectic phase transition. Surface alkali-metal doping can suppress the smectic phase and induce high-T superconductivity in bilayer FeSe. Our observations provide evidence that the monolayer FeSe is in close proximity to the smectic phase, and its superconductivity is likely enhanced by this electronic instability as well.
生长在 SrTiO₃ 上的单层 FeSe 的超导性显著增强，但多层膜并非如此，多层膜中会出现很强的向列性。然而，单层中高转变温度超导性与多层 FeSe 膜中与关联相关的向列性之间的联系尚未得到很好的理解。在此，我们使用低温扫描隧道显微镜来研究通过分子束外延生长的少层 FeSe 薄膜。我们观察到一种非公度的长程近晶相，它仅出现在双层 FeSe 膜中。近晶序仍然局部存在，并随着膜厚度的增加而逐渐消失，而在单层 FeSe 中它突然消失，这表明存在近晶相的突然转变。表面碱金属掺杂可以抑制双层 FeSe 中的近晶相并诱导高温超导性。我们的观察结果提供了证据，表明单层 FeSe 与近晶相非常接近，并且其超导性也可能通过这种电子不稳定性得到增强。
