Coupling quantum anomalous Hall (QAH) state with superconducting state offers an attractive approach to detect the signature of alluding topological superconducting state [Science {357}, 294 (2017)], but its explanation could be clouded by disorder effects in magnetic doped QAH materials. On the other hand, an antiferromagnetic (AFM) quantum spin Hall (QSH) state has been identified in the well-known high-temperature 2D superconductor of monolayer FeSe [Nat. Mater. {15}, 968 (2016)]. Here we report a light-induced type-II band inversion (BI) and a QSH-to-QAH phase transition in the monolayer FeSe. Depending on the handedness of light, a spin-tunable QAH state with a high Chern number of $\pm2$ is realized. In contrast to the conventional type-I BI resulting from intrinsic spin-orbital coupling (SOC) that inverts band odd number of times and respects time reversal symmetry, the type-II BI results from a light-induced handedness-dependent effective SOC, which inverts band even number of times and does not respect time reversal symmetry. The interplay between these two SOC terms makes the spin-up and -down bands of an AFM QSH state respond oppositely to a circularly polarized light, leading to the type-II BI and an exotic topological phase transition. Our finding affords an exciting opportunity to detect Majorana fermions in one single material without magnetic doping.

中文翻译：

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单层FeSe中光诱导的II型能带反转和量子异常霍尔态

量子异常霍尔（QAH）状态与超导状态的耦合为检测暗示拓扑超导状态的特征提供了一种有吸引力的方法[Science {357}，294（2017）]，但其解释可能会因掺杂QAH磁性材料中的无序效应而模糊不清。 。另一方面，已经在单层FeSe的高温2D超导体中发现了反铁磁（AFM）量子自旋霍尔（QSH）状态。母校 {15}，968（2016）]。在这里，我们报告了单层FeSe中光诱导的II型能带反转（BI）和QSH到QAH的相变。根据光的惯用性，可以实现具有$ \ pm2 $的高Chern值的可自旋可调QAH状态。与由固有自旋轨道耦合（SOC）反转带奇数倍并尊重时间反转对称性的常规I型BI相比，II型BI由光感应的依赖于手性的有效SOC产生，将带反转次数为偶数，并且不考虑时间反转对称性。这两个SOC项之间的相互作用使AFM QSH状态的自旋上升和下降带对圆偏振光产生相反的响应，从而导致II型BI和奇异的拓扑相变。我们的发现提供了令人兴奋的机会，无需磁掺杂就可以在一种材料中检测马约拉纳费米子。它使带偶数反转，并且不考虑时间反转对称性。这两个SOC项之间的相互作用使AFM QSH状态的自旋上升和下降带对圆偏振光产生相反的响应，从而导致II型BI和奇异的拓扑相变。我们的发现提供了令人兴奋的机会，无需磁掺杂就可以在一种材料中检测马约拉纳费米子。它使带偶数反转，并且不考虑时间反转对称性。这两个SOC项之间的相互作用使AFM QSH状态的自旋上升和下降带对圆偏振光产生相反的响应，从而导致II型BI和奇异的拓扑相变。我们的发现提供了令人兴奋的机会，无需磁掺杂就可以在一种材料中检测马约拉纳费米子。