Topological Fermi-arc surface state covered by floating electrons on a two-dimensional electride.

Two-dimensional electrides can acquire topologically non-trivial phases due to intriguing interplay between the cationic atomic layers and anionic electron layers. However, experimental evidence of topological surface states has yet to be verified. Here, via angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM), we probe the magnetic Weyl states of the ferromagnetic electride [Gd₂C]²⁺·2e⁻. In particular, the presence of Weyl cones and Fermi-arc states is demonstrated through photon energy-dependent ARPES measurements, agreeing with theoretical band structure calculations. Notably, the STM measurements reveal that the Fermi-arc states exist underneath a floating quantum electron liquid on the top Gd layer, forming double-stacked surface states in a heterostructure. Our work thus not only unveils the non-trivial topology of the [Gd₂C]²⁺·2e⁻ electride but also realizes a surface heterostructure that can host phenomena distinct from the bulk. Electrides are an emerging family of materials with potential for hosting topological states. Here, the authors demonstrate the presence of Fermi-arc surface states in close proximity to the floating electron layer on the surface of an electride. [ABSTRACT FROM AUTHOR]