Realizing Topological Superconductivity in Ultrathin FeTeSe Films

Topological superconductors are the material platforms to realize emergent quasiparticles called Majorana zero modes, which are the basic units for nonabelian statistics and topological quantum computation. Earlier material realizations of topological superconductors are semiconductor nanowires coated with low-temperature superconductors, which not only bear substantial manufacturing complications but also remain under debate in terms of the fundamental physics picture. Here we introduce our latest effort using molecular beam epitaxy to fabricate ultrathin FeTe_xSe_1-xthin films on SrTiO₃ substrates, where the topological surface state is proximity-coupled to its own superconducting bulk. This leads to a robust topological superconducting state which is manifested in the observation of a superconducting Dirac surface state. By systematically varying the Te:Se ratio and the film thickness, we observe the Dirac surface state in ultrathin FeTe_xSe_1-xfilms with x > 0.7 and thickness > 2 layers. As we thin down the FeTe_xSe_1-xfilms, a drastic band structure evolution is identified due to the hybridization of the top and bottom Dirac surface states, which transforms the band structure quickly into a correlated insulating state in the monolayer limit. Our study not only marks the thinnest topological superconducting film ever fabricated so far, but also paves the road towards topological quantum devices based on ultrathin films.