Landau-Level Quantization and Band Splitting of FeSe Monolayers Revealed by Scanning Tunneling Spectroscopy.

Two-dimensional (2D) superconductors that reside on substrates must be influenced by Rashba spin-orbit coupling (SOC). The intriguing effect of Rashba-type SOCs on iron-based superconductors (IBSs) has remained largely a mystery. In this work, we unveil modified Landau-level spectroscopy and the intricate band splitting of FeSe monolayers through the precision of scanning tunneling spectroscopy, which unequivocally demonstrates the presence of Rashba SOC. The discovery sheds light on a nonparabolic electron band at the X and/orY point, displaying a distinctive Landau quantization behavior characterized by En ∝ (nB)4/3. The theoretical model aligns with our experimental insights, positing that the k4-term of the electron band becomes predominant and profoundly reshapes the band structure. Our results underscore the pivotal role of the Rashba SOC effect on 2D superconductors and set the stage to probe new quantum states in systems with remarkably low carrier concentrations.