Landau-Level\nQuantization and Band Splitting of FeSe\nMonolayers Revealed by Scanning Tunneling Spectroscopy

Two-dimensional (2D) superconductors that reside on substrates\nmust be influenced by Rashba spin–orbit coupling (SOC). The\nintriguing effect of Rashba-type SOCs on iron-based superconductors\n(IBSs) has remained largely a mystery. In this work, we unveil modified\nLandau-level spectroscopy and the intricate band splitting of FeSe\nmonolayers through the precision of scanning tunneling spectroscopy,\nwhich unequivocally demonstrates the presence of Rashba SOC. The discovery\nsheds light on a nonparabolic electron band at the X and/orY point,\ndisplaying a distinctive Landau quantization behavior characterized\nby <i>E</i><sub><i>n</i></sub> ∝ (<i>nB</i>)<sup>4/3</sup>. The theoretical model aligns with our\nexperimental insights, positing that the <i>k</i><sup>4</sup>-term of the electron band becomes predominant and profoundly reshapes\nthe band structure. Our results underscore the pivotal role of the\nRashba SOC effect on 2D superconductors and set the stage to probe\nnew quantum states in systems with remarkably low carrier concentrations.