Unusual temperature evolution of quasiparticle band dispersion in electron-doped fese films

The discovery of high-temperature (high-T_c) superconductivity in one-monolayer FeSe on SrTiO₃ has attracted tremendous attention. Subsequent studies suggested the importance of cooperation between intra-FeSe-layer and interfacial interactions to enhance T_c. However, the nature of intra-FeSe-layer interactions, which would play a primary role in determining the pairing symmetry, remains unclear. Here we have performed high-resolution angle-resolved photoemission spectroscopy of one-monolayer and alkaline-metal-deposited multilayer FeSe films on SrTiO₃, and determined the evolution of quasiparticle band dispersion across T_c. We found that the band dispersion in the superconducting state deviates from the Bogoliubov-quasiparticle dispersion expected from the normal-state band dispersion with a constant gap size. This suggests highly anisotropic pairing originating from small momentum transfer and/or mass renormalization due to electron–boson coupling. This band anomaly is interpreted in terms of the electronic interactions within the FeSe layers that may be related to the high-T_c superconductivity in electron-doped FeSe.