Particle-hole mixed Bogoliubov quasiparticles and Cooper instability in single-unit-cell FeSe/SrTiO3 films

In conventional superconductors, Bogoliubov quasiparticles and Cooper instability provide a paradigm to describe the superconducting state and the superconducting transition, respectively. However, whether these concepts can be adapted to describe Fe-based superconductors requires rigorous examinations from experiments. Here, we report angle-resolved photoemission studies on single-layer FeSe films grown on SrTiO3 substrate. Due to the improved clarity, our results reveal both particle and hole branches of the energy band with clear quasiparticles. The dispersion and coherence factors are extracted, which unveil the particle-hole mixed Bogoliubov quasiparticles in the superconducting state of the FeSe/STO films. Effective pairing susceptibility is also deduced as a function of temperature, which indicates the persistence of Cooper instability in Fe-based superconductors. It is still under debate whether unconventional superconductors can still be described in terms of Bogoliubov quasiparticles in the superconducting state. Here, angle-resolved photoemission spectroscopy measurements on FeSe/SrTiO3 films reveal particle-hole mixed Bogoliubov quasiparticles, despite the likely unconventional pairing mechanism.