Observation of discrete CdGM states inthe vortex core of single layer FeSe/SrTiO₃

For conventional type-II superconductors, confined quasi-particles in the vortex cores will give rise to Caroli-de Gennes-Matricon (CdGM) bound states with energies of E=μΔ²/E_F (where μ is a half integer, ±1/2, ±3/2…). However, discrete CdGM states were seldom identified due to the small value of Δ²/E_F, which is usually in the micro-eV range. Here we report a clear observation of multiple discrete CdGM states in single layer FeSe film on SrTiO₃, through high energy resolved tunneling spectrum. We found that the energy of these CdGM states can be well described by E=μΔ²/E_F with μ equal to half integers (no zero-bias conductance peak observed). Therefore our results support a simple s-wave pairing scenario for this high Tc system. Besides, we studied magnetic impurity (Fe) induced in-gap state with high energy resolution (T=0.4K). We found that all the in-gap state are off zero bias and split at high field, despite their intensity and location may vary for different impurities. Our results would help to further unveil the mechanism underlying the high Tc single layer FeSe/SrTiO₃ superconductor.