Direct observation of a flat band and a partially filled interlayer band in a Cs/graphene/Cs trilayer

We investigate, using angle-resolved photoemission spectroscopy (ARPES), the electronic structure of graphene sandwiched in between two Cs layers with $2times 2$ and $sqrt{3}timessqrt{3}$ structures. ARPES reveals that this trilayer has a flat electron energy dispersion at the Fermi level and a partially occupied alkali metal $s$ band. The joint occurence of these two features together has proven impossible to achieve so far. Here, we introduce a technique to induce these features by the backfolding of graphene bands in a $2times 2$ superstructure and their subsequent hybridization with Cs derived states. The large band renormalization reduces the $pi$ to $pi^*$ transition energy at the $M$ point to only $4.15$~eV. Hence the excitations of this system can be probed by optical methods with ultraviolet (UV) excitation energy. Performing UV, ultra-high vacuum Raman spectroscopy of this trilayer, we find the Raman $G$ band of graphene is shifted down in frequency by almost 60~cm$^{-1}$ and sharply reduces its Fano asymmetry as the Fermi level is pushed beyond the Lifshitz transition.