Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO${}_3$

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO${}_3$(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a $\mathrm{\Gamma }$-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at $\mathrm{\Gamma }$ and suggest that the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.