Electron–phonon interactions in the SrTiO3(001) 2D electron liquid

We use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, our ARPES data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime, the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range interaction.