Tailoring the nature and strength of electron phonon interactions in the SrTiO3 001 2D electron liquid

Surfaces and interfaces offer new possibilities for tailoring the many body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, 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, where superconductivity is found in the analogous 2DEL at the LaAlO3 SrTiO3 interface, our angle resolved photoemission data show replica bands separated by 100 amp; 8201;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 preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short range electron phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3 based 2DEL