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Z. Wang, S. McKeown Walker, A. Tamai, Y. Wang, Z. Ristic, F. Bruno, A. de la Torre, S. Riccò, N. Plumb, M. Shi, P. Hlawenka, J. Sánchez-Barriga, A. Varykhalov, T. Kim, M. Hoesch, P. King, W. Meevasana, U. Diebold, J. Mesot, B. Moritz, T. Devereaux, M. Radovic, F. Baumberger:
"Tailoring the nature and strength of electron-phonon interactions in the SrTiO₃(001) 2D electron liquid";
Nature Materials, 15 (2016), 835 - 840.

the many-body interactions that dominate the electrical
and thermal properties of transition metal oxides1-4. Here,
we use the prototypical two-dimensional electron liquid
(2DEL) at the SrTiO3(001) surface5-7 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 interface8-13, our angle-resolved photoemission
data show replica bands separated by 100meV 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 2DELs.