Physical Review Letters ( IF 8.385 ) Pub Date : 2020-09-15 , DOI: 10.1103/physrevlett.125.126401
Andrey Geondzhian; Alessia Sambri; Gabriella M. De Luca; Roberto Di Capua; Emiliano Di Gennaro; Davide Betto; Matteo Rossi; Ying Ying Peng; Roberto Fumagalli; Nicholas B. Brookes; Lucio Braicovich; Keith Gilmore; Giacomo Ghiringhelli; Marco Salluzzo

Despite its simple structure and low degree of electronic correlation, ${\mathrm{SrTiO}}_{3}$ (STO) features collective phenomena linked to charge transport and, ultimately, superconductivity, that are not yet fully explained. Thus, a better insight into the nature of the quasiparticles shaping the electronic and conduction properties of STO is needed. We studied the low-energy excitations of bulk STO and of the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ two-dimensional electron gas (2DEG) by Ti ${\mathrm{L}}_{3}$ edge resonant inelastic x-ray scattering. In all samples, we find the hallmark of polarons in the form of intense $dd+\text{phonon}$ excitations, and a decrease of the LO3-mode electron-phonon coupling when going from insulating to highly conducting STO single crystals and heterostructures. Both results are attributed to the dynamic screening of the large polaron self-induced polarization, showing that the low-temperature physics of STO and STO-based 2DEGs is dominated by large polaron quasiparticles.

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