Observation of a two-dimensional liquid of Fröhlich polarons at the bare SrTiO3 surface

Abstract

The polaron is a quasi-particle formed by a conduction electron (or hole) together with its self-induced polarization in a polar semiconductor or an ionic crystal. Among various polarizable examples of complex oxides, strontium titanate (SrTiO3) is one of the most studied. Here we examine the carrier type and the interplay of inner degrees of freedom (for example, charge, lattice, orbital) in SrTiO3. We report the experimental observation of Fröhlich polarons, or large polarons, at the bare SrTiO3 surface prepared by vacuum annealing. Systematic analyses of angle-resolved photoemission spectroscopy and X-ray absorption spectra show that these Fröhlich polarons are two-dimensional and only exist with inversion symmetry breaking by two-dimensional oxygen vacancies. Our discovery provides a rare solvable field theoretical model, and suggests the relevance of large (bi)polarons for superconductivity in perovskite oxides, as well as in high-temperature superconductors.

Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2

“…As found in similar systems, e.g. rutile TiO2 35 and SrTiO3 36 , the observed changes can be directly linked to the number of oxygen vacancies.…”

Floquet analysis of excitations in materials

“…The best known example being the electron-phonon kinks in the ARPES spectrum of superconducting materials [85]. However, electron-phonon coupling is also known to result in observable distinct satellite, or shadow, bands in the photo-electron spectral function [86][87][88], also known as Polaron replicas [89][90][91][92][93]. The avenue to create a phonon-driven Floquet material is to selectively excite such strongly coupled phonon modes and thus to create new properties of the electronic structure or to better understand its properties, as for example realized in [94].…”

dxz/yz orbital subband structures and chiral orbital angular momentum in the (001) surface states of SrTiO3

“…Several theoretical and experimental studies of the surface states of STO have been performed so far [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Especially, exploiting its surface sensitivity, ARPES has been used to directly measure the band structures of STO metallic states.…”