Spectroscopic investigation of frustrated ground states in mixed-valence spinel oxides

AB₂O₄ spinel oxides have a corner-sharing B-site pyrochlore sublattice which gives rise to inherent geometric frustration. Of particular interest are lithium based spinel oxides (LiB₂O₄) where the mixed valence B-site may give rise to charge frustration. The interplay of the geometric and charge degrees of freedom have led to non-trivial ground states like unconventional superconductivity in LiTi₂O₄ and unusual d-electron heavy fermion behavior in LiV₂O₄. Here, we present a multimodal investigation of Li-based spinel oxide materials, combining ARPES, RIXS and ab-initio many body theoretical calculations. Our combined spectroscopic approach allows us to explore how the interplay of charge and geometric frustration in these materials results in a rich, competing landscape of many-body interactions. In particular, our work on LiTi₂O₄ reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations that persist far above the superconducting transition. The cooperation between the two interaction scales suggests an unconventional origin to superconductivity in LiTi₂O₄. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems and further promotes the mixed-valence spinel-oxide structure as an under-explored platform for correlated physics.