Kavli Affiliate: Darrell G. Schlom
| First 5 Authors: Zubia Hasan, Grace A. Pan, Harrison LaBollita, Austin Kaczmarek, Suk Hyun Sung
| Summary:
Geometrically frustrated lattices can display a range of correlated
phenomena, ranging from spin frustration and charge order to dispersionless
flat bands due to quantum interference. One particularly compelling family of
such materials is the half-valence spinel Li$B_2$O$_4$ materials. On the
$B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic
behavior and charge frustration leads to a superconducting state in
LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however,
LiTi$_2$O$_4$ has primarily been understood as a conventional BCS
superconductor despite a lattice structure that could host more exotic
groundstates. Here, we present a multimodal investigation of LiTi$_2$O$_4$,
combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body
theoretical calculations. Our data reveals a novel mobile polaronic ground
state with spectroscopic signatures that underlie co-dominant electron-phonon
coupling and electron-electron correlations also found in the lightly doped
cuprates. The cooperation between the two interaction scales distinguishes
LiTi$_2$O$_4$ from other superconducting titanates, suggesting an
unconventional origin to superconductivity in LiTi$_2$O$_4$. Our work deepens
our understanding of the rare interplay of electron-electron correlations and
electron-phonon coupling in unconventional superconducting systems. In
particular, our work identifies the geometrically frustrated, mixed-valence
spinel family as an under-explored platform for discovering unconventional,
correlated ground states.
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