Hund flat band in a frustrated spinel oxide

TL;DR

This study uncovers a correlation-induced flat band in the frustrated spinel LiV2O4, revealing a new mechanism for heavy fermion behavior driven by intra-atomic Hund coupling and orbital-selective Mott physics, with implications for exotic quantum phases.

Contribution

It demonstrates the emergence of a flat band in a d-orbital spinel oxide due to Hund coupling, advancing understanding of heavy fermion phenomena without f orbitals.

Findings

Identification of a correlation-induced flat band via ARPES and DMFT.

Evidence of orbital-selective Mott state with fluctuating local moments.

Observation of heavy quasiparticles and high-temperature bad metal behavior.

Abstract

Electronic flat bands associated with quenched kinetic energy and heavy electron mass have attracted great interest for promoting strong electronic correlations and emergent phenomena such as high-temperature charge fractionalization and superconductivity. Intense experimental and theoretical research has been devoted to establishing the rich non-trivial metallic and heavy fermion phases intertwined with such localized electronic states. Here, we investigate the transition metal oxide spinel LiV2O4, an enigmatic heavy fermion compound lacking localized f orbital states. We use angle-resolved photoemission spectroscopy and dynamical mean field theory to reveal a new kind of correlation-induced flat band with suppressed inter-atomic electron hopping arising from intra-atomic Hund coupling. The appearance of heavy quasiparticles is ascribed to a proximate orbital-selective Mott state characterized by fluctuating local moments as evidenced by complementary magnetotransport measurements. The spectroscopic fingerprints of long-lived quasiparticles and their disappearance with increasing temperature further support the emergence of a high-temperature bad metal state observed in transport data. This work resolves a long-standing puzzle on the origin of heavy fermion behavior and unconventional transport in LiV2O4. Simultaneously, it opens a new path to achieving flat bands through electronic interactions in d-orbital systems with geometrical frustration, potentially enabling the realization of exotic phases of matter such as the fractionalized Fermi liquids.