Non-Fermi liquid behavior in a correlated flatband pyrochlore lattice

TL;DR

This study reports the discovery of a correlated flat band at the Fermi level in a pyrochlore metal, revealing non-Fermi liquid behavior due to combined electron interactions and geometric frustration, advancing understanding of correlated topological materials.

Contribution

It demonstrates the realization of correlated flat bands in a 3d electron system through combined experimental and theoretical approaches, highlighting a pathway to correlated topological phases.

Findings

Observation of a flat band at the Fermi level in CuV₂S₄

Deviation from Fermi-liquid transport behavior

Large Sommerfeld coefficient indicating strong correlations

Abstract

Electronic correlation effects are manifested in quantum materials when either the onsite Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in the cuprate superconductors or heavy fermion systems while the latter in twisted bilayer graphene or geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quantum material--the design principle to produce a correlated topological flat bands pinned at the Fermi level--remains rare. Here, using angle-resolved photoemission spectroscopy, we report the observation of a flat band at the Fermi level in a 3$d$ pyrochlore metal CuV$_2$S$_4$. From a combination of first-principles calculations and slave-spin calculations, we understand the origin of this band to be a destructive quantum-interference effect associated with the V pyrochlore sublattice and further renormalization to the Fermi level by electron interactions in the partially filled V $t_{2g}$ orbitals. As a result, we find transport behavior that indicates a deviation from Fermi-liquid behavior as well as a large Sommerfeld coefficient. Our work demonstrates the pathway into correlated topology by constructing and pinning correlated flat bands near the Fermi level out of a pure $d$-electron system by the combined cooperation of local Coulomb interactions and geometric frustration in a pyrochlore lattice system.