Good plasmons in a bad metal

TL;DR

This paper demonstrates the existence of long-lived hyperbolic plasmon polaritons in a highly correlated, incoherent metal, revealing how electronic correlations influence collective charge excitations.

Contribution

It provides direct optical imaging of hyperbolic plasmon polaritons in MoOCl2 and links their properties to the material's orbital-selective incoherent electronic phase.

Findings

HPPs are long-lived in MoOCl2 despite strong correlations.

Electronic interactions renormalize the plasma frequency.

MoOCl2 exhibits an orbital-selective incoherent Peierls phase.

Abstract

Correlated materials may exhibit unusually high resistivity increasing linearly in temperature, breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed. The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies suggest plasmons are overdamped while others detect unrenormalized plasmons. Here, we present direct optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl2. HPPs are plasmon-photon modes that waveguide through extremely anisotropic media and are remarkably long-lived in MoOCl2. Many-body theory supported by photoemission results reveals that MoOCl2 is in an orbital-selective and highly incoherent Peierls phase. Different orbitals acquire markedly different bonding-antibonding character, producing a highly-anisotropic, isolated Fermi surface. The Fermi surface is further reconstructed and made partly incoherent by electronic interactions, renormalizing the plasma frequency. HPPs remain long-lived in spite of this, allowing us to uncover previously unseen imprints of electronic correlations on plasmonic collective modes.