Plasmon-driven exciton formation in a non-equilibrium Fermi liquid

TL;DR

This study demonstrates that under optical excitation, bulk plasmons in a Fermi liquid can mediate the formation of correlated excitonic states, revealing a non-equilibrium regime where collective modes stabilize bound states.

Contribution

It provides experimental evidence that plasmons can facilitate exciton formation in a non-equilibrium Fermi liquid, challenging the traditional view of plasmons solely as dissipation channels.

Findings

Plasmons transfer energy from bulk bands to surface states at high excitation density.

A long-lived spectral feature consistent with a Mahan exciton is observed.

Collective modes can stabilize correlated bound states in non-equilibrium conditions.

Abstract

Collective modes in Fermi liquids are usually regarded as dissipation channels that relax electronic excitations through Landau damping. Whether such modes can instead mediate the formation of correlated electronic states under non-equilibrium conditions remains an open question. Here we show that, under optical photo-doping, a bulk plasmon can drive correlated inter-band transfer within a transient electronic continuum. Using time- and angle-resolved photoemission spectroscopy (Tr-ARPES) on EuCd$_2$As$_2$ supported by electronic structure calculations, we observe that at high excitation density, plasmons transfer energy from a weakly dispersing bulk band into unoccupied surface states. This bulk-to-surface redistribution stabilizes a long-lived, energy-localized spectral feature consistent with a Mahan exciton. Our results uncover a non-equilibrium regime of Fermi-liquid physics in which collective modes do not merely dissipate energy, but also stabilize correlated bound states.