Exceptional Excitons

TL;DR

This paper introduces a new class of excitonic quasiparticles called exceptional excitons, arising from non-Hermitian physics in non-equilibrium semiconductors, with unique properties and potential for experimental observation.

Contribution

It uncovers exceptional excitons as a novel non-Hermitian quasiparticle emerging from non-equilibrium correlations in photoexcited semiconductors, supported by ab initio calculations.

Findings

Exceptional excitons are localized and long-lived.

They appear at population inversion and support excitonic superfluidity.

Distinct optical signatures enable their experimental detection.

Abstract

Non-Hermitian physics is reshaping our understanding of quantum systems by revealing states and phenomena without Hermitian counterparts. While non-Hermiticity is typically associated with gain-loss processes in open systems, we uncover a fundamentally different route to non-Hermitian behavior emerging from non-equilibrium correlations. In photoexcited semiconductors, the effective interaction between electrons and holes gives rise to a pseudo-Hermitian Bethe-Salpeter Hamiltonian (PH-BSH) that governs excitonic states in the presence of excited populations. Within this framework, we identify a previously unknown class of excitonic quasiparticles - exceptional excitons - corresponding to exceptional points embedded inside the electron-hole continuum. Exceptional excitons emerge at the onset of population inversion, and represent the strongly renormalized counterparts of the system's equilibrium excitons. They are spatially localized, protected against hybridization with the continuum, and remain long-lived even in regimes where conventional excitons undergo a Mott transition. Crucially, exceptional excitons appear only when the PH-BSH is evaluated with non-thermal, resonantly generated carrier populations that support an excitonic superfluid. Ab initio results for monolayer WS_2 explicitly demonstrate this scenario and show that exceptional excitons can be realized with existing ultrafast pumping techniques. We also identify distinctive optical and photoemission signatures that enable their unambiguous detection.