Bright excitons with negative-mass electrons

TL;DR

This paper reports the discovery of negative-mass excitons in monolayer WSe2, revealing novel optical phenomena and potential for exotic quasiparticles through band structure tuning.

Contribution

It identifies and explains negative-mass excitons in WSe2, combining experimental photoluminescence and ab initio calculations, a novel finding in excitonic physics.

Findings

Observation of narrow-band UV photoluminescence at 1.66 eV

Detection of cascaded phonon peaks up to ninth order

Explanation of exciton structure via GW-BSE calculations

Abstract

Bound electron-hole excitonic states are generally not expected to form with charges of negative effective mass. We identify such excitons in a single layer of the semiconductor WSe2, where they give rise to narrow-band upconverted photoluminescence in the UV, at an energy of 1.66 eV above the first band-edge excitonic transition. Negative band curvature and strong electron-phonon coupling result in a cascaded phonon progression with equidistant peaks in the photoluminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations unmask and explain the admixture of upper conduction-band states to this complex many-body excitation: an optically bright, bound exciton in resonance with the semiconductor continuum. This exciton is responsible for atomic-like quantum-interference phenomena such as electromagnetically induced transparency. Since band curvature can be tuned by pressure or strain, synthesis of exotic quasiparticles such as flat-band excitons with infinite reduced mass becomes feasible.