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.
@article{arxiv.2006.14705,
title = {Bright excitons with negative-mass electrons},
author = {Kai-Qiang Lin and Chin Shen Ong and Sebastian Bange and Paulo E. Faria Junior and Bo Peng and Jonas D. Ziegler and Jonas Zipfel and Christian Bäuml and Nicola Paradiso and Kenji Watanabe and Takashi Taniguchi and Christoph Strunk and Bartomeu Monserrat and Jaroslav Fabian and Alexey Chernikov and Diana Y. Qiu and Steven G. Louie and John M. Lupton},
journal= {arXiv preprint arXiv:2006.14705},
year = {2021}
}