  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.
