"Observations of plasmarons in a two-dimensional system: Tunneling measurements using time-domain capacitance spectroscopy", Phys. Rev. B 85, 081306(R) (2012)
Calculations of the single-particle density of states (SPDOS) of electron liquids have long predicted that there exist two distinct charged excitations: the usual quasiparticle consisting of an electron or hole, and a plasmaron consisting of a hole resonantly bound to real plasmons in the Fermi sea. Using tunneling spectroscopy to measure the SPDOS of a 2D electronic system, we demonstrate the detection of a plasmaron in a 2D system in which electrons have mass. With the application of a magnetic field we discover unpredicted magnetoplasmarons which resemble Landau levels with a negative index.
Two bright features (they look like green diagonal lines) appear in the figure on the right. The upper feature corresponds to the band-edge of a 2D electron system. The lower feature exists below the band-edge, and this feature is the long elusive "plasmaron".