The Quadruplon in a Monolayer Semiconductor

So far, composite particles involving two or three constituent particles have been experimentally identified, such as the Cooper pairs, excitons, and trions in condensed matter physics, or diquarks and mesons in quantum chromodynamics. Although the four-body irreducible entities have long been predicted theoretically in a variety of physical systems alternatively as quadruplons, quadrons, or quartets, the closely related experimental observation so far seems to be restricted to the field of elementary particles (e.g. the recent tetraquark at CERN). In this article, we present the first experimental evidence for the existence of a four-body irreducible entity, the quadruplon, involving two electrons and two holes in a monolayer of Molybdenum Ditelluride. Using the optical pump-probe technique, we discovered a series of new spectral features that are distinct from those of trions and bi-excitons. By solving the four-body Bethe-Salpeter equation in conjunction with the cluster expansion approach, we are able to explain these spectral features in terms of the four-body irreducible cluster or the quadruplons. In contrast to a bi-exciton which consists of two weakly bound excitons, a quadruplon consists of two electrons and two holes without the presence of an exciton. Our results provide experimental evidences of the hitherto theorized four-body entities and thus could impact the understanding of the structure of matter in a wide range of physical systems or new semiconductor technologies

Threshold-like Superlinear Accumulation of Excitons in a Gated Monolayer Transition Metal Dichalcogenide

Coexistence and mutual conversion of various excitonic species in semiconductors are the essence of Mott physics and underpin important device applications such as excitonic or polaritonic lasers. The emergence of monolayer semiconductors provides unprecedented opportunities to study these fundamental issues due to the much larger exciton binding energies. In this paper, we study the evolution of the coupled exciton–trion system in electrically gated monolayer MoTe2 devices. Contrary to the conventional linear scaling, we found that exciton density exhibits an abnormal three-stage scaling behavior: a conventional linear scaling at low pumping levels, followed by a superlinear behavior accompanied by a strong saturation of trion emission. In the third stage, the exciton emission returns to the linear scaling with the further increase of pumping. Although such behavior has a rare similarity in other physical systems, surprisingly we discovered a complete analogy of this behavior with the threshold of a conventional laser and proved mathematically that the exciton–trion equations are identical to the laser equations. We further showed that the power-law index increases with the charge density experimentally and can be as high as 40 at a density of ∼1 × 1012/cm2 in principle, leading to extremely nonlinear behavior of exciton accumulation. Our results reveal a new behavior of exciton accumulation and provide an alternative mechanism for multiplying exciton population, in analogy to a condensate. The intricate dynamics between excitons and trions will also enrich our understanding of the complex Mott physics in 2D semiconductors and may lead to new devices based on the exciton nonlinearity