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

Optical Gain from Biexcitons in CsPbBr₃ Nanocrystals Revealed by Two-dimensional Electronic Spectroscopy

Perovskite semiconductor nanocrystals (NCs) exhibit highly efficient optical gain, which is promising for laser applications. However, the intrinsic mechanism of optical gain in perovskite NCs, particularly whether more than one exciton per NCs is required, remains poorly understood. Here, we use two-dimensional electronic spectroscopy to resonantly probe the interplay between near-band-edge transitions during the buildup of optical gain in CsPbBr3 NCs. We find compelling evidence to conclude that optical gain in CsPbBr3 NCs is generated through stimulated emission from strongly interacting biexcitons. The threshold is largely determined by the competition between stimulated emission from biexcitons and excited-state absorption from single exciton to biexciton states. The findings in this work may guide future explorations of NC materials with low-threshold optical gain

Experimental Determination of the Rotational Constants of High-Lying Vibrational Levels of Ultracold Cs₂ in the 0_g^– Purely Long-Range State

We report on a quantitative experimental determination of the rotational constants for the high-lying vibrational levels of the ultracold pure long-range Cesium molecules formed via photoassociation. The scheme relies on a precise reference of frequency difference in a double photoassociation spectroscopy, which is induced by two laser beams based on an acoustic-optical modulator. The rotational constants are obtained by fitting a nonrigid rotor model into the frequency intervals of the neighboring rotational levels deduced from the reference