The Quadruplon in a Monolayer Semiconductor

TL;DR

This paper reports the first experimental evidence of quadruplons, four-body entities involving two electrons and two holes, in a monolayer semiconductor, expanding understanding of complex correlated particles in condensed matter physics.

Contribution

It provides the first experimental observation and theoretical explanation of quadruplons in a monolayer material, a previously unobserved four-body entity.

Findings

Discovery of spectral features distinct from trions and bi-excitons.

Theoretical modeling confirms the existence of quadruplons as four-body irreducible entities.

First experimental evidence of quadruplons in condensed matter physics.

Abstract

Understanding the structure of matter or materials and interaction or correlations among the constituent elementary particles are the central tasks of all branches of science, from physics, chemistry, to biology. In physics, this ultimate goal has spurred a constant search for high-order correlated entities or composite particles for nearly all states and forms of matter, from elementary particles, nuclei, cold atoms, to condensed matter. So far, such 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 materials 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) only. 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.