Scientists led by Professor Cun-Zheng Ning have made a groundbreaking discovery of a genuine four-body quasi-particle called a “quadruplon” in a two-dimensional semiconductor material (monolayer Molybdenum Ditelluride). This represents the first experimental evidence of such a four-body entity in condensed matter physics.
The research team employed an optical pump-probe technique, where a strong ultrashort pulse excites electrons in the semiconductor, followed by a weaker probe pulse that reveals how the excited material absorbs photons after a brief delay. This allowed them to study the spectral properties and identify new quasi-particles.
While scientists had previously only observed quasi-particles like excitons (electron-hole pairs), trions (three-particle states), and bi-excitons (weakly coupled pairs of excitons), the new experiments revealed at least six unexpected spectral peaks that couldn’t be explained by these known entities. The researchers conducted extensive testing across multiple samples and varying conditions to rule out defects and confirm these were intrinsic features of the material.
When existing theories proved insufficient to explain these new spectral features, the team developed advanced theoretical approaches. They found that fully accounting for all Coulomb interaction terms between two electrons and two holes could reproduce the experimental results. Through cluster expansion techniques that allow for intuitive analysis, they identified a critical fourth-order cluster that was necessary and sufficient to explain the observations – the quadruplon.
Unlike a bi-exciton (which consists of two weakly interacting excitons), a quadruplon is a tightly bound four-particle entity without well-defined excitons. This fundamental distinction makes it a truly new type of quasi-particle with potentially novel properties.
This discovery could significantly impact our understanding of material structures across various physical systems and may lead to new photonic applications. The researchers plan to extend their investigation to other material systems and explore the quantum nature and light emission properties of these newly discovered quadruplons.
Reference: “The quadruplon in a monolayer semiconductor” by Jiacheng Tang, Cun-Zheng Ning, Hao Sun, Qiyao Zhang, Xingcan Dai and Zhen Wang, 26 March 2025, eLight. DOI: 10.1186/s43593-025-00081-1
