Molecular origins of exciton condensation in van der Waals heterostructure bilayers.

Recent experiments have realized exciton condensation in bilayer materials such as graphene double layers and the van der Waals heterostructure MoSe-WSe with the potential for nearly frictionless energy transport. Here we computationally observe the microscopic beginnings of exciton condensation in a molecular-scale fragment of MoSe-WSe, using advanced electronic structure methods based on reduced density matrices. We establish a connection between the signature of exciton condensation-the presence of a large eigenvalue in the particle-hole reduced density matrix-and experimental evidence of exciton condensation in the material. The presence of a "critical seed" of exciton condensation in a molecular-scale fragment of a heterostructure bilayer provides insight into how local short-range strongly correlated effects may give rise to macroscopic exciton condensation. We find that molecular-scale properties such as layer alignment and interlayer distance can impact the formation of nonclassical long-range order in heterostructure bilayers, demonstrating the importance of geometric considerations for the rational design of exciton condensate materials. Mechanistic insights into the microscopic origins of exciton condensation have potential implications for the design and development of new materials with enhanced energy transport properties.