Enhanced Interactions of Interlayer Excitons in Free-Standing Hetero-Bilayers

Strong, long-range dipole-dipole interactions between interlayer excitons (IXs) can lead to novel multi-particle correlation regimes that drive the system into distinct quantum and classical phases¹ including dipolar liquids, crystals, and superfluids. Both repulsive and attractive dipole-dipole interactions have been theoretically predicted between IXs in a semiconductor bilayer²^,³, but only repulsive interactions have been experimentally reported so far⁴^,⁵. This study⁶ investigated free-standing, twisted (51^o, 53^o, 45^o), tungsten diselenide/tungsten disulfide (WSe₂/WS₂) hetero-bilayers, where we observed a transition in the nature of dipolar interactions among IXs from repulsive to attractive. This was caused by quantum-exchange-correlation effects, leading to the appearance of a robust interlayer biexciton (formed by two IXs) phase that was theoretically predicted⁷ but never observed in experiments before. The reduced dielectric screening in a free-standing hetero-bilayer not only resulted in a much higher formation efficiency of IXs, but also led to strongly enhanced dipole-dipole interactions, which allowed us to observe the many-body correlations of pristine IXs at the two-dimensional (2D) quantum limit. In addition, we first observed multiple emission peaks from moiré-trapped IXs at room temperature in a well-aligned, free-standing WSe₂/WS₂ hetero-bilayer. Our findings can open avenues for exploring new quantum phases with potential for application in nonlinear optics.<!--![endif]---->