Towards Exciton Bose-Einstein Condensate with Atomically Thin Heterostructures

In 1964, L.V. Keldysh predicted that excitons - quasiparticles formed by bound electron-hole pairs - could undergo Bose-Einstein condensation (BEC) below a critical temperature. However, realizing this macroscopic quantum state in conventional semiconductors proved difficult due to the ultrashort lifetime of optically excited excitons. While spatially indirect excitons in GaAs quantum wells provided an example of this physics under high magnetic fields (the Quantum Hall exciton condensate), a true BEC of dilute excitons at zero magnetic field remained elusive. In this colloquium, I will demonstrate that atomically thin van der Waals heterostructures provide the long-sought platform to engineer stable, strongly interacting electron-hole bilayers. By separating carriers by only a few atomic sheets, we realize strongly interacting electron-hole systems with ultralong lifetime necessary for thermal equilibration. It enabled the observation of various quantum phases composed of multiparticle complexes, such as excitons, trions, and exciton molecules. I will discuss the use of optical spectroscopy to probe the compressibility, transport, and spin susceptibility of these emergent phases. Our measurements reveal that the excitons can form a two-component Bose-Einstein condensate characterized by unusual spin and valley textures, opening a new frontier in the study of quantum fluids.

Speaker: Feng Yang, UC Berkeley