This paper reports experimental evidence that bound pairs of electrons and holes, called excitons, can form a multicomponent Bose–Einstein condensate (BEC) in a layered semiconductor device. The team studied a van der Waals stack made from molybdenum diselenide (MoSe2), hexagonal boron nitride (hBN) and tungsten diselenide (WSe2) and used magnetic and optical probes to reveal collective quantum states with more than one condensed component.

An exciton is a particle-like bound state of an electron and the missing electron (a hole) it leaves behind. Like atoms in ultracold gas experiments, excitons can act like bosons and, under the right conditions, all occupy the same quantum state to form a BEC. In solids, exciton condensates could combine strong interactions and electrical tunability and might exist at much higher temperatures than atomic BECs. Until now, clear experimental evidence for such condensation in a solid-state system has been hard to obtain.

To look for condensation, the researchers built an electron–hole bilayer by stacking MoSe2 and WSe2 with thin hBN between them. This device supports excitons with four “flavors,” a label that combines the particle’s spin and valley degree of freedom (a valley is a momentum-space location in the crystal). They cooled the device in a dilution refrigerator and used magneto-optical spectroscopy to measure the spin susceptibility — how the spins of the electrons and holes respond to an applied magnetic field. Those measurements are sensitive to whether and how many exciton flavors are condensed.

The data reveal three distinct exciton condensate phases, each characterized by a different pattern of flavor polarization. At zero magnetic field, the ground state appears to be a coherent superposition of two intravalley exciton flavors (two components condensed at once). Applying a magnetic field drives a first-order quantum phase transition at a relatively weak critical field to a two-component intervalley condensate (where the paired electrons and holes come from different valleys). At even higher fields the system becomes a fully polarized single-component condensate. The two-component condensate phases are seen to survive up to about 1.8 kelvin.