Three voices merging into a single chord. In the physics of quantum materials, this image takes on a literal meaning: three layers of graphene, separated by thin spacers, force electrons and holes from different layers to move as a single fluid. Exciton condensate, a state in which electron-hole pairs flow without resistance, has previously been observed only in two-layer systems. Now Chinese physicists have shown that coherence can unite three independent electronic layers at once.
The sandwich of three graphene monolayers was assembled with spacers of hexagonal boron nitride, 1.3 and 2 nanometers thick. On top, they placed a monolayer of tungsten disulfide (WSe2), whose Rydberg excitons, hydrogen-like excited states with a large radius, act as an ultrasensitive optical probe. These excitons sense charge in neighboring layers through Coulomb interaction, and their response decays with distance, allowing the contributions of different layers to be distinguished. At a temperature of 1.8 K and a magnetic field of 9 T, they measured the reflectance contrast while scanning the filling and the displacement field.
When tuning the displacement field and the filling, the three pairwise channels converge to a common point where the levels of all layers are nearly degenerate. There, the incompressibility persists, and the energy and spectral weight of the 2s exciton take intermediate values between the pairwise limits. This indicates a unified three-component state, not a mixture of independent pairs. Previously, it was unclear whether a coherent state uniting three independent electronic layers could be realized, and how to detect it. Now, for the first time, optical evidence of such a condensate has been obtained.
Three layers of graphene form a single quantum fluid with two phases, much like three voices in a chord retain their individuality but sound together. The architecture can be extended to more layers, creating a programmable synthetic dimension with tunable energies and couplings. This would allow simulating strongly correlated lattice models and searching for exotic phases such as anyonic superfluidity or fractional states with irrational charge. The exciton-based optical probe could be adapted for real-time imaging of phase diagrams.
Rydberg excitons in WSe2 are so sensitive that they can distinguish graphene layers separated by just 1.3 nanometers, which is tens of thousands of times thinner than a human hair. At a magnetic field of 9 T, the magnetic length (the characteristic size of the electron orbit) is about 8.5 nm, comparable to the interlayer distance.
The idea of a coherent state with three components echoes the 'tricorder' in the series 'Star Trek': a being with three independent personalities acting as one. In physics, three layers of graphene form a single quantum liquid with two phases, much as the tricorder unites three minds.