Musicians in an orchestra each play their own part, not hearing the others. In a perfect crystal of tungsten disulfide on graphene, excitons, bound electron-hole pairs, behave the same way: they barely mix due to a hidden symmetry called the valley. The energy levels of such a system resemble notes sorted into separate folders — the chaos is incomplete.

But introduce disorder (say, randomly shifting atoms) and the symmetry disappears. Electronic flat bands emerge, acting as a catalyst: excitons begin to actively mix between valleys. The orchestra turns into a jazz ensemble where every instrument engages in dialogue with the rest. The system becomes fully ergodic: its state covers all available possibilities.

The researchers also estimated the Thouless time — the characteristic time over which exciton mixing spans the entire width of the energy band. In a disordered sample, this process happens faster. The layered material becomes a tunable platform for studying quantum chaos: by varying the degree of disorder, one can smoothly transition from regular dynamics to fully chaotic and observe this in optical spectra.