In a two-dimensional crystal of tungsten diselenide (WSe₂), light creates pairs — an electron and a hole that stick together. Such pairs are called excitons. Among them are "dark" ones: they barely emit, but they set the tone at low temperatures. After a short laser flash, hot dark excitons scatter across the sheet, gradually cooling down.
The authors placed a WSe₂ monolayer between layers of protective boron nitride and cooled it to cryogenic temperatures. Then they stretched or compressed the crystal and watched how exciton motion changed in the first picoseconds. It turned out that stretching increases the gap between bright and dark states. As a result, scattering, the transition from a bright exciton to a dark one with the emission of a lattice vibration, is suppressed. Hot particles stay fast longer and run farther.
Under compression, the picture is different. The deformation makes the lattice more actively emit special vibrations, so-called M-phonons. This causes a periodic change in exciton speed: their run becomes not uniform but pulsating.
This is the first time we've seen how mechanical action controls not just the glow but the very dynamics of particles in a two-dimensional semiconductor. It's a step toward chips where a signal can be modulated not by an electric field but by simply bending the substrate.
Dark excitons in WSe₂ are so "quiet" that their presence is revealed only by weak phonon replicas in the spectrum, like an echo of the main signal.