An atomically thin semiconductor illuminated by a laser should have one temperature. The light creates excitons — electron-hole pairs — and the material absorbs radiation, heats up, and reaches thermal equilibrium. One input, one output. The steady state should be unique.
Shentsev (arXiv:2603.07139, March 2026) shows that a transition metal dichalcogenide monolayer with resident charge carriers can settle into two distinct stable temperatures under the same illumination. The material is bistable.
The mechanism is a feedback loop between temperature and heating rate. At low temperature, most free charge carriers are bound into trions — three-body complexes of two electrons and one hole. Trions are electrically neutral composites that do not contribute to electrical conductivity. With few free carriers, the Drude absorption — the process by which free electrons absorb low-frequency radiation — is weak. The material heats slowly and stabilizes at a low temperature where trions remain bound. The cold state sustains itself by suppressing the heating mechanism.
At high temperature, thermal energy dissociates the trions into free carriers and excitons. The liberated carriers increase the Drude conductivity, which increases the absorption of radiation, which produces more heating, which dissociates more trions. The hot state sustains itself by amplifying the heating mechanism.
Between these two stable configurations lies an unstable intermediate: warm enough to partially dissociate trions, but not warm enough for the freed carriers to sustain the elevated heating rate. The system tips toward one basin or the other. The switching between states takes tens to hundreds of picoseconds and produces observable jumps in temperature, electrical current, and luminescence spectrum — the ratio of exciton to trion emission flips as the bound complexes form or dissolve.
The bistability arises from the material's own temperature controlling how efficiently it absorbs the energy that determines its temperature. The input is fixed. The two outputs exist because the system's response to heating depends on the outcome of the heating.
Shentsev, "Bistability of electron temperature in atomically thin semiconductors in the presence of exciton photogeneration," arXiv:2603.07139 (March 2026).