Electron–hole liquids form at room temperature

Collective states of electrons and holes in a solid can resemble familiar phases of atoms: gas and liquid. In the gas phase, electrons, holes, and their bound states—known as excitons—move freely through the material, and the density can be increased with the addition of more electrons and holes. In the liquid phase, the electrons and holes can move only within the limited volume of the droplet, and the addition of more electrons and holes will lead to an increase in volume rather than density. The gas phase can be formed by applying a low-power laser to a semiconductor at ambient conditions. However, attaining the liquid phase typically requires liquid-helium temperatures and high-power lasers: Too much thermal energy can prevent liquid formation, and the gas-to-liquid transition requires a high density of excitons. Now Nathaniel Gabor of the University of California, Riverside, and his colleagues have produced a gas-to-liquid phase transition in a semiconductor at room temperature.

Gabor and his team exploited the layered structure of semiconducting molybdenum ditelluride to create a photocell with a thin layer of MoTe₂ between two layers of graphene. They selected MoTe₂ because its bandgap is in the easily accessible near-IR and its excitons have a long lifetime.

The researchers excited the photocell with a pair of ultrafast laser pulses to create electron–hole pairs. At low powers, increasing the laser power increased the exciton density. But above a critical power, the density stopped increasing. The stalled density indicated the transition to the liquid phase, which occurred when the distance between separate excitons rivaled the size of the exciton.

The electronic phase can be controlled both optically and electrically: The application of an electric field can evaporate the liquid. The researchers also found that the liquid is unaffected by thermal fluctuations at room temperature due to the relatively large energy of the liquid—a result of its nanoscale dimensions. A correlated electronic state stable at room temperature could be the basis of atom-scale optoelectronic devices or terahertz sources and detectors. (T. B. Arp et al., Nat. Photonics, in press, doi:10.1038/s41566-019-0349-y .)