Superconductivity’s new contender

Researchers have experimentally observed a new kind of particle in transition‑metal dichalcogenide bilayers called doubly charged excitons, or quaternions. A single exciton is an electron bound to a hole, and combining an even number of fermions can create a boson with integer spin. In this system, one electron and three holes (or one hole and three electrons) bind together into a stable, doubly charged bosonic complex. Because bosons can occupy the same quantum state, these quaternions could in principle form a Bose-Einstein condensate, a collective phase in which all particles share a single macroscopic wavefunction. For charged bosons, such a condensate could carry electrical current with zero resistance, opening a pathway to a new kind of superconductivity.

The researchers confirmed the existence of quaternions through two key measurements. By continuously tuning the electron and hole densities, they observed the expected population behaviour of the bound state, and by applying magnetic fields, they identified the complex as a spin‑triplet. These signatures match theoretical predictions for a doubly charged exciton.

Unlike exciton or polariton condensates, a quaternion condensate is not expected to emit coherent light, and the experiments indeed show no signs of spectral narrowing or other coherence effects. Achieving condensation will require overcoming practical challenges, including heating from the optical pump and nonradiative Auger recombination at high densities, both of which raise the critical density for condensation. Better cooling and possible lateral confinement could help reach the required regime.

Although true Bose-Einstein condensation is not possible in an infinite two‑dimensional system, finite 2D systems can still undergo a transition that is effectively indistinguishable from condensation if the coherence length exceeds the system size. This makes it reasonable to search for superfluidity, and potentially superconductivity, in this platform. The strong long‑range Coulomb repulsion between quaternions also raises the possibility of entirely different quantum phases, such as a bosonic Wigner crystal or even a supersolid.

The establishment of these doubly charged exciton complexes in screened transition‑metal dichalcogenide bilayers opens a promising new direction in quantum materials research, with the real prospect of discovering a non‑BCS form of superconductivity (one that does not rely on the conventional Cooper‑pair mechanism) and other exotic states of matter.

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Bose–Einstein condensation and indirect excitons: a review by Monique Combescot, Roland Combescot and François Dubin (2017)

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