An equilibrium trion liquid in atomic double layers

A trion is a bound state of two electrons and one hole or two holes and one electron. To date, trions have been observed only as optically excited states in doped semiconductors. We report the emergence of an equilibrium trion liquid in Coulomb-coupled molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) monolayers. By electrically tuning the hole density in WSe₂ to be two times the electron density in MoSe₂, we generated equilibrium interlayer trions with binding energy in the milli-electron volt energy scale at temperatures two orders of magnitude below the Fermi temperature. We observed a density-tuned phase transition to an electron-hole plasma, spin-singlet correlations for the constituent holes, and Zeeman field-induced dissociation of the trions. Our results pave the way for exploration of the correlated phases of composite particles in solids.