  The combination of repulsive and attractive Coulomb interactions in a quantum
electron(e)-hole(h) fluid can give rise to novel correlated phases of
multiparticle charge complexes such as excitons, trions and biexcitons. Here we
report the first experimental realization of an electrically controlled
interlayer trion fluid in two-dimensional van der Waals heterostructures. We
demonstrate that in the strong coupling regime of electron-hole bilayers,
electrons and holes in separate layers can spontaneously form three-particle
trion bound states that resemble positronium ions in high energy physics. The
interlayer trions can assume 1e-2h and 2e-1h configurations, where electrons
and holes are confined in different transition metal dichalcogenide layers. We
show that the two correlated holes in 1e-2h trions form a spin-singlet state
with a spin gap of ~1meV. By electrostatic gating, the equilibrium state of our
system can be continuously tuned into an exciton fluid, a trion fluid, an
exciton-trion mixture, a trion-charge mixture or an electron-hole plasma. Upon
optical excitation, the system can host novel high-order multiparticle charge
complexes including interlayer four-particle complex (tetrons) and
five-particle complex (pentons). Our work demonstrates a unique platform to
study novel correlated phases of tunable Bose-Fermi mixtures and opens up new
opportunities to realize artificial ions/molecules in electronic devices.

Mesoscale and Nanoscale Physics;Strongly Correlated Electrons