Andrew Y. Joe

The combination of repulsive and attractive Coulomb interactions in a quantum electron-hole (e-h) fluid can produce correlated phases of multiparticle charge complexes, such as excitons, trions, and biexcitons. We report an experimental realization of an electrically controlled interlayer trion fluid in van der Waals heterostructures. In strongly coupled e-h bilayers, electrons and holes spontaneously form three-particle trion bound states. The interlayer trions can assume 1e-2h and 2e-1h configurations. We show that the two holes in 1e-2h trions form a spin-singlet with a spin gap of approximately one milli–electron volt. By electrostatic gating, the equilibrium state can be continuously tuned into an exciton fluid, a trion fluid, an exciton-trion mixture, or a trion-charge mixture. Our work demonstrates a platform to study correlated phases of tunable Bose-Fermi mixtures.

Interlayer excitons—bound electron–hole pairs residing in separate layers—feature long lifetimes and Stark-tunable energies, making them promising for excitonic devices and dipolar quantum phenomena. While most prior studies have focused on the 1s ground state, here we reveal the 2p interlayer exciton along with a Rydberg series of interlayer states extending from 1s to 4s in bilayer MoS₂. The 2p state, optically dark at zero field, acquires oscillator strength under an applied electric field through hybridization with the intralayer A exciton. Due to its finite angular momentum, it exhibits distinct selection rules and g-factors in agreement with theoretical expectations. Strikingly, whereas the intralayer Rydberg series fades beyond the 2s state, the interlayer series remains optically visible up to 4s. This extended visibility arises from hybridization with the intralayer B exciton, which transfers oscillator strength to higher interlayer states and effectively brightens the entire series. Simulations further uncover a novel coupling mechanism between s-like intralayer and p-like interlayer excitons—one driven not by conventional electron–hole Coulomb interactions, but by interlayer conduction band mixing. These findings establish bilayer MoS₂ as a versatile platform for exploring tunable excitonic phenomena.

Strongly coupled electron-hole bilayers can host quantum states of interlayer excitons, such as high-temperature exciton condensates at zero magnetic field. This state is predicted to feature perfect Coulomb drag, where a current in one layer is accompanied by an equal but opposite current in the other. We used an optical technique to probe the electrical transport of correlated electron-hole bilayers based on MoSe ₂ /hBN/WSe ₂ heterostructures. We observed perfect Coulomb drag in the excitonic insulator phase at low temperatures; the counterflow resistance of interlayer excitons remained finite. These results indicate the formation of an exciton gas that does not condense into a superfluid. Our work demonstrates that dynamic optical spectroscopy provides a powerful tool for probing exciton transport behavior in correlated electron-hole fluids.

Atomically thin semiconductor heterostructures provide a two-dimensional (2D) device platform for creating high densities of cold, controllable excitons. Interlayer excitons (IEs), bound electrons and holes localized to separate 2D quantum well layers, have permanent out-of-plane dipole moments and long lifetimes, allowing their spatial distribution to be tuned on demand. Here, we employ electrostatic gates to trap IEs and control their density. By electrically modulating the IE Stark shift, electron-hole pair concentrations above 2 × 10¹² cm⁻² can be achieved. At this high IE density, we observe an exponentially increasing linewidth broadening indicative of an IE ionization transition, independent of the trap depth. This runaway threshold remains constant at low temperatures, but increases above 20 K, consistent with the quantum dissociation of a degenerate IE gas. Our demonstration of the IE ionization in a tunable electrostatic trap represents an important step towards the realization of dipolar exciton condensates in solid-state optoelectronic devices.

Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton condensates. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe₂/hBN/WSe₂ heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~0.8 × 10¹² cm⁻² and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

Strongly coupled two-dimensional electron-hole bilayers can give rise to novel quantum Bosonic states: electrons and holes in electrically isolated layers can pair into interlayer excitons, which can form a Bose-Einstein condensate below a critical temperature at zero magnetic field. This state is predicted to feature perfect Coulomb drag, where a current in one layer must be accompanied by an equal but opposite current in the other, and counterflow superconductivity, where the excitons form a superfluid with zero viscosity. Electron-hole bilayers in the strong coupling limit with an excitonic insulator ground state have been recently achieved in semiconducting transition metal dichalcogenide heterostructures, but direct electrical transport measurements remain challenging. Here we use a novel optical spectroscopy to probe the electrical transport of correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe perfect Coulomb drag in the excitonic insulator phase up to a temperature as high as ~15K. Strongly correlated electron and hole transport is also observed at unbalanced electron and hole densities, although the Coulomb drag is not perfect anymore. Meanwhile, the counterflow resistance of interlayer excitons remains finite. These results indicate the formation of an exciton gas in the excitonic insulator which does not condensate into a superfluid at low temperature. Our work also demonstrates that dynamic optical spectroscopy provides a powerful tool for probing novel exciton transport behavior and possible exciton superfluidity in correlated quantum electron-hole fluids.

Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton superfluids. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~$0.8\times {10}^{12} \mathrm{cm}^{-2}$ and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

Ultrafast charge transfer processes provide a facile way to create interlayer excitons in directly contacted transition metal dichalcogenide (TMD) layers. More sophisticated heterostructures composed of TMD/hBN/TMD enable new ways to control interlayer exciton properties and achieve novel exciton phenomena, such as exciton insulators and condensates, where longer lifetimes are desired. In this work, we experimentally study the charge transfer dynamics in a heterostructure composed of a 1 nm thick hBN spacer between MoSe₂ and WSe₂ monolayers. We observe the hole transfer from MoSe₂ to WSe₂ through the hBN barrier with a time constant of 500 ps, which is over 3 orders of magnitude slower than that between TMD layers without a spacer. Furthermore, we observe strong competition between the interlayer charge transfer and intralayer exciton-exciton annihilation processes at high excitation densities. Our work opens possibilities to understand charge transfer pathways in TMD/hBN/TMD heterostructures for the efficient generation and control of interlayer excitons.