Observation of an exciton crystal in a moiré excitonic insulator

Strong Coulomb interactions can drive electrons to crystallize into a Wigner lattice. Achieving the bosonic analogue - a crystal of excitons - has remained elusive due to their short lifetimes and weaker interactions. Here, we report the observation of a thermodynamically stable exciton crystal in an excitonic insulator coupled to a moiré potential. Using an electron-hole bilayer composed of a monolayer MoSe2 and a WS2/WSe2 moiré superlattice, we construct a tunable extended Bose-Hubbard model with electrical control over exciton and charge doping in thermal equilibrium. Optical spectroscopy reveals spontaneous crystallization of long-lived excitons at one exciton filling per three moiré sites, evidenced by strong Umklapp scattering peaks in the optical spectrum. Exciton transport measurements further show a pronounced exciton resistance peak at the same filling, consistent with suppressed exciton hopping in a crystalline phase. When doped away from net charge neutrality, this moiré electron-hole bilayer can host new correlated insulating phases where dipolar excitonic insulators form on top of the background of a hole Mott insulator or generalized Wigner crystals in the moiré superlattice. These findings establish moiré excitonic insulators as a versatile platform for realizing correlated crystalline phases of bosons and fermions.

💡 Research Summary

The authors report the discovery of a thermodynamically stable exciton crystal in a moiré‑engineered excitonic insulator. The device consists of a monolayer MoSe₂ electron layer and an angle‑aligned WS₂/WSe₂ moiré superlattice that serves as a hole layer, separated by a ~2 nm hBN tunneling barrier. Independent top‑gate (V_TG) and bottom‑gate (V_BG) voltages allow separate control of the vertical electric field (which tunes the band gap) and the inter‑layer bias (which sets the net electron‑hole imbalance). By adjusting these voltages the system can be driven from a band insulator, through a dipolar excitonic insulator (EI) where electrons and holes spontaneously bind into long‑lived interlayer excitons, to an electron‑hole plasma (EHP) where the excitons dissociate.

The moiré superlattice imposes an ~8 nm periodic potential on the hole layer, strongly suppressing the kinetic bandwidth of the holes (and the bound excitons) while preserving the large dipole‑dipole repulsion between excitons. This creates an effective extended Bose‑Hubbard model with a tunable filling factor ν = n_exc/n_moire. Optical reflectance contrast measurements focused on the MoSe₂ intralayer exciton X₀ (≈1.65 eV) reveal a distinct high‑energy satellite peak that appears only when ν ≈ 1/3 (one exciton per three moiré sites). The satellite is interpreted as an Umklapp scattering feature: the emergent exciton crystal defines a new 3 × 3 superlattice, folding the exciton dispersion back to the Brillouin‑zone centre and producing an additional absorption line. The measured energy separation (~8 meV) matches the theoretical estimate based on the exciton density (≈6 × 10¹⁰ cm⁻²) and an effective mass of 1.1 m₀.

Transport experiments were performed in a dilution refrigerator (T ≈ 10 mK) using separate Pt contacts to the WS₂/WSe₂ layer and graphene contacts to the MoSe₂ layer. In a closed‑circuit Coulomb‑drag configuration, a small drive voltage applied to the electron layer generates a drive current I_drive; the induced drag current I_drag in the shorted hole layer is measured simultaneously. Within the EI regime the drag ratio I_drag/I_drive reaches unity, confirming perfect drag and indicating that charge transport is entirely mediated by neutral excitons. As the exciton density is increased, a pronounced peak in the exciton resistance R_exc (defined as ΔV_exc/I_exc) emerges precisely at ν = 1/3. This resistance peak signals a dramatic reduction of exciton mobility due to crystallization, providing an electrical signature that complements the optical Umklapp peak.

Further gate‑induced doping away from charge neutrality reveals additional correlated insulating phases. When extra electrons (or holes) are added, the exciton crystal coexists with a background hole Mott insulator or with generalized Wigner crystals at commensurate hole fillings (e.g., ½ or ¼ per moiré site). These mixed boson‑fermion states exhibit insulating behavior despite the presence of mobile carriers, highlighting the rich interplay between dipolar excitons and strongly correlated fermions in a moiré lattice.

Overall, the work demonstrates (i) the creation of long‑lived interlayer excitons in a moiré‑confined environment, (ii) the ability to tune exciton filling to a fractional value where a crystal forms, (iii) simultaneous optical (Umklapp scattering) and transport (exciton resistance peak) evidence for the exciton crystal, and (iv) the emergence of novel correlated insulating states when bosonic excitons are embedded in fermionic Mott or Wigner backgrounds. This establishes moiré excitonic insulators as a versatile platform for exploring strongly correlated bosonic matter, quantum simulation of extended Bose‑Hubbard physics, and potentially for designing devices that exploit exciton crystallization, such as low‑dissipation excitonic circuits or quantum‑information platforms based on ordered dipolar exciton arrays.