Is the $\mathrm{MoSe_2}$/$\mathrm{WSe_2}$ dipolar excitonic insulator an exciton superfluid?
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
In two semiconductor layers separated by a thin insulator, electrons in one layer and holes in the other bind into long-lived excitons and make the system insulating. Drag experiments show these excitons move as units, but frictionless superflow has not been shown.
Precise statement
In $\mathrm{MoSe_2}$/hBN/$\mathrm{WSe_2}$ electron-hole double layers at equal densities $n_e = n_h$ of order $10^{11} \text{ to } 10^{12}\,\mathrm{cm}^{-2}$, 2025 Coulomb-drag experiments found a drag ratio near 1 (perfect drag), showing transport by bound excitons. Determine whether the exciton fluid has a finite superfluid stiffness below a Berezinskii-Kosterlitz-Thouless temperature $T_{\mathrm{BKT}}$, i.e. vanishing counterflow resistance or a Josephson-like interlayer response, and give $T_{\mathrm{BKT}}$ versus density. An answer is yes or no with measured $T_{\mathrm{BKT}}(n)$.
What would settle it
A counterflow transport measurement showing resistance vanishing below a density-dependent $T_{\mathrm{BKT}}$, or a bound on stiffness excluding superfluidity down to the lowest accessible temperature.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Two 2025 Science papers (Nguyen et al.; Qi et al.) reported perfect Coulomb drag, establishing exciton transport but not superfluidity.
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