Lengthy-sustained superfluidity of dark excitons in strained double layer nanoribbons of transition metal dichalcogenide

We propose the occurrence of superfluidity of dipolar excitons in a double-layer of strained transition metal dichalcogenide (TMDC) nanoribbons [1]. We show that strain causes a shift in k space between the minimum in the conduction band and the maximum in the valence band. Therefore, we expect that applying strain to this system can lead to the existence of dark excitons. We numerically calculate the energy spectrum of dark dipolar excitons in strained MoS₂, and we determine their binding energies and effective masses. We show that dark dipolar excitons in strained TMDC heterostructures form superfluids, and we obtain the sound velocity in the energy spectrum of collective excitations, as well as the mean-field critical temperature for superfluidity. Two separate superfluid flows moving in opposite directions will appear in the system, one on each edge of the ribbon, forming the double layer. The critical temperature for superfluidity increases with the concentration of dark excitons and the interlayer separation. Since dark excitons cannot decay via the simple emission of photons, the superfluids and condensates which they form have a much longer lifetime than that formed by bright excitons.

[1] G. P. Martins, O. L. Berman, G. Gumbs, and G. Grosso, Physical Review B 112, 075429 (2025).