Long-living superfluidity of dark excitons in a strip of strained transition metal dichalcogenides double layer

Abstract: We have proposed the superfluidity of dipolar excitons in a strip of double-layer transition metal dichalcogenides (TMDCs) heterostructures. We have shown that strain causes a shift in k-space between the minimum of the conduction band and the maximum of the valence band. Therefore, we expect that applying strain to this system can cause dark excitons to be created. We have numerically calculated the energy spectrum of dark dipolar excitons in strained MoSe$_2$, and we have calculated their binding energies and effective masses. We have shown that the dark dipolar excitons in strained TMDC heterostructures form superfluids, and we have calculated the sound velocity in the energy spectrum of collective excitations, as well as the mean-field critical temperature for superfluidity. We have shown that two separate superfluid flows moving in opposite directions will appear in the system, one on each edge of the strip, forming the double layer. We have seen that the critical temperature for superfluidity increases with the concentration of dark excitons, as well as with the inter-layer separation. The fact that dark excitons cannot decay by the simple emission of photons, makes it so that the superfluids and condensates formed by them have a much longer lifetime than that formed by bright excitons. We propose a way to experimentally verify the predicted phenomena.