Enhanced excitonic emission from single layer MoSe₂ interfaced with superconductor NbN

We analyze emissive properties of a coupled monolayer (1L) transition metal dichalcogenide (MoSe₂) and bulk superconductor (NbN) using a combined time-dependent density-functional theory (TDDFT) and many-body theory approach. After obtaining the band structure of both subsystems using density functional theory (DFT), we proceed with analysis of the excited states, including excitons and trions, of 1L MoSe₂ using TDDFT. Superconducting (SC) properties of NbN were calculated from the Fröhlich Hamiltonian with a screened Coulomb pseudopotential μ=0.21, the electron-phonon coupling constant λ=1.2, and an effective optical phonon frequency ω₀=25meV, that give SC critical temperature T_c=25K. To calculate the photoluminescence spectrum we first derived the semiconductor luminescence equations (SLEs) for a system with excitons and trions coupled to a superconductor through an effective semiconductor-superconductor Hamiltonian. On solving the SLEs for different excited carrier densities, that correspond to specific laser pulse strength, we find that coupling between the two subsystems leads to transformation of Cooper pairs into excitons and as result to a significant enhancement of the excitonic emission in 1L MoSe₂ when the system is below T_c, in qualitative agreement with our experimental data. Obtained results may provide a better understanding of the ultrafast properties of novel hybrid materials with potential application in optical devices.