Interlayer excitonic insulator in two-dimensional double-layer semiconductor junctions: An explicitly solvable model

Excitonic insulators conduct neither electrons nor holes but bound electron-hole pairs (excitons). Unfortunately, it is not possible to inject and detect the electron and hole currents independently within a single semiconducting layer. However, interlayer excitonic insulators provide a spatial separation of electrons and holes enabling exciton current measurements. The problem is that the spatial separation weakens electron-hole pairing and may lead to interlayer exciton disassociation. Here we develop an explicitly solvable model to determine an interlayer separation that is strong enough to prevent electron and hole hopping across the layers but still allows for electron-hole pairing sufficient for transition into an interlayer excitonic insulator state. An ideal junction to realize such a state would comprise a pair of identical narrow-gap two-dimensional semiconductors separated by a wide-gap dielectric layer with low dielectric permittivity. The present study quantifies parameters of such a junction by taking into account interlayer coherence effects.