Nonreciprocal Perfect Coulomb Drag in Electron–Hole Bilayers: The Excitonic Diode Effect

In an electron–hole bilayer coupled via Coulomb interaction, an exciton condensate can emerge at low temperatures, where the resulting excitonic supercurrent gives rise to dissipationless, perfect counterflow Coulomb drag. We propose a model for such an electron–hole bilayer in which both parity and time-reversal symmetries are simultaneously broken by spin–orbit coupling and an external magnetic field. In this setting, a finite-momentum exciton condensate is stabilized. Moreover, the system exhibits a nonreciprocal perfect Coulomb drag—an excitonic diode effect—corresponding to a supercurrent diode effect of electric dipoles, with an efficiency reaching up to $2\%$. We further estimate the magnitude of the counterflow supercurrent and argue that the predicted diode effect should be observable in transport experiments.