Evidence for a Stable Excitonic Ground State in a Spatially Separated Electron-Hole System

Far-infrared magnetospectroscopy on $\mathrm{InAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{Sb}$ quantum-well structures has revealed new transitions ($X$ lines) when sufficient holes (in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{Sb}$) coexist with electrons (in InAs) and the magnetic field is high enough. The electron-related $X$ line is about 3 meV above electron cyclotron resonance, has roughly the same slope vs magnetic field, and increases in intensity with increasing hole density or decreasing temperature. This line is assigned to an internal excitonic transition and provides evidence for the existence of an excitonic ground state.