Discovery of the Immobility of Electron-Hole Drops in Ge at Low Excitation

Using a new experimental technique based on the hysteresis effect, the spatial distribution of a localized packet of electron-hole drops in a spatially uniform free-exciton gas near threshold is measured for times ∼${10}^4$ sec. No significant motion is found, determining an upper limit $D\lesssim {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ to the drop diffusion constant. These results require that drops bind to crystal defects; for electrical impurities we estimate the binding energy $B\gtrsim 5$ meV.