Capillary-Wave Resonances of Electron-Hole Droplets

We have studied in the gigahertz range the ultrasonic attenuation by electron-hole droplets in Ge as function of their size. A broad bump is attributed to the resonant excitation of nonresolved capillary modes. From a detailed analysis of the line shape we derive a value of 3× ${10}^{-4\mathrm{}}$ erg/${\mathrm{cm}}^2$ for the surface tension of droplets. This is the first mechanical measurement of this quantity.