abstract
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A detailed study is presented of the far-infrared absorption of electron-hole drops in germanium. Experimental observations in the frequency range 1 to 50 meV and at superfluid He temperatures are reported. The lineshape agrees well with Mie's theory of scattering of electromagnetic radiation from spheres with a dielectric constant that includes interband hole transitions and a frequency dependent damping, produced by electron-hole interaction. The electron-hole density is found to be (2.02±0.05)x10¹⁷ cm‾³. The lineshape changes with excitation level. At low excitation surface effects are important, since drops are very small: within a layer of the excitonic Bohr radius the drops have a lower density, and the absorption is damped by collisions with the surface. At high excitation levels, the drops are larger and the absorption lineshape is explained including higher multipole terms in the Mie expansion. Estimates for the drop sizes are presented.
A study of the effect of uniaxial stress on the far-infrared absorption is conducted. Large variations in positions and lineshape are observed for different stress values and direction of polarization of the electric field with respect to the stress direction. The fit to the experimental results to a theory with band structure modified by stress, allows the determination of the electron-hole density as a function of [111] stress, as well as a clearer understanding of the general properties of the fluid under the different conditions.