Size effects in the far-infrared magneto-optical absorption of small bismuth particles

Small bismuth particles are studied as a model system for a three-dimensionally confined solid-state plasma. Information on the carrier dynamics is obtained from far-infrared magneto-optical absorption measurements. Samples consisted of pellets pressed from a mixture of bismuth particles, prepared by inert gas evaporation, and an insulating host with the volume fraction of bismuth less than 0.01. The spectra were taken from 10 to 350 ${\mathrm{cm}}^{\mathrm{-}1}$ by Fourier-transform spectroscopy with the samples typically immersed in liquid helium. Particles with diameters around 0.5 μm show an abundance of magnetic-field-dependent resonances that can be successfully described by a semiclassical treatment for the solid-state plasma using bulk bismuth parameters. As the mean particle diameter is reduced to 150 Å, the data show a dramatic change from a rich spectrum of field-dependent resonances to strong field-independent absorption. This is markedly different behavior than that observed for semiconductor quantum dots, for which field-dependent absorption is seen even in the smallest structures. Despite relatively broad size distributions within the bismuth powders, a size-specific sum-rule analysis associates the field-dependent absorption with particles larger than approximately 200 nm in diameter.