Microwave propagation in InSb powder: Magnetoplasma and helicon-wave excitations

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Abstract
Microwave propagation in narrow‐gap semiconductor powders shows a strong gyrotropic behavior in the presence of an externally applied dc magnetic field B. This behavior is interesting as a possible tool for the study of electrical transport in small grains; it shows some promise for application in microwave nonreciprocal devices; and, finally, the interaction of electromagnetic waves with a particulate magnetoplasma is interesting as an electrodynamic phenomenon in its own right. Microwave propagation was investigated experimentally in powdered pure n‐type InSb at 35 GHz, in magnetic fields up to 22 kG, and in the temperature range 90–300 K. The experiments were carried out as a function of particle size and conductivity, using irregular powders, as well as aggregates of spherical particles. Measurements of transmitted amplitude and phase were performed with incident circular polarization in the Faraday geometry (k∥B, where k is the wave vector). Faraday rotation arising from the gyrotropy was also investigated. The above experiments reveal that the observed gyrotropy is associated with two distinct resonant interactions: Magnetoplasma resonance (MPR) and dimensional resonance (DR). These powder resonances occur at the condition for resonant scattering of individual particles, shifted through multiparticle effects. At microwave frequencies, powder grains satisfy the dipole limit (D≪λ0, where λ0 is the free‐space wavelength and D is the largest dimension of the particle). Using the recently developed theory of electromagnetic resonances in a gyrotropic sphere, we identify MPR as an electric dipole interaction occurring when D≪λ, where λ is the wavelength in the particle interior (Rayleigh scattering). DR, on the other hand, is a magnetic dipole interaction, occurring when λ is comparable to the particle dimensions. Thus, although the particles are nonmagnetic, the powder as a whole must be described by an effective permeability, particularly in the vicinity of DR. In our parameter range, the DR interaction involves resonant absorption of heliconlike waves. Finally, in the case of Rayleigh scattering, particle‐particle interaction can be expressed through a ’’powder depolarizing factor’’, which provides a measure of the local Lorentz field in a dense particle distribution.