Modes for transmission of microwaves through alkali metals at cryogenic temperatures

We present experimental results for three different modes of magnetic-field-dependent microwave transmission through thin foils of pure alkali metals at cryogenic temperatures. The first of these modes occurs when the magnetic field is parallel to the surface, and results in propagation windows above the field for cyclotron resonance Hc and its subharmonics. The mode is identified as the transmission analog of cyclotron waves as observed in reflection. The data are compared with extensive computer simulations in an effort to deduce values for the coefficients of the momentum part of the Landau correlation function. Results are presented for Na, K, and Rb, and compared with other values. The second mode contains two series, each characterized by a rapid oscillatory dependence of the transmitted field on the angle between the applied dc magnetic field and the sample surface. They occur when the dc field is much greater than Hc. We identify one of these series with a model dependent upon the simple time of flight for electrons along the field lines. The other appears to be more complicated and we suggest warrants theoretical attention. The third mode, for the dc field normal to the sample, is identified as the Gantmakher-Kaner oscillations at microwave frequencies. Attention is focused on the behavior of this mode near Hc and for fields much greater than Hc. The signal-to-noise ratio available experimentally for observation of all of the modes discussed is very high. Consequently it is suggested that if any electronic properties of interest of the alkali metals can be theoretically identified with an observed mode, they could be further measured and analyzed.