Large—Wave-Number, Almost Transverse Helicon Propagation and the Specific Heat of Helicons

Abstract

The frequency and damping of helicons with large—wave-number $q$ propagating almost transversely to a strong dc magnetic field have been investigated. These modes have not previously been studied under the conditions considered in the present work, namely $\omega \tau \gg 1$ and ${\omega }_c-\left|q_z{v}_F\right|>\mathrm{}\omega >\left|q_z{v}_F\right|$, where $\omega$ and ${\omega }_c$ are the wave frequency and cyclotron frequency, respectively; $\tau$ is the mean collision time for electrons at the Fermi surface, and $v_F$ is the Fermi velocity. They have entirely different propagation characteristics from those of the previously studied Landau damped modes for $\omega \tau \ll 1$. These large-$q$ helicons with $\omega \tau \gg 1$ exhibit no significant geometric-resonance features, whereas the large-$q$, Landau-damped helicons with $\omega \tau \ll 1$ exhibit geometric resonance in an exaggerated form, as "propagation spikes." This striking difference is due to the existence, in the latter case, of a group of electrons traveling along the magnetic field in synchronism with the helicon, and the total absence of any such synchronous group of electrons in the former case. It is found that the number of large-$q$ modes with $\omega \tau \gg 1$ which can be counted as elementary excitations in the face of collision damping is so small that the helicon specific heat is probably unobservable against the over-whelmingly dominant electron and phonon specific heats.