  We present a microscopic theory on the observation of a beating pattern in
the radiation-induced magnetoresistance oscillations at very low magnetic
field. We con- sider that such a beating pattern develops as a result of the
coupling between two oscillatory components: the first is a system of electron
Landau states being harmon- ically driven by radiation. The second is a lattice
oscillation, i.e., an acoustic phonon mode. We analyze the dependence of the
beating pattern on temperature, radiation frequency and power. We conclude that
the beating pattern is an evidence of the radiation-driven nature of the
irradiated Landau states that makes them behave as a collective plasma
oscillation at the radiation frequency. Thus, the frequency of such plasmons
could be tuned from microwave to terahertz in the same nanodevice with an
apparent technological application.
