  The response of the system, consisting of two kinds of opposite-charged
fermions and their bound states (hydrogen-like atoms), to the perturbation by
the external electromagnetic field in low particle kinetic energies region is
studied. Expressions for Green functions that describe the system response to
the external electromagnetic field and take into account the presence of
particle bound states (atoms) are found. Macroscopic parameters of the system,
such as conductivity, permittivity and magnetic permeability in terms of these
Green functions are introduced. As an example, the perturbation of the ideal
hydrogen-like plasma by the external electromagnetic field in low temperature
region is considered. Such approach also enables to study the propagation
properties of the signal, tuned up to the transition between two hyperfine
ground state levels of alkali atoms that are considered in Bose-Einstein
condensation (BEC) state. It is shown that the signal can propagate in such
system with rather small energy loss. Such fact allows to introduce the group
velocity concept and to study the slowing down conditions for the microwave
signal that propagates in BEC.
