  The magnetic field and temperature dependence of the transverse and
longitudinal plasmons in SmLa_0.8Sr_0.2CuO_{4-\delta} have been measured for
frequencies 5 < nu < 30 cm^{-1} and magnetic fields 0 < B < 7 T. A transition
between a vortex glass and a vortex liquid regime, which revealed different
field dependencies of the resonance frequencies, could be clearly observed. The
positions and the spectral weights of the plasmons were successfully described
using the multilayer model (Phys. Rev. B 64, 024530, 2001), which takes the
compressibility of the electronic liquid into account. The compressibility is
shown to remain constant in the broad range of field and temperature, which
strongly supports the applicability of the model. The absolute value of the
compressibility is close to that of a two-dimensional noninteracting electron
gas.
