Magnetic Field Dependence of the Transverse Plasmon in SmLa₀.8Sr₀.2CuO₄

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.