The properties of plasma excitations (plasmons) in freely suspended graphene in zero magnetic field are studied for the first time using inelastic light scattering. The dependences of the plasmon energy EPL on the electron density n and wave vector q are measured. It is shown that the dependence of the plasmon energy on the electron density is close to EPL ~ n1/4. The dispersion of plasma excitations in suspended graphene with a nearby gate is measured. It is found that the plasmon energy for small wave vectors depends linearly on the momentum transfer, while the dispersion for large wave vectors is described by the law EPL ~ q1/2. It is shown that the effective dielectric constant, which enters the dispersion relation of plasma excitations for large q, turns out to be close to 5.1 rather than unity, as might be expected for the case of freely suspended graphene. The unexpectedly large renormalization of the effective dielectric constant discovered in freely suspended graphene may probably be associated with a large interband contribution to the effective screening of Coulomb interaction.
Kirpichev et al. (Thu,) studied this question.