Collective excitations of Dirac electrons in a graphene layer with spin-orbit interactions

Coulomb screening and the excitation spectra of electrons in a graphene layer with spin-orbit interaction (SOI) are studied in the random phase approximation. The SOI opens a gap between the valence and conduction bands and between the intraband and interband electron-hole excitation continuum of the semimetal Dirac system. As a result, we have observed a dramatic change in the long-wavelength dielectric function of the system. An undamped plasmon mode appears in the electron-hole continuum gap reflecting the interplay between the intraband and interband electron correlations.