Criterion for the appearance of negative dielectric areas in crystals

Damping of the polar vibrations determines the real limits of negative dielectric permittivity (DP) region as shown in this work. We obtain the simple quantitative criterion for the negative DP existence in the single oscillatory approximation model. We introduce this criterion as inequality between the damping constant and T-L frequency splitting of the polar phonon. This inequality is the sufficient condition for existence of negative dielectric permittivity and it is shown its applicability to the many oscillation cases.