Collective Excitation Modes in an Excitonic Phase Under Strong Magnetic Fields

Collective excitation modes in an excitonic phase under strong magnetic fields are investigated theoretically. Dispersion relations of such modes that are associated with oscillations of charge density and phase of the order parameter are derived. Special attention is paid to revealing how the one-dimensional spectrum and the degeneracy of the one-particle state are reflected in the dispersion relations. It is predicted that the phase oscillation mode yields a characteristic attenuation peak slightly below the phase transition temperature in the case where the ultrasound propagates perpendicular to the magnetic field. In the normal phase there are no resonant attenuations in this configuration.