Effect of spin-orbit interaction on the plasma excitations in a quantum wire

Incorporating the spin-orbit interaction into the Hamiltonian of a quantum wire, we have calculated the plasma excitation energies. We include both the Rashba term (alpha coupling) arising from the asymmetry of the heterostructure forming the two-dimensional electron gas, and the beta coupling due to the quantum wire confinement which we model with a harmonic potential. The alpha coupling lifts the degeneracy of the spin states and the beta coupling causes the quantized transverse single-particle energy to have negative dispersion. Our model yields several interesting features which may be observed with the use of inelastic light scattering and electron energy loss spectroscopy (EELS). The collective excitations for the quantum wire are determined by the allowed transitions between subbands. The collective plasma excitations split off from each branch of allowed particle-hole modes. The subband structure gives rise to a plasmon mode with a negative group velocity.