Dipole and quadrupole plasmon in confined quasi-one-dimensional electron gas systems

•Use eigen-equation to calculate plasmon excitations in confined Q1DEG systems.•Quadrupole plasmons besides dipole plasmons are found in the systems.•Quadrupole plasmons originate from symmetric eigen-charge density distribution.•The size dependence of the plasmons can be well explained by the RPA.

An eigen-equation of plasmon excitation in confined quasi-one-dimensional systems is presented. Besides dipole plasmons, quadrupole plasmons are found in the systems by comparing the eigen-solutions with the dipole response. For both dipole and quadrupole plasmons, the plasmon frequencies decrease with the increase of the system's length, and their size dependence can be well fitted by the plasmon dispersion in the infinite systems calculated by the random phase approximation. Through extensively studies of eigen-charge density and induced charge density, we find that quadrupole plasmon corresponds to symmetric charge density distribution, and can only be excited by non-uniform electric field.