Plasmonic transverse dipole moment in chiral fermion nanowires

Plasmons are elementary quantum excitations of conducting materials with Fermi surfaces. In two dimensions they may carry a static dipole moment that is transverse to their momentum which is quantum geometric in nature, the quantum geometric dipole (QGD). We show that this property is also realized for such materials confined in nanowire geometries. Focusing on the gapless, intrasubband plasmon excitations, we compute the transverse dipole moment Dx of the modes for a variety of situations. We find that single chiral fermions generically host nonvanishing Dx, even when there is no intrinsic gap in the two-dimensional spectrum, for which the corresponding two-dimensional QGD vanishes. In the limit of very wide wires, the transverse dipole moment of the highest velocity plasmon mode matches onto the two-dimensional QGD. Plasmons of multivalley systems that are time-reversal symmetric have a vanishing transverse dipole moment but can be made to carry nonvanishing values by breaking the valley symmetry, for example, via a magnetic field. The presence of a nonvanishing transverse dipole moment for nanowire plasmons in principle offers the possibility of continuously controlling their energies and velocities by the application of a static transverse electric field.