Luis Brey

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, intra-subband 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 non-vanishing 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 multi-valley systems that are time-reversal symmetric have vanishing transverse dipole moment, but can be made to carry non-vanishing values by breaking the valley symmetry, for example via magnetic field. The presence of a non-vanishing 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.

Graphene nanoribbons with zigzag terminated edges have a magnetic ground state characterized by edge ferromagnetism and antiferromagnetic inter edge coupling. This broken symmetry state is degenerate in the spin orientation and we show that, associated with this degeneracy, the system has topological solitons. The solitons appear at the interface between degenerate ground states. These solitons are the relevant charge excitations in the system. When charge is added to the nanoribbon, the system energetically prefers to create magnetic domains and accommodate the extra electrons in the interface solitons rather than setting them in the conduction band.

We study a two-dimensional electron gas exchange coupled to a system of classical magnetic ions. For large Rashba spin–orbit coupling, a single electron can become self-trapped in a skyrmion spin texture self-induced in the magnetic ions system. This new quasiparticle carries electrical and topological charge as well as a large spin, and we named it as magnetic skyrmionic polaron. We study a range of parameters; temperature, exchange coupling, Rashba coupling, and magnetic field, for which the magnetic skyrmionic polaron is the fundamental state in the system. The dynamics of this quasiparticle is studied using the collective coordinate approximation, and we obtain that in the presence of an electric field the new quasiparticle shows, due to the chirality of the skyrmion, a Hall effect. Finally, we argue that the magnetic skyrmionic polarons can be found in large Rashba spin–orbit coupling semiconductors as GeMnTe.

We study a two-dimensional electron gas exchanged-coupled to a system of classical magnetic ions. For large Rashba spin-orbit coupling a single electron can become self-trapped in a skyrmion spin texture self-induced in the magnetic ions system. This new quasiparticle carries electrical and topological charge as well as a large spin, and we named it as magnetic skyrmionic polaron. We study the range of parameters; temperature, exchange coupling, Rashba coupling and magnetic field, for which the magnetic skyrmionic polaron is the fundamental state in the system. The dynamics of this quasiparticle is studied using the collective coordinate approximation, and we obtain that in presence of an electric field the new quasiparticle shows, because the chirality of the skyrmion, a Hall effect. Finally we argue that the magnetic skyrmionic polarons can be found in large Rashba spin-orbit coupling semiconductors as GeMnTe.

Twisted bilayer graphene is a chiral system which has been recently shown to present circular dichroism. In this work we show that the origin of this optical activity is the rotation of the Dirac fermions’ helicities in the top and bottom layer. Starting from the Kubo formula, we obtain a compact expression for the Hall conductivity that takes into account the dephasing of the electromagnetic field between the top and bottom layers and gathers all the symmetries of the system. Our results are based in both a continuum and a tight-binding model, and they can be generalized to any two-dimensional Dirac material with a chiral stacking between layers.