Alexander López

Photoinduced pseudospin effects in silicene beyond the off-resonant condition

We study the photoinduced manipulation of charge carriers in monolayer silicene subject to intense electromagnetic terahertz radiation. Considering the Dirac cone approximation and going beyond the off-resonant condition for large frequencies of the radiation field, where only virtual photon processes are allowed, we present the exact zero-momentum pseudospin polarization and numerical results for the quasienergy band structure and time-averaged density of states. We find that resonant processes, due to real photon emission and absorbtion processes, induce a band inversion that qualitatively modifies the quasienergy spectrum. These band-structure changes manifest themselves as an inversion of the averaged pseudospin polarization. Through the analysis of the time-averaged density of states we find that effective photoinduced gap manipulation can only be achieved in the intermediate and strong matter-radiation coupling regime where the off-resonant approximation breaks down.

Gauge symmetry breaking and topological quantization for the Pauli Hamiltonian

We discuss the Pauli Hamiltonian within a SU(2) gauge theory interpretation, where the gauge symmetry is broken. This interpretation carries directly over to the structural inversion asymmetric spin-orbit interactions in semiconductors and offers new insight into the problem of spin currents in the condensed-matter environment. The central result is that symmetry breaking leads to zero spin conductivity in contrast to predictions of gauge symmetric treatments. Computing the translation operator commutation relations comprising the simplest possible structural inversion asymmetry due to an external electric field, we derive a new condition for orbit quantization. The relation between the topological nature of this effect is consistent with our non-Abelian gauge symmetry breaking scenario.

Floquet spin states in graphene under ac driven spin-orbit interaction

We study the role of periodically driven time-dependent Rashba spin-orbit coupling (RSOC) on a monolayer graphene sample. After recasting the originally

Spin superfluidity and spin-orbit gauge symmetry fixing

4\times 4

Zero-field magnetization reversal of two-body Stoner particles with dipolar interaction

system of dynamical equations as two time-reversal related two-level problems, the quasi-energy spectrum and the related dynamics are investigated via various techniques and approximations. In the static case the system is a gapped at the Dirac point. The rotating wave approximation (RWA) applied to the driven system unphysically preserves this feature, while the Magnus-Floquet approach as well as a numerically exact evaluation of the Floquet equation show that this gap is dynamically closed. In addition, a sizable oscillating pattern of the out-of-plane spin polarization is found in the driven case for states which completely unpolarized in the static limit. Evaluation of the autocorrelation function shows that the original uniform interference pattern corresponding to time-independent RSOC gets distorted. The resulting structure can be qualitatively explained as a consequence of the transitions induced by the ac driving among the static eigenstates, i.e., these transitions modulate the relative phases that add up to give the quantum revivals of the autocorrelation function. Contrary to the static case, in the driven scenario, quantum revivals (suppresions) are correlated to spin up (down) phases.

Gauge transformations of spin-orbit interactions in graphene

The Hamiltonian describing 2D electron gas, in a spin-orbit active medium, can be cast into a consistent non-Abelian gauge field theory leading to a proper definition of the spin current. The generally advocated gauge symmetric version of the theory results in current densities that are gauge covariant, a fact that poses severe concerns on their physical nature. We show that in fact the problem demands gauge fixing, leaving no room to ambiguity in the definition of physical spin currents. Gauge fixing also allows for polarized edge excitations not present in the gauge symmetric case. The scenario here is analogous to that of superconductivity gauge theory. We develop a variational formulation that accounts for the constraints between U(1) physical fields and SU(2) gauge fields, show that gauge fixing renders physical matter and radiation currents and derive the particular consequences for the Rashba SO interaction.

Interplay between spin-orbit interactions and a time-dependent electromagnetic field in monolayer graphene

We investigate magnetization reversal in a system of two Stoner particles with uniaxial anisotropies both subject to a static and antiparallel magnetic field, and taking into account their mutual dipolar interaction. We identify an interesting regime of stable synchronized magnetic dynamics where the two particles are implementing a single information bit. Here a modified Stoner-Wohlfarth limit occurs which results in a dramatically lower critical switching field Hc (including Hc=0) and also a substantially shorter reversal time. Our analytical results are verified by numerical simulations and offer new technological perspectives regarding devices for information storage and/or fast magnetic response.

A perfect spin filtering device through Mach–Zehnder interferometry in a GaAs/AlGaAs electron gas

Inclusion of spin-dependent interactions in graphene in the vicinity of the Dirac points can be posed in terms of non-Abelian gauge potentials. Such gauge potentials being surrogates of physical electric fields and material parameters, only enjoy a limited gauge freedom. A general gauge transformation thus changes the physical model. We argue that this property can be useful in connecting reference physical situations, such as free particle or Rashba interactions to non-trivial physical Hamiltonians with a new set of spin-orbit interactions, albeit constrained to being isoenergetic. We analyse different combinations of spin-orbit interactions in the case of monolayer graphene and show how they are related by means of selected non-Abelian gauge transformations.

Two-electron entanglement in quasi-one-dimensional systems: Role of resonances

We apply a circularly and linearly polarized terahertz field on a monolayer of graphene, taking into account spin-orbit interactions of the intrinsic and Rashba types. It turns out that the field can be used not only to induce a gap in the energy spectrum, but also to close an existing gap due to the different reaction of the spin components with circularly polarized light. Signatures of spin-orbit coupling in the density of states of the driven system can be observed even for energies where the static density of states is independent of spin-orbit interactions. Furthermore it is shown that the time evolution of the spin polarization and the orbital dynamics of an initial wave packet can be modulated by varying the ratio of the spin-orbit-coupling parameters. Assuming that the system acquires a quasistationary state, the optical conductivity of the irradiated sample is calculated. Our results confirm the multistep nature of the conductivity obtained recently, where the number of intermediate steps can be changed by adjusting the spin-orbit-coupling parameters and the orientation of the field.

Laser-induced modulation of the Landau level structure in single-layer graphene

A spin filtering device based on quantum spin interference is addressed, for use with a two-dimensional GaAs/AlGaAs electron gas that has both Rashba and Dresselhaus spin-orbit (SO) couplings and an applied external magnetic field. We propose an experimentally feasible electronic Mach-Zehnder interferometer and derive a map, in parameter space, that determines perfect spin filtering conditions. We find two broad spin filtering regimes: one where filtering is achieved in the original incoming quantization basis, that takes advantage of the purely non-Abelian nature of the spin rotations; and another where one needs a tilted preferential axis in order to observe the polarized output spinor. Both solutions apply for arbitrary incoming electron polarization and energy, and are only limited in output amplitude by the randomness of the incoming spinor state. Including a full account of the beam splitter and mirror effects on spin yields solutions only for the tilted basis, but encompasses a broad range of filtering conditions.