L. E. F. Foa Torres

Multiterminal Conductance of a Floquet Topological Insulator

We report on simulations of the dc conductance and quantum Hall response of a Floquet topological insulator using Floquet scattering theory. Our results reveal that laser-induced edge states lead to quantum Hall plateaus once imperfect matching with the nonilluminated leads is lessened. The magnitude of the Hall plateaus, however, is not directly related to the number and chirality of all the edge states at a given energy, as usual. Instead, the plateaus are dominated by those edge states adding to the time-averaged density of states. Therefore, the dc quantum Hall conductance of a Floquet topological insulator is not directly linked to topological invariants of the full Floquet bands.

Irradiated graphene as a tunable Floquet topological insulator

In the presence of a circularly polarized mid-infrared radiation graphene develops dynamical band gaps in its quasienergy band structure and becomes a Floquet insulator. Here, we analyze how topologically protected edge states arise inside these gaps in the presence of an edge. Our results show that the gap appearing at

GaAs-Al x Ga 1¿x As double-barrier heterostructure phonon laser: A full quantum treatment

\ensuremath{\hbar}\ensuremath{\Omega}/2

Floquet topological transitions in a driven one-dimensional topological insulator

, where

Antiresonances as precursors of decoherence

\ensuremath{\hbar}\ensuremath{\Omega}

Tuning a resonance in Fock space: Optimization of phonon emission in a resonant-tunneling device

is the photon energy, is bridged by two chiral edge states whose propagation direction is set by the direction of the polarization of the radiation field. Therefore, both the propagation direction and the energy window where the states appear can be controlled externally. We present both analytical and numerical calculations that fully characterize these states. This is complemented by simple topological arguments that account for them and by numerical calculations for the case of the semi-infinite sample, thereby eliminating finite-size effects.

Floquet interface states in illuminated three-dimensional topological insulators

The aim of this work is to describe the behavior of a device capable to generate high frequency (∼ THz) acoustic phonons. This device consists in a GaAs-AlGaAs double barrier heterostructure that, when an external bias is applied, produces a high rate of longitudinal optical LO phonons. These LO phonons are confined and they decay by stimulated emission of a pair of secondary longitudinal optical (L̃O) and transversal acoustic (TA) phonons. The last ones form an intense beam of coherent acoustic phonons. To study this effect, we start from a tight binding Hamiltonian that take into account the electron-phonon (e-ph) and phonon-phonon (ph-ph) interactions. We calculate the electronic current through the double barrier and we obtain a set of five coupled kinetic equations that describes the electron and phonon populations. The results obtained here confirm the behavior of the terahertz phonon laser, estimated by rougher treatments [1].

Coherent versus sequential electron tunneling in quantum dots.

The Su--Schrieffer--Heeger model of polyacetylene is a paradigmatic Hamiltonian exhibiting nontrivial edge states. By using Floquet theory we study how the spectrum of this one-dimensional topological insulator is affected by a time-dependent potential. In particular, we provide evidence of the competition among different photon-assisted processes and the native topology of the unperturbed Hamiltonian to settle the resulting topology at different driving frequencies. While some regions of the quasienergy spectrum develop new gaps hosting Floquet edge states, the native gap can be dramatically reduced and the original edge states may be destroyed or replaced by new Floquet edge states. Our study is complemented by an analysis of the Zak phase applied to the Floquet bands. Besides serving as a simple example for understanding the physics of driven topological phases, our results could find a promising testing ground in cold-matter experiments.

Hierarchy of Floquet gaps and edge states for driven honeycomb lattices

We show that, in the presence of a complex spectrum, antiresonances act as a precursor for dephasing enabling the crossover to a fully decoherent transport even within a unitary Hamiltonian description. This general scenario is illustrated here by focusing on a quantum dot coupled to a chaotic cavity containing a finite, but large, number of states using a Hamiltonian formulation. For weak coupling to a chaotic cavity with a sufficiently dense spectrum, the ensuing complex structure of resonances and antiresonances leads to phase randomization under coarse graining in energy. Such phase instabilities and coarse graining are the ingredients for a mechanism producing decoherence and thus irreversibility. For the present simple model one finds a conductance that coincides with the one obtained by adding a ficticious voltage probe within the Landauer-Buttiker picture. This sheds new light on how the microscopic mechanisms that produce phase fluctuations induce decoherence.

Towards a time reversal mirror for quantum systems

Phonon-assisted tunneling in a double barrier resonant tunneling device can be seen as a resonance in the electron-phonon Fock space which is tuned by the applied voltage. We show that the geometrical parameters can induce a symmetry condition in this space that can strongly enhance the emission of longitudinal optical phonons. For devices with thin emitter barriers this is achieved by a wider collectors barrier.