Gloria Platero

Photon-assisted transport in semiconductor nanostructures

Abstract In this review we focus on electronic transport through semiconductor nanostructures which are driven by ac fields. Along the review we describe the available experimental information on different nanostructures, like resonant tunneling diodes, superlattices or quantum dots, together with different theoretical techniques used in the study of photon-assisted transport. These theoretical tools such as, for instance, the Floquet formalism, the nonequilibrium Greens function technique or the density matrix technique, are suitable for tackling with problems where the interplay of different aspects like nonequilibrium, nonlinearity, quantum confinement or electron–electron interactions gives rise to many intriguing new phenomena. Along the review we give many examples which demonstrate the possibility of using appropriate ac fields to control/manipulate coherent quantum states in semiconductor nanostructures.

Theoretical approach to microwave-radiation-induced zero-resistance states in 2D electron systems.

We present a theoretical model in which the existence of radiation-induced zero-resistance states is analyzed. An exact solution for the harmonic oscillator wave function in the presence of radiation, and a perturbation treatment for elastic scattering due to randomly distributed charged impurities, form the foundations of our model. Following this model most experimental results are reproduced, including the formation of resistivity oscillations, their dependence on the intensity and frequency of the radiation, temperature effects, and the locations of the resistivity minima. The existence of zero-resistance states is thus explained in terms of the interplay of the electron microwave-driven orbit dynamics and the Pauli exclusion principle.

ac-Driven double quantum dots as spin pumps and spin filters.

We propose and analyze a new scheme of realizing both spin filtering and spin pumping by using ac-driven double quantum dots in the Coulomb blockade regime. By calculating the current through the system in the sequential tunneling regime, we demonstrate that the spin polarization of the current can be controlled by tuning the parameters (amplitude and frequency) of the ac field. We also discuss spin relaxation and decoherence effects in the pumped current.

Floquet-Bloch theory and topology in periodically driven lattices

We propose a general framework to solve tight binding models in D dimensional lattices driven by ac electric fields. Our method is valid for arbitrary driving regimes and allows us to obtain effective Hamiltonians for different external field configurations. We establish an equivalence with time-independent lattices in D+1 dimensions and analyze their topological properties. Furthermore, we demonstrate that nonadiabaticity drives a transition from topological invariants defined in D+1 to D dimensions. Our results have potential applications in topological states of matter and nonadiabatic topological quantum computation, predicting novel outcomes for future experiments.

ac-driven localization in a two-electron quantum dot molecule

We investigate the dynamics of two interacting electrons confined to a pair of coupled quantum dots driven by an external ac field. By numerically integrating the two-electron Schrodinger equation in time, we find that for certain values of the strength and frequency of the ac field the electrons can become localized within just one of the dots, in spite of the Coulomb repulsion. Reducing the system to an effective two-site model of Hubbard type, and applying Floquet theory, leads to a detailed understanding of this effect. This demonstrates the possibility of using appropriate ac fields to manipulate entangled states in mesoscopic devices on extremely short time scales, which is an essential component of practical schemes for quantum information processing.

From zero resistance states to absolute negative conductivity in microwave irradiated two-dimensional electron systems

Recent experimental results regarding a two-dimensional electron gas subjected to microwave radiation reveal that magnetoresistivity, apart from presenting oscillations and zero resistance states, can evolve to negative values at minima. Here the authors present a theoretical model which explains the transition from zero resistance states to absolute negative conductivity in terms of multiphoton assisted electron scattering due to charged impurities and shows how this transition can be driven by tuning microwave frequency and intensity. This opens the possibility of controlling the magnetoconductivity in microwave driven nanodevices and understanding the novel optical and transport properties of such devices.

Temperature effects on microwave-induced resistivity oscillations and zero-resistance states in two-dimensional electron systems

In this work we address theoretically a key issue concerning microwave-induced longitudinal resistivity oscillations and zero resistance states, as is tempoerature. In order to explain the strong temperature dependence of the longitudinal resistivity and the thermally activated transport in 2DEG, we have developed a microscopic model based on the damping suffered by the microwave-driven electronic orbit dynamics by interactions with the lattice ions yielding acoustic phonons. Recent experimental results show a reduction in the amplitude of the longitudinal resistivity oscillations and a breakdown of zero resistance states as the radiation intensity increases. In order to explain it we have included in our model the electron heating due to large microwave intensities and its effect on the longitudinal resistivity.

Dynamical detection of Majorana fermions in current-biased nanowires

We analyze the current-biased Shapiro experiment in a Josephson junction formed by two one-dimensional nanowires featuring Majorana fermions. Ideally, these junctions are predicted to have an unconventional

MICROSCOPIC MODEL FOR SEQUENTIAL TUNNELING IN SEMICONDUCTOR MULTIPLE QUANTUM WELLS

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Merging of Dirac points and Floquet topological transitions in ac-driven graphene

-periodic Josephson effect and thus only Shapiro steps at even multiples of the driving frequency. Taking additionally into account overlap between the Majorana fermions, due to the finite length of the wire, renders the Josephson junction conventional for any dc-experiments. We show that probing the current-phase relation in a current biased setup dynamically decouples the Majorana fermions. We find that besides the even integer Shapiro steps there are additional steps at odd and fractional values. However, different from the voltage biased case, the even steps dominate for a wide range of parameters even in the case of multiple modes thus giving a clear experimental signature of the presence of Majorana fermions.