Could a laser field lead to the much sought-after tunable band gaps in graphene? By using Floquet theory combined with Green's functions techniques, we predict that a laser field in the mid-infrared range can produce observable band gaps in the electronic structure of graphene. Furthermore, we show how they can be tuned by using the laser polarization. Our results could serve as a guidance to design optoelectronic nanodevices.

We investigate charge and spin transport through an adiabatically driven, strongly interacting quantum dot weakly coupled to two metallic contacts with finite bias voltage. Within a kinetic equation approach, we identify coefficients of response to the time-dependent external driving and relate these to the concepts of charge and spin emissivities previously discussed within the time-dependent scattering matrix approach. Expressed in terms of auxiliary vector fields, the response coefficients allow for a straightforward analysis of recently predicted interaction-induced pumping under periodic modulation of the gate and bias voltage [Reckermann et al., Phys. Rev. Lett. 104, 226803 (2010)]. We perform a detailed study of this effect and the related adiabatic Coulomb blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic formulas for the pumped charge and spin in the regimes of small and large driving amplitude are provided for arbitrary bias. In the absence of a magnetic field, we obtain a striking, simple relation between the pumped charge at zero bias and at bias equal to the Coulomb charging energy. At finite magnetic field, there is a possibility to have interaction-induced pure spin pumping at this finite bias value, and generally, additional features appear in the pumped charge. For large-amplitude adiabatic driving, the magnitude of both the pumped charge and spin at the various resonances saturates at values which are independent of the specific shape of the pumping cycle. Each of these values provides an independent, quantitative measure of the junction asymmetry.

Recent experiments showed that the surface of a three-dimensional topological insulator develops gaps in the Floquet-Bloch band spectrum when illuminated with a circularly polarized laser. These Floquet-Bloch bands are characterized by nontrivial Chern numbers which only depend on the helicity of the polarization of the radiation field. Here we propose a setup consisting of a pair of counterrotating lasers, and show that one-dimensional chiral states emerge at the interface between the two lasers. These interface states turn out to be spin polarized and may trigger interesting applications in the field of optoelectronics and spintronics.Introduction. Amid the excitement sparked by graphene [1,2] and its record properties [3], the discovery of topological insulators (TIs) [4,5] developed with surprising speed. Indeed, TIs were predicted two years earlier in graphene [6], but the necessary spin-orbit interactions were too weak for this to be observed and a different playground was needed to realize them [7]. Most TIs are three-dimensional (3D) materials as are usual solids, but with a special property: They have a bulk band gap while keeping states that propagate with unprecedented robustness at the periphery of the sample [8,9]. These peculiar states hold great promise for quantum computation [10] but at the same time open up a major challenge: Controlling them is particularly demanding for 3D TIs.Encompassing the rapid progress in graphene photonics [11] and optoelectronics [12,13], theoretical studies predicted the formation of laser-induced band gaps [14] in graphene when properly tuning the laser polarization, frequency, and intensity [15][16][17][18]. More recently, these gaps were unveiled at the surface of a TI through angle-resolved photoemission spectroscopy (ARPES) [19]. This triggered great expectations for achieving laser-assisted control not only in the form of an on-off switch for the available states, but also because theoretically nontrivial topological states [14,20,21] can be induced on a diversity of materials [22][23][24][25][26], and also in cold matter physics [27,28]. Exciting questions arise about the nature of these novel states [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44], the possibilities for manipulating them [31], the associated topological invariants [32-36], their statistical properties [37-40], and their twoterminal [41,42] and multiterminal (Hall) responses [43,44]. Still, an experimental realization of the Floquet chiral edge states is missing. Most studies considered two-dimensional (2D) systems, except for Refs. [45,46], where the target was a 3D semiconductor.Here, we show that, besides opening a band gap as in Ref. [19], illuminating a 3D TI with a suitable set of lasers can confine the surface states into one-dimensional states which also bear a topological origin. The proposed setup is represented in Fig. 1: two lasers with opposite circular polarization incident perpendicularly to a face of a 3D TI. This can be devised through, e.g., a single laser beam...

During the last years there has been an increasing excitement in nanomotors and particularly in current-driven nanomotors. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. Here we consider nanoelectromechanical devices composed by a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. We use a real-time diagrammatic approach to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We prove our expressions accomplish with Onsager's reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes. The obtained results are illustrated in a nanomotor consisting of a double quantum dot capacitively coupled to some rotating charges. We analyze the dynamics and performance of the motor as function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

In this article we review aspects of charge and heat transport in interacting quantum dots and molecular junctions under stationary and time-dependent non-equilibrium conditions due to finite electrical and thermal bias. In particular, we discuss how a discrete level spectrum can be beneficial for thermoelectric applications, and investigate the detrimental effects of molecular vibrations on the efficiency of a molecular quantum dot as an energy converter.In addition, we consider the effects of a slow timedependent modulation of applied voltages on the transport properties of a quantum dot and show how this can be used as a spectroscopic tool complementary to standard dc-measurements. Finally, we combine timedependent driving with thermoelectrics in a doublequantum dot system -a nanoscale analogue of a cyclic heat engine -and discuss its operation and the main limitations to its performance.

In this article, we briefly review dynamical and thermodynamical aspects of different forms of quantum motors and quantum pumps. We then extend previous results to provide new theoretical tools for a systematic study of those phenomena at far-from-equilibrium conditions. We mainly focus on two key topics: (1) The steady-state regime of quantum motors and pumps, paying particular attention to the role of higher-order terms in the nonadiabatic expansion of the current-induced forces.(2) The thermodynamical properties of such systems, emphasizing systematic ways of studying the relationship between different energy fluxes (charge and heat currents, and mechanical power) passing through the system when beyond-first-order expansions are required. We derive a general order-by-order scheme based on energy conservation to rationalize how every order of the expansion of one form of energy flux is connected with the others. We use this approach to give a physical interpretation of the leading terms of the expansion. Finally, we illustrate the above-discussed topics in a double quantum dot within the Coulomb-blockade regime and capacitively coupled to a mechanical rotor. We find many exciting features of this system for arbitrary nonequilibrium conditions: A definite parity of the expansion coefficients with respect to the voltage or temperature biases; negative friction coefficients; and the fact that, under fixed parameters, the device can exhibit multiple steady states where it may operate as a quantum motor or as a quantum pump depending on the initial conditions.

Abstract. The use of Floquet theory combined with a realistic description of the electronic structure of illuminated graphene and graphene nanoribbons is developed to assess the emergence of non-adiabatic and non-perturbative effects on the electronic properties. Here, we introduce an efficient computational scheme and illustrate its use by applying it to graphene nanoribbons in the presence of both linear and circular polarization. The interplay between confinement due to the finite sample size and laserinduced transitions is shown to lead to sharp features on the average conductance and density of states. Particular emphasis is given to the emergence of the bulk limit response.PACS numbers: 73.23.-b, 72.10.-d, 73.63.-b arXiv:1301.6629v1 [cond-mat.mes-hall]

We present a theoretical study of quantum charge pumping with a single ac gate applied to graphene nanoribbons and carbon nanotubes operating with low resistance contacts. By combining Floquet theory with Green's function formalism, we show that the pumped current can be tuned and enhanced by up to two orders of magnitude by an appropriate choice of device length, gate voltage intensity and driving frequency and amplitude. These results offer a promising alternative for enhancing the pumped currents in these carbon-based devices.