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We explore the energy transfer dynamics in an array of two chains of identical rigid interacting dipoles. A crossover between two different ground state (GS) equilibrium configurations is observed with varying distance between the two chains of the array. Linearizing around the GS configurations, we verify that interactions up to third nearest neighbors should be accounted for accurately describe the resulting dynamics. Starting with one of the GS, we excite the system by supplying it with an excess energy DK located initially on one of the dipoles. We study the time evolution of the array for different values of the system parameters b and DK. Our focus is hereby on two features of the energy propagation: the redistribution of the excess energy DK among the two chains and the energy localization along each chain. For typical parameter values, the array of dipoles reaches both the equipartition between the chains and the thermal equilibrium from the early stages of the time evolution. Nevertheless, there is a region in parameter space (b,DK) where even up to the long computation time of this study, the array does neither reach energy equipartition nor thermalization between chains. This fact is due to the existence of persistent chaotic breathers.

We investigate irradiated high-mobility two-dimensional electron systems (2DES) under low or moderated magnetic fields. These systems present microwave-induced magnetoresistance oscillations (MIRO) which, as we demonstrate, reveal the presence of coherent states of the quantum harmonic oscillator. We also show that the principle of minimum uncertainty of coherent states is at the heart of MIRO and zero resistance states (ZRS). Accordingly, we are able to explain, based on coherent states, important experimental evidence of these photo-oscillations. Such as their physical origin, their periodicity with the inverse of the magnetic field and their peculiar oscillations minima and maxima positions in regards of the magnetic field. Thus, remarkably enough, we come to the conclusion that 2DES, under low magnetic fields, become a system of quasiclassical states or coherent states and MIRO would be the smoking gun of the existence of these peculiar states is these systems., Comment: 6 pages and 10 figures. arXiv admin note: substantial text overlap with arXiv:2306.12160

We report a novel theoretical approach on the microwave-induced resistance oscillations based on the coherent states of the quantum harmonic oscillator. We first obtain an expression for the coherent states of driven-quantum harmonic oscillators that are used, in the model of microwaveinduced electron orbits, to calculate magnetoresistance under radiation. Thus, we find that the principle of minimum uncertainty of coherent states, involving time and energy, is at the heart of photo-oscillations and zero resistance states. Accordingly, we are able to explain important experimental evidence of this remarkable effect. Such as the physical origin of oscillations, their periodicity with the inverse of the magnetic field, their peculiar minima and maxima positions and the existence of zero resistance states. We apply our theory to the case of ultra-high mobility samples where we appeal to the principle of quantum superposition of coherent states and obtain that Schrodinger cat states (even and odd coherent states) are key to explain magnetoresistance at these extreme mobilities. With them we explain the, experimentally obtained, magnetoresistance resonance peak shift to a magnetic field where the cyclotron frequency equals half the radiation frequency. This effect is similar to the one described in quantum optics as a second harmonic generation process. We also explain the magnetoresistance collapse, that take place in the dark and with light. This effect is known as giant negative magnetoresistance. We generalize our results to study the case of a three-component or triangular Schrodinger cat state., Comment: 11 pages, 7 figures

We explore the connection between chaos, thermalization and ergodicity in a linear chain of $N$ interacting dipoles. Starting from the ground state, and considering chains of different numbers of dipoles, we introduce single site excitations with energy $\Delta K$. The time evolution of the chaoticy of the system and the energy localization along the chain is analyzed by computing, up to very long times, the statistical average of the finite time Lyapunov exponent $\lambda(t)$ and of the participation ratio $\Pi(t)$. For small $\Delta K$, the evolution of $\lambda(t)$ and $\Pi(t)$ indicates that the system becomes chaotic at roughly the same time as $\Pi(t)$ reaches a steady state. For the largest values of $\Delta K$, the system becomes chaotic at an extremely early stage in comparison with the energy relaxation times. We find that this fact is due to the presence of chaotic breathers that keep the system far from equipartition and ergodicity. Finally, we show that the asymptotic values attained by the participation ratio $\Pi(t)$ fairly corresponds to thermal equilibrium., Comment: 7 pages, 6 figures

We report on a theoretical study on the rise of radiation-induced magnetoresistance oscillations in two-dimensional systems of massive Dirac fermions. We study the bilayer system of monolayer graphene and hexagonal boron nitride (h-BN/graphene) and the trilayer system of hexagonal boron nitride encapsulated graphene (h-BN/graphene/h-BN). We extend the radiation-driven electron orbit model that was previously devised to study the same oscillations in two-dimensional systems of Schr\"odinger electrons (GaAs/AlGaAS heterostructure) to the case of massive Dirac fermions. In the simulations we obtain clear oscillations for radiation frequencies in the terahertz and far-infrared bands. %which contrasts with the two-dimensional Schrodinger electrons case, %that are mainly sensitive to microwave frequencies. We investigate also the power and temperatures dependence. For the former we obtain similar results as for Schr\"odinger electrons and predict the rise of zero resistance states. For the latter we obtain a similar qualitatively dependence but quantitatively different when increasing temperature. While in GaAs the oscillations are wiped out in a few degrees, interestingly enough, for massive Dirac fermions, we obtain observable oscillations for temperatures above $100$ K and even at room temperature for the higher frequencies used in the simulations., Comment: 8 pages and 6 figures

We report on a theoretical study on the rise of strong peaks at the harmonics of the cyclotron resonance in the irradiated magnetoresistance in ultraclean two-dimensional electron systems. The motivation is the experimental observation of a totally unexpected strong resistance peak showing up at the second harmonic. We extend the radiation-driven electron orbit model (previously developed to study photocurrent oscillations and zero resistance states) to a ultraclean scenario that implies longer scattering time and longer mean free path. Thus, when the mean free path is equivalent, in terms of energy, to twice the cyclotron energy ($2\hbar w_{c}$), the electron behaves as under an effective magnetic field twice the one really applied. Then, at high radiation power and/or low temperature, a resistance spike can be observed {\it at the second harmonic}. For even cleaner samples the energy distance could increase to three or four times the cyclotron energy giving rise to resistance peaks at higher harmonics (third, fourth, etc.), i.e., a resonance peak shift to lower magnetic fields as the quality of the sample increases. Thus, by selecting the sample mobility one automatically would select the radiation resonance response without altering the radiation frequency., Comment: Accepted in Physical Review B 6 pages, 5 figures

We investigate the structure and the nonlinear dynamics of two rigid polar rotors coupled through the dipole-dipole interaction in an external homogeneous electric field. In the field-free stable head-tail configuration, an excess energy is provided to one of the dipoles, and we explore the resulting three-dimensional classical dynamics. This dynamics is characterized in terms of the kinetic energy transfer between the dipoles, their orientation along the electric field, as well as their chaotic behavior. The field-free energy transfer mechanism shows an abrupt transition between equipartition and non-equipartition regimes, which is independent of the initial direction of rotation due to the existence of an infinite set of equivalent manifolds. The field-dressed dynamics is highly complex and strongly depends on the electric field strength and on the initial conditions. In the strong field regime, the energy equipartition and chaotic behavior dominate the dynamics.

We present a microscopic theory on the observation of a beating pattern in the radiation-induced magnetoresistance oscillations at very low magnetic field. We con- sider that such a beating pattern develops as a result of the coupling between two oscillatory components: the first is a system of electron Landau states being harmon- ically driven by radiation. The second is a lattice oscillation, i.e., an acoustic phonon mode. We analyze the dependence of the beating pattern on temperature, radiation frequency and power. We conclude that the beating pattern is an evidence of the radiation-driven nature of the irradiated Landau states that makes them behave as a collective plasma oscillation at the radiation frequency. Thus, the frequency of such plasmons could be tuned from microwave to terahertz in the same nanodevice with an apparent technological application., Comment: 5 pages, 5 figures

We present a theoretical study on the effect of radiation on the mangetoresistance of two-dimensional electron systems with strong Rashba spint-orbit coupling. We want to study the interplay between two well-known effects in these electron systems: the radiation-induced resistance oscillations and the typical beating pattern of systems with intense Rashba interaction. We analytically derive an exact solution for the electron wave function corresponding to a total Hamiltonian with Rashba and radiation terms. We consider a perturbation treatment for elastic scattering due to charged impurities to finally obtain the magnetoresistance of the system. Without radiation we recover a beating pattern in the amplitude of the Shubnikov de Hass oscillations: a set of nodes and antinodes in the magnetoresistance. In the presence of radiation this beating pattern is strongly modified following the profile of radiation-induced magnetoresistance oscillations. We study their dependence on intensity and frequency of radiation, including the teraherzt regime., Comment: 10 pages, 5 figures Mathematical derivation of equation [30]. New figure explaining the mathematical derivation of equation [30]

We present a microscopic model on radiation-induced zero resistance states according to a novel approach: Franck-Condon physics and blockade. Zero resistance states rise up from radiation-induced magnetoresistance oscillations when the light intensity is strong enough. The theory starts off with the {\it radiation-driven electron orbit model} that proposes an interplay of the swinging nature of the radiation-driven Landau states and the presence of charged impurity scattering. When the intensity of radiation is high enough it turns out that the driven-Landau states (vibrational states) involved in the scattering process are spatially far from each other and the corresponding electron wave functions do not longer overlap. As a result, it takes place a drastic suppression of the scattering probability and then current and magnetoresistance exponentially drop. Finally zero resistance states rise up. This is an application to magnetotransport in two dimensional electron systems of the Franck-Condon blockade, based on the Franck-Condon physics which in turn stems from molecular vibrational spectroscopy., Comment: 5 pages, 6 figures

We present an experimental study of the microwave power and the linear polarization angle dependence of the microwave-induced magnetoresistance oscillations in the high-mobility GaAs/AlGaAs two-dimensional electron system. Experimental results show the sinusoidal dependence of the oscillatory magnetoresistance extrema as a function of the polarization angle. Yet, as the microwave power increases, the angular dependence includes additional harmonic content, and it begins to resemble the absolute value of the cosine function. We present a theory to explain such peculiar behavior.

We explore the classical dynamics of two interacting rotating dipoles that are fixed in the space and exposed to an external homogeneous electric field. Kinetic energy transfer mechanisms between the dipoles are investigated varying both the amount of initial excess kinetic energy of one of them and the strength of the electric field. In the field-free case, and depending on the initial excess energy an abrupt transition between equipartition and non-equipartition regimes is encountered. The study of the phase space structure of the system as well as the formulation of the Hamiltonian in an appropriate coordinate frame provide a thorough understanding of this sharp transition. When the electric field is turned on, the kinetic energy transfer mechanism is significantly more complex and the system goes through different regimes of equipartition and non-equipartition of the energy including chaotic behavior., Comment: 21 pages, 11 figures

We report on a theoretical work on magnetotransport under terahertz radiation with high mobility two-dimensional electron systems. We focus on the interaction between the obtained radiation-induced magnetoresistance oscillations (RIRO) and the Shubnikov-de Haas (SdHO) oscillations. We study two effects experimentally obtained with this radiation. First, the observed disappearance of the SdHO oscillations simultaneously with the vanishing resistance at the zero resistance states region. And secondly the strong modulation of the SdHO oscillations at sufficient terahertz radiation power. We conclude that both effects share the same physical origin, the interference between the average advanced distance by the scattered electron between irradiated Landau states, (RIRO), and the available initial density of states at a certain magnetic field, (SdHO). Thus, from a physical standpoint, what the terahertz experiments and theoretical simulations reveal is, on the one hand, the oscillating nature of the Landau states subjected to radiation and, on the other hand, how they behave in the presence of scattering., Comment: 5 pages and 3 figures

We provide the ultimate explanation of one of the core features of microwave-induced magnetoresistance oscillations in high mobility two dimensional electron systems: the 1/4-cycle phase shift of minima. We start with the radiation-driven electron orbits model with the novel concept of scattering flight-time between Landau states. We calculate the extrema and nodes positions obtaining an exact coincidence with the experimental ones. The main finding is that the physical origin of the phase shift is a delay of $\frac{\pi}{2}$ of the radiation-driven Landau guiding center with respect to radiation, demonstrating the oscillating nature of the irradiated Landau states. We analyze the dependence of this minima on radiation frequency and power and its possible shift with the quality of the sample, Comment: 5 pages, 3 figures

We present a theoretical approach to study the effect of microwave radiation on the magnetoresistance of a one-dimensional superlattice. In our proposal the magnetoresistance of a unidirectional spatial periodic potential (superlattice), is modulated by microwave radiation due to an interference effect between both, space and time-dependent potentials. The final magnetoresistance will mainly depend on the spatial period of the superlattice and the radiation frequency. %Then, by tuning either the spatial period of the superlattice or the radiation %frequency, the magnetoresistance can be strongly modified. We consider an approach to study these effects based on the fast Fourier transform of the obtained magnetorresistance profiles in function of the inverse of the magnetic field. Based on this theory we propose the design of a novel radiation sensor for the Terahertz band.} % We first study the FFT of the system for each potential individually. Then we study jointly the FFT of the system when the two types of potentials are simultaneously acting., Comment: 5 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:0808.2370

We report on a theoretical insight about the microwave-induced resistance oscillations and zero resistance states when dealing with p-type semiconductors and holes instead of electrons. We consider a high-mobility two-dimensional hole gas hosted in a pure Ge/SiGe quantum well. Similarly to electrons we obtain radiation-induce resistance oscillations and zero resistance states. We analytically deduce a universal expression for the irradiated magnetoresistance, explaining the origin of the minima positions and their $1/4$ cycle phase shift. The outcome is that these phenomena are universal and only depend on radiation and cyclotron frequencies. We also study the possibility of having simultaneously two different carriers driven by radiation: light and heavy holes. As a result the calculated magnetoresistance reveals an interference profile due to the different effective masses of the two types of carriers., Comment: 9 pages and 9 figures

In high-mobility two-dimensional electron gases Landau levels are already formed at very small magnetic field values. Such two-dimensional electron gases show a huge negative magnetoresistance at low temperatures and an unexpected and very strong non-linear behavior with the applied current. This non-linearity depends on carrier concentration and is explained by the subtle interplay of elastic scattering within Landau levels and in between Landau levels.

Recent experiments on re-emission of radiation and magnetotransport in photoexcited two-dimensional electron systems are theoretically analyzed. These experiments have concluded that there exists a strong correlation between the re-emitted radiation and the radiation-driven current through the sample. We study these remarkable experimental results using the radiationdriven electron orbit model. According to it, two-dimensional electrons under a magnetic field move back and forth driven by radiation. Therefore, they behave as oscillating dipoles re-emitting part of the radiation previously absorbed. Then, this theory would explain in a simple way the experimental results., Comment: 6 pages, 6 figures

This paper studies the problem of optimally allocating a cash injection into a financial system in distress. Given a one-period borrower-lender network in which all debts are due at the same time and have the same seniority, we address the problem of allocating a fixed amount of cash among the nodes to minimize the weighted sum of unpaid liabilities. Assuming all the loan amounts and asset values are fixed and that there are no bankruptcy costs, we show that this problem is equivalent to a linear program. We develop a duality-based distributed algorithm to solve it which is useful for applications where it is desirable to avoid centralized data gathering and computation. We also consider the problem of minimizing the expectation of the weighted sum of unpaid liabilities under the assumption that the net external asset holdings of all institutions are stochastic. We show that this problem is a two-stage stochastic linear program. To solve it, we develop two algorithms based on: Benders decomposition algorithm and projected stochastic gradient descent. We show that if the defaulting nodes never pay anything, the deterministic optimal cash injection allocation problem is an NP-hard mixed-integer linear program. However, modern optimization software enables the computation of very accurate solutions to this problem on a personal computer in a few seconds for network sizes comparable with the size of the US banking system. In addition, we address the problem of allocating the cash injection amount so as to minimize the number of nodes in default. For this problem, we develop two heuristic algorithms: a reweighted l1 minimization algorithm and a greedy algorithm. We illustrate these two algorithms using three synthetic network structures for which the optimal solution can be calculated exactly. We also compare these two algorithms on three types random networks which are more complex.

We explore the classical dynamics of atoms in an optical dipole trap formed by two identical Gaussian beams propagating in perpendicular directions. The phase space is a mixture of regular and chaotic orbits, the later becoming dominant as the energy of the atoms increases. The trapping capabilities of these perpendicular Gaussian beams are investigated by considering an atomic ensemble in free motion. After a sudden turn on of the dipole trap, a certain fraction of atoms in the ensemble remains trapped. The majority of these trapped atoms has energies larger than the escape channels, which can be explained by the existence of regular and chaotic orbits with very long escape times.

We report on theoretical studies of the recently discovered negative giant magnetoresistance in ultraclean two-dimensional electron systems at low temperatures. We adapt a transport model to a ultraclean scenario and calculate the elastic scattering rate (electron-charged impurity) in a regime where the Landau level width is much smaller than the cyclotron energy. We obtain that for low magnetic fields the scattering rate and, as a consequence, the longitudinal magnetoresistance dramatically drop because of the small density of states between Landau levels. We also study the dependence of this striking effect on temperature and an in-plane magnetic field., Comment: 5pages, 5 figures

Light-matter coupling is investigated by rotating, by an angle \theta, the polarization of linearly polarized microwaves with respect to the long-axis of GaAs/AlGaAs Hall-bar electron devices. At low microwave power, P, experiments show a strong sinusoidal variation in the diagonal resistance R_{xx} vs. \theta at the oscillatory extrema, indicating a linear polarization sensitivity in the microwave radiation-induced magnetoresistance oscillations. Surprisingly, the phase shift \theta_{0} for maximal oscillatory R_{xx} response under photoexcitation appears dependent upon the radiation-frequency f, the extremum in question, and the magnetic field orientation or sgn(B)., Comment: 6 pages, 5 figures

We study the influence of the polarization angle of linear radiation on the radiation-induced magnetoresistance oscillations in two-dimensional electron systems and examine the polarization immunity on the temperature and quality of the sample. We have applied the radiation-driven electron orbits model obtaining that the magnetoresistance is affected by the orientation of the electric field of linearly polarized radiation when dealing with high quality samples and low temperatures. Yet, for lower quality samples and higher temperature, we recover polarization immunity in the radiation driven magnetoresistance oscillations. This could be of interest for future photoelectronics in high quality mesoscopic devices. VC 2012 American Institute of Physics, Comment: 4 pages, 3 figures

We report on theoretical studies of a recently discovered strong radiation-induced magnetoresistance spike obtained in ultraclean two-dimensional electron systems at low temperatures. The most striking feature of this spike is that it shows up on the second harmonic of the cyclotron resonance and with an amplitude that can reach an order of magnitude larger than the radiation-induced resistance oscillations. We apply the radiation-driven electron orbits model in the ultraclean scenario. Accordingly, we calculate the elastic scattering rate (charged impurity) which will define the unexpected resonance spike position. We also obtain the inelastic scattering rate (phonon damping), that will be responsible of the large spike amplitude. We present a microscopical model to explain the dependence of the Landau level width on the magnetic field for ultraclean samples. We find that this dependence explains the experimental shift of the resistance oscillations with respect to the magnetic field found in this kind of samples. We study also recent results on the influence of an in-plane magnetic field on the spike. We are able to reconcile the obtained different experimental response of both spike and resistance oscillations versus an increasing in-plane field. The same model on the variation of the LL width, allows us to explain such surprising results based in the increasing disorder in the sample caused by the in-planed magnetic field. Calculated results are in good agreement with experiments. These results would be of special interest in nanophotonics; they could lead to the design of novel ultrasensitive microwave detectors., Comment: 16 pages, 12 figures. arXiv admin note: text overlap with arXiv:1109.5946

We report on theoretical studies of a recently discovered strong microwave-induced magnetoresistance peak obtained in ultra-clean two-dimensional electron systems at low temperatures. The most striking feature of such a peak is that it shows up on the second harmonic of the cyclotron resonance and with an amplitude that can reach an order of magnitude larger than the microwave-induced resistance oscillations. We apply the microwave-driven electron orbits model in the ultra-clean scenario. Accordingly, we calculate the elastic scattering rate (charged impurity) which will define the unexpected resonance peak position. We also obtain the inelastic scattering rate (phonon damping), that will be responsible of the large peak amplitude. Calculated results are in good agreement with experiments. These results would be of special interest in nanophotonics; they could lead to the design of novel ultrasensitive microwave detectors., Comment: 4 pages, 5 figures,To be published in Physica Status Solidi-rapid research letters

We report on theoretical studies of recently discovered microwave-induced resistance oscillations and zero resistance states in Hall bars with two occupied subbands. In the same results, resistance presents a peculiar shape which appears to have a built-in interference effect not observed before. We apply the microwave-driven electron orbit model, which implies a radiation-driven oscillation of the two-dimensional electron system. Thus, we calculate different intra and inter-subband electron scattering rates and times that are revealing as different microwave-driven oscillations frequencies for the two electronic subbands. Through scattering, these subband-dependent oscillation motions interfere giving rise to a striking resistance profile. We also study the dependence of irradiated magnetoresistance with power and temperature. Calculated results are in good agreement with experiments., Comment: 7 pages, 6 figures

We analyze theoretically magnetoresistance of high mobility two-dimensional electron systems being illuminated by multiple radiation sources. In particular, we study the influence on the striking effect of microwave-induced resistance oscillations. We consider moderate radiation intensities without reaching the zero resistance states regime. We use the model of radiation-driven Larmor orbits extended to several light sources. First, we study the case of two different radiations polarized in the same direction with different or equal frequencies. For both cases we find a regime of superposition or interference of harmonic motions. When the frequencies are different, we obtain a modulated magnetoresistance response with pulses and beats. On the other hand, when the frequencies are the same, we find that the final result will depend on the phase difference between both radiation fields going from an enhanced response to a total collapse of oscillations, reaching an outcome similar to darkness. Finally, we consider a multiple photoexcitation case (three different frquencies) where we propose the two-dimensional electron system as a potential nanoantenna device for microwaves., Comment: 5 pages, 2 figures

We find that the amplitude of the $R_{xx}$ radiation-induced magnetoresistance oscillations in GaAs/AlGaAs system grows nonlinearly as $A \propto P^{\alpha}$ where $A$ is the amplitude and the exponent $\alpha < 1$. %, with $\alpha \rightarrow 1/2$ in %the low temperature limit. This striking result can be explained with the radiation-driven electron orbits model, which suggests that the amplitude of resistance oscillations depends linearly on the radiation electric field, and therefore on the square root of the power, $P$. We also study how this sub-linear power law varies with lattice temperature and radiation frequency., Comment: 5 pages, 3 figures

We analyze electron spin relaxation in electronic transport through coherently coupled double quantum dots in the spin blockade regime. In particular, we focus on hyperfine interaction as the spin relaxation mechanism. We pay special attention to the effect of the dynamical nuclear spin polarization induced by the electronic current on the nuclear environment. We discuss the behaviour of the electronic current and the induced nuclear spin polarization versus an external magnetic field for different hyperfine coupling intensities and interdot tunnelling strengths. We take into account, for each magnetic field, all hyperfine mediated spin relaxation processes coming from the different opposite spin levels approaches. We find that the current as a function of the external magnetic field shows a peak or a dip, and that the transition from a current dip to a current peak behaviour is obtained by decreasing the hyperfine coupling or by increasing the interdot tunnelling strength. We give a physical picture in terms of the interplay between the electrons tunnelling out of the double quantum dot and the spin flip processes due to the nuclear environment., Comment: 25 pages and 8 figures. To be published in New Journal of Physics

In this work we present a theoretical approach to study the effect of an in-plane (parallel) magnetic field on the microwave-assisted transport properties of a two-dimensional electron system. Previous experimental evidences show that microwave-induced resistance oscillations and zero resistance states are differently affected depending on the experimental set-up: two magnetic fields (two-axis magnet) or one tilted magnetic field. In the first case, experiments report a clear quenching of resistance oscillations and zero resistance states. In a tilted field, one obtains oscillations displacement and quenching but the latter is unbalanced and less intense. In our theoretical proposal we explain these results in terms of the microwave-driven harmonic motion performed by the electronic orbits and how this motion is increasingly damped by the in-plane field., Comment: Figure 1 has been changed

In this work we present a theoretical model to study the effect of microwave radiation on Weiss oscillations. In our proposal Weiss oscillations, produced by an spatial periodic potential, are modulated by microwave radiation due to an interference effect between both, space and time-dependent, potentials. The final magnetoresistance depends mainly on the spatial period of the spatial potential and the frequency of radiation. Depending on the values of these parameters, we predict that Weiss oscillations can reach zero resistance states. On the other hand, these dissipationless transport states, created just by radiation, can be destroyed by the additional presence of a periodic space-dependent potential. Then by tuning the spatial period or the radiation frequency, the magnetoresistance can be strongly modified., Comment: 4 pages and 5 figures

Experimental results on microwave-induced magnetoresistance oscillation in two-dimensional electron systems show a similar behavior of these systems regarding temperature and microwave frequency. It is found that these oscillations tend to quench when frequency or temperature increase, approaching magnetoresistance to the response of the dark system. In this work we show that this experimental behavior can be addressed on the same theoretical basis. Microwave radiation forces the electron orbits to move back and forth being damped by interaction with the lattice. We show that this damping depends dramatically on microwave frequency and also on temperature. An increase in frequency or temperature gives rise to an increase in the lattice damping producing eventually a quenching effect in the magnetoresistance oscillations., Comment: 3 pages and 3 figures

Double quantum dot systems in the spin blockade regime exhibit leakage currents that have been attributed to the Hyperfine interaction. We model weakly coupled double-dot transport using a rate equation approach which accounts for Hyperfine flip-flop transitions. The rate equations allow us to obtain self-consistently the time evolution for electronic charge occupations and for the nuclei polarizations in each dot. We analyze the current in the spin blockade region as a function of magnetic field and observe hysteretic behavior for fields corresponding to the crossing between triplet and singlet states., Comment: 3 pages and 4 figures. Accepted in APL

We analyze theoretically the oscillations that the magnetoresistivity of two-dimensional electron systems present when a high intensity direct current is applied. In the model presented here we suggest that a plasma wave is excited in the system producing an oscillating motion of the whole two-dimensional electron gas at the plasma frequency. This scenario affects dramatically the way that electrons interact with scatterers giving rise to oscillations in the longitudinal resistivity. With this theoretical model experimental results are well reproduced and explained., Comment: 4 pages, 5 figures, Accepted in Applied Physics Letters

We analyzed the magnetoresistivity of a two-dimensional electron system excited by microwave radiation in a regime of high intensities and low frequencies. In such a regime, recent experiments show that different features appear in the magnetoresistivity response which suggest an anharmonic behavior. These features consist mainly in distorted oscillations and new resonance peaks at the subharmonics of the cyclotron frequency. We follow the model of microwave-driven electron orbits motion which become anharmonic when the ratio of microwave intensity to microwave frequency is large enough., Comment: 3 pages, 2 figures. Accepted in Applied Physics Letters

We theoretically analyzed the microwave-induced modification of the Hall magnetoresistivity in high mobility two-dimensional electron systems. These systems present diagonal magnetoresistivity oscillations and zero-resistance states when are subjected to microwave radiation. The most surprising modification of the Hall magnetoresistivity is a periodic reduction which correlates with a periodic increase in the diagonal resistivity. We present a model that explains the experimental results considering that radiation affects directly only the diagonal resistivity and the observed Hall resistivity changes are coming from the tensor relationship between both of them., Comment: 3 pages, 2 figures

We analyze theoretically the dependence of the microwave polarization sate and sense on the magnetoresistivity response of two-dimensional electron systems. Linear and circular polarization have been considered with different senses and directions. We discuss the polarization dependence of the longitudinal magnetoresistivity and propose an explanation for the experimentally observed polarization immunity, i.e., resistivity oscillations and zero resistance state regions are unaffected by the sense of circular polarization or by the direction of microwave electric field., Comment: 4 pages and 1 figure

We analyzed the electronic transport through a double quantum dot in the spin blockade regime. Experiments of current rectification by Pauli exclusion principle in double quantum dots were discussed. The electron and nuclei spin dynamics and their interplay due to the Hyperfine interaction were self-consistently analyzed within the framework of rate equations. Our results show that the current leakage experimentally observed in the spin-blockade region, is due to spin-flip processes induced by Hyperfine interaction through Overhauser effect. We show as well how a magnetic field applied parallel to the current allows excited states to participate in the electronic current and removes spin blockade. Our model includes also a self-consistent description of inelastic transitions where the energy is exchanged through interactions with acoustic phonons in the environment. It accounts for spontaneous emission of phonons which results in additional features in the current characteristics. We develop a microscopical model to treat the Hyperfine interaction in each dot. Using this model we study the dynamical nuclear polarization as a function of the applied voltage., Comment: 7 pages 7 figures, PRB 76, 085329 (2007)

We analyze the effect of bichromatic microwave irradiation on the magnetoresistivity of a two dimensional electron system. We follow the model of microwave driven Larmor orbits in a regime where two different microwave lights with different frequencies are illuminating the sample ($w_{1}$ and $w_{2}$). Our calculated results demonstrate that now the electronic orbit centers are driven by the superposition of two harmonic oscillatory movements with the frequencies of the microwave sources. As a result the magnetoresisitivity response presents modulated pulses in the amplitude with a frequency of $\frac{w_{1}-w_{2}}{2}$, whereas the main response oscillates with $\frac{w_{1}+w_{2}}{2}$., Comment: 4 pages, 3 figures Accepted in Applied Physics Letters

Recent experimental results regarding a 2D electron gas subjected to microwave radiation reveal that magnetoresistivity, apart from presenting oscillations and zero resistance states, can evolve to negative values at minima. In other words, the current can evolve from flowing with no dissipation, to flow in the opposite direction of the dc bias applied. Here we present a theoretical model in which the existence of radiation-induced absolute negative conductivity is analyzed. Our model explains the transition from zero resistance states to absolute negative conductivity in terms of multiphoton assisted electron scattering due to charged impurities. It shows as well, how this transition can be driven by tuning microwave frequency and intensity. Then it opens the possibility of controlling the electron Larmor orbits dynamics (magnetoconductivity) in microwave driven nanodevices. The analysis of zero resistance states is therefore promising because new optical and transport properties in nanodevices will be expected., Comment: 5 pages and 4 figures

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., Comment: 4 pages and 4 figures. Accepted in Phys Rev B

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 MW-driven orbit dynamics and the Pauli exclusion principle., Comment: 4 pages, 5 figures Phys. Rev. Lett. in press

(Abridged) Deep VLT spectropolarimetric observations are presented for two powerful radio galaxies, 0850-206 and 1303+091. These observations cover the rest-frame wavelength range ~ 1450 - 3750 A. New radio observations and continuum images of the same sources are also presented. These galaxies are the first two observed from a complete sample of nine radio sources with redshifts in the range 1.3 < z < 1.5 (selected from the equatorial sample of powerful radio sources of Best, Rottgering & Lehnert), as part of a project aimed to investigate the multi-component nature of the UV continuum in radio galaxies and any variations of the continuum properties with the radio source age. The larger radio source of the two, 0850-206, presents a high continuum fractional polarization, averaging 17% across the observed wavelength range and reaching 24% at rest-frame wavelengths of <2000 A. The smaller radio source, 1303+091, shows a lower continuum polarization, averaging 8% and rising to 11% for rest-frame wavelengths >3000 A. For both galaxies, the position angle of the electric vector is generally constant with wavelength and within ~15 degrees of perpendicular to the radio axis. Both their total flux spectra and polarized flux spectra reveal the 2200 A dust feature, and comparison with dust scattering models suggests that the composition of the dust in these galaxies is similar to that of Galactic dust. In 0850-206, scattered quasar radiation dominates the UV continuum emission, with the nebular continuum accounting for no more than ~22% and no requirement for any additional emission component such as emission from young stars. By contrast, in 1303+091, unpolarized radiation could be a major constituent of the UV continuum emission, with starlight accounting for up to ~50% and the nebular continuum accounting for ~11%., Comment: 20 pages, including 14 figures. MNRAS accepted

Integral-field spectroscopic observations are presented for three powerful radio galaxies, namely 3C277.3 (Coma A; z=0.0857), 3C171 (z=0.2384) and 3C265 (z=0.811), which are known to be undergoing jet-cloud interactions. The morphology, kinematics and ionization of the gas in the emission-line structure of these sources are mapped and analysed. One-dimensional spectra are also extracted and integrated over the different emission-line regions in each galaxy. In two of the galaxies (3C277.3 and 3C171) the radio sizes are of similar extent to the emission-line structure. For these, enhanced emission-line regions are found associated with the radio structures, in addition to complex kinematics and low ionization states close to the radio hotspots, indicating that jet-induced shocks disturb and ionize the gas. Interestingly, the bright -- presumably shock-ionized -- emission-line region coincident with the radio jet knot in 3C277.3 shows quiescent kinematics and high ionization state. Possible explanations for this puzzling result are proposed. The images of 3C171 and 3C265 indicate that the lateral expansion of the cocoon has a significant effect on the kinematics and ionization of the gas, showing for the first time that the effects of the radio source are felt far from the jet axis. In addition, the presence of a stellar-photoionized HII region is detected in the extended emission-line nebula of the radio galaxy 3C277.3., Comment: 19 pages, including 16 figures (4 in colour), MNRAS accepted

We present deep, narrow-band and continuum images of the powerful z=0.811 radio galaxy 3C265, taken with the TTF on the WHT, together with detailed long-slit spectroscopic observations along the axis defined by the UV/optical emission elongation. The deep images reveal the existence of cones in the ionization structure within ~7 arcsec (58 kpc) of the nucleus, where the emission-line structure is not closely aligned with the radio axis. This indicates that anisotropic illumination from the central AGN dominates on a small scale. At larger distances (>10 arcsec) from the nucleus, low-ionization emission gas is closely aligned with the radio axis, suggesting that jet-cloud interactions may be the dominant mechanism in the line-emitting gas on a larger scale. Moreover, the presence of a high-velocity cloud at 2.5 arcsec from the nucleus, close to the radio axis, indicates that even close to the nucleus jet-induced shocks have an important kinematic effect. While on a large scale the low-ionization emission-line structures are aligned with the radio axis, on a smaller scale, where AGN-photoionization dominates, the highest surface brightness structure is aligned with the closest companion galaxy. This suggest that much of the emission-line structure reflects the intrinsic gas distribution, rather than the ionization pattern imprinted by the radio jets or by AGN illumination. Overall, our results underline the need for a variety of mechanisms to explain the properties of the extended emission-line gas of radio galaxies (abridged)., Comment: 12 pages, including 6 figures, MNRAS in press

We present the result obtained from a study, based on long-slit spectroscopy, of the kinematics and ionization mechanisms of the line-emitting gas for a sample of four high/intermediate-redshift radio galaxies. In two of the galaxies (3C352 and 3C435A) the radio sources are of the same scale as the emission-line regions, whereas in the other two (3C34 and 3C330) the radio sources are extended on a larger scale than the emission-line structures. We see evidence for shock-acceleration of the emission-line gas in the extended regions of all the galaxies, even in the largest radio sources of our sample, in which the radio hot spots have passed the extended gas of the galaxies. The extended regions present highly disturbed kinematics (line-splitting and/or underlying broad components), which are difficult to explain if we do not consider a strong interaction between the radio-emitting components and the ambient gas. However, the dominant ionization mechanism of the line-emitting gas remains uncertain. We have compared the optical diagnostic line ratios of the galaxies in our sample with both AGN-photoionization and shock-ionization models. We find a lack of consistency in explaining the main ionization mechanism of the emission-line gas. This suggest that, if the extended regions are shock-ionized, some of the assumptions implicit in the shock models may need to be reconsidered. In addition, we have investigated the nebular continuum cointribution to the UV excess in the galaxies of our sample. We find a substantial nebular emission contribution to the UV continuum in all the cases. However, after the subtraction iof the nebular component, a significant UV excess remains in the extended nebulae of most of the objects., Comment: 33 pages, 24 figures, accepted for publication in MNRAS. (Abstract shortened for astro-ph)

We develope a model to describe the transmission coefficient and tunneling current in the presence of photon-electron coupling in a resonant diode. Our model takes into account multiphoton processes as well as the transitions between electronic states with different wave numbers. This is crutial to explain the experimental features observed in the tunneling current through a double barrier which cannot be reproduced with more simplified established models. According to our results, what experiments show in the current density are quantum photon-assisted features coming from multiphoton transitions which are not related with sample heating., Comment: 8 pages,2 Postscript Figures