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  • Publication
    Effect of Al addition to Rapidly Solidified Mg-Cu-Rare Earth Alloys
    (ABM, ABC, ABPol, 2016-12-01) Rozenberg, Silvia; Audebert, Fernando; Galano, Marina; Vergara Ogando, Isabel; Mendive, Cecilia
    Rapidly solidified Mg based alloys are of interest for industrial applications as a structural material and for hydrogen storage. Mg-Cu-Rare Earth alloys have shown high glass forming ability; full amorphous structure with thickness of mm size can be obtained within these systems. However, their brittle behavior limits their industrial applications. In the present work, the Al effect in substitution of Cu in the Mg-65 Cu-25 MM10 (at%, MM: mischmetal) was studied. Samples up to 15at% Al were prepared by splat cooling and their microstructure, stability and mechanical properties were characterised. The crystallization temperature increases with the Al addition; the amorphous phase with different Al content has a Young's modulus of similar to 55GPa; the microhardness increases with the Al content in the amorphous and crystallized samples and the fracture of the alloy containing 10at% Al showed ductile vein patterns characteristics of ductile metallic glasses. The partial Cu substitution by Al can improve the stability and mechanical properties of the amorphous Mg-65 Cu-25 MM10 alloy.
  • Publication
    Temperature effects on the absorption properties in II-VI semiconductor core-shell nanocrystals
    (Elsevier, 2012-04-01) Cruz Fernández, Rosa María de la; Kanyinda-Malu, Climent
    By using Varshni's law, we have determined the temperature effects in the effective dielectric function of II-VI semiconductor nanocrystals distribution of type I. For each temperature, we have also used an extended Maxwell-Garnett theory to determine the effective dielectric function. We have taken into account the confinement of charge carriers by considering appropriate dielectric function for the cores in the distribution. For the shells, we have considered dielectric function similar to that in bulk semiconductors. Besides, the effects of (i) the cores sizes and (ii) the volume fraction of the distribution are investigated in the real and imaginary parts of the effective dielectric function. By increasing the temperature from 50 K to 300 K, we have found a red shift of the core resonant peaks as a consequence of its shrinkage bandgap along with a decreasing of its peak intensities and an increasing of its peak FWHM. Also, by increasing the core sizes, we have obtained a red shift of its resonant peak, while a decreasing of their sizes induces a blue shift, similar to that observed in other low-dimensional systems. Finally, by increasing the volume fraction, the intensities of the resonant peaks are higher and they are slightly red-shifted.
  • Publication
    Phonons contribution to the infrared and visible spectra of II-VI semiconductor core-shell nanocrystals
    (Elsevier, 2012-06) Cruz Fernández, Rosa María de la; Kanyinda-Malu, C; Rodríguez Aumente, Pedro Acisclo
    We have investigated the phonons contribution in the infrared and visible optical properties in II-VI semiconductor nanoshells of type I. For this, we use Mie scattering theory by defining appropriate dielectric functions for the constitutive materials of the nanoshells. Indeed, for the core we have considered dielectric function taking into account the spatial confinement of the charge carriers along with the phonons contribution. For the shell, we have considered dielectric function similar to that used in bulk semiconductor. Independently of the core and shell sizes and the embedding medium, we obtain in the infrared (IR) spectra, three resonant peaks ascribed to the CdS stretching vibration, the longitudinal optical (LO)-CdS and surface optical (SO)-ZnS phonon modes, respectively. The increase of core and shell sizes induces a red shift of the Cd-S stretching vibration and the SO ZnS branches, while a blue-shift is obtained for the LO CdS branch. If the phonons contribution is not considered in the IR spectrum, the CdS stretching vibration is disappeared. On the other hand, in the visible (VIS) spectra, we obtain one sharp resonant peak related to the 1s(e)-1s(h) optical transition, whose localization is characterized by the core size, essential parameter to evaluate the exciton energy. Phonons contribution in the VIS range yields information about the exciton-phonon coupling in II-VI semiconductor nanoshells. When the embedding medium is glass, where the dielectric constants at high frequency of core, shell and islanding materials are similar, we obtain two effects on the IR as well as the VIS optical properties: (i) the phonon peaks (IR range) or the exciton peak (VIS range) are red-shifted, and (ii) the peaks intensities are greater. Therefore, in the light of these results, it can be concluded that the phonons contribution is primordial if the optical properties are investigated in the low-dimensional systems.
  • Publication
    Simulations of optical reflectance in vertically aligned GaAs nanowires array: The effect of the geometrical structural parameters
    (Elsevier, 2022-08-15) Cruz Fernández, Rosa María de la; Kanyinda-Malu, C.; Muñoz Santiuste, Juan Enrique; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    We report the effects of radius-, length- and pitch-sizes on the optical reflectance of a periodic square array of GaAs nanowires embedded in epoxy. The simulated system is a multilayer array constituted by alternating layers of epoxy and an effective medium of GaAs nanowires embedded in epoxy. For both s- and p-polarizations, we observe an oscillating behavior in the reflectance spectra, as a consequence of interferences in periodical systems. We found that the radius- and pitch-sizes significantly affect the reflectance of GaAs nanowires array, while the length-sizes do not present evidence of changes in the optical reflectance. For higher radius, the number of oscillations increases and consequently, the peak-to-peak distance decreases. Besides, there is a red-shift of the reflectance for increasing radius. For higher pitch, the number of oscillations also increases, and a red-shift is observed. We obtain dependence laws for the peak-to-peak distance and red-shift versus radius and versus pitch. These dependences obey approximate quadratic relations. Attending to the reflectance dependence on the light incidence angle, we have found that for s-polarized light, the reflectance is higher with increasing angles, in comparison to p-polarized light cases, independently of the radius and pitch values. For both polarizations, we found that the reflectance is increasing for greater radii and smaller pitches, independently of the incident angle.
  • Publication
    Quantum simulation of multiphoton and nonlinear dissipative spin-boson models
    (APS, 2019-03) Puebla Antunes, Ricardo; Casanova, J.; Houhou, O.; Solano, E.; Paternostro, M.; European Commission; Ministerio de Economía y Competitividad (España)
    We present a framework for the realization of dissipative evolutions of spin-boson models, including multiphoton exchange dynamics, as well as nonlinear transition rates. Our approach is based on the implementation of a generalized version of a dissipative linear quantum Rabi model. The latter comprises a linearly coupled spin-boson term, spin rotations, and standard dissipators. We provide numerical simulations of illustrative cases supporting the good performance of our method. Our work allows for the simulation of a large class of fundamentally different quantum models where the effect of distinct dissipative processes can be easily investigated.
  • Publication
    Spatial dispersion in two-dimensional plasmonic crystals: Large blueshifts promoted by diffraction anomalies
    (American Physical Society, 2016-10-12) Christin, David; Christensen, Johan; Mortensen, N. Asger
    We develop a methodology to incorporate nonlocal optical response of the free electron gas due to quantum-interaction effects in metal components of periodic two-dimensional plasmonic crystals and study the impact of spatial dispersion on promising building blocks for photonic circuits. Within the framework of the hydrodynamic model, we observe significant changes with respect to the commonly employed local-response approximation, but also in comparison with homogeneous metal films where nonlocal effects have previously been considered. Notable are the emergence of a contribution from nonlocality at normal incidence and the surprisingly large structural parameters at which finite blueshifts are observable, which we attribute to diffraction that offers nonvanishing in-plane wave vector components and increases the penetration depth of longitudinal (nonlocal) modes.
  • Publication
    Obtaining statistics of cascading line outages spreading in an electric transmission network from standard utility data
    (IEEE, 2016-11-01) Dobson, Ian; Carreras Verdaguer, Benjamin A.; Newman, David E.; Reynolds Barredo, José Miguel
    We show how to use standard transmission line outage historical data to obtain the network topology in such a way that cascades of line outages can be easily located on the network. Then we obtain statistics quantifying how cascading outages typically spread on the network. Processing real outage data is fundamental for understanding cascading and for evaluating the validity of the many different models and simulations that have been proposed for cascading in power networks.
  • Publication
    A robust scheme for the implementation of the quantum Rabi model in trapped ions
    (IOP Publishing, 2016-11-23) Puebla Antunes, Ricardo; Casanova, Jorge; Plenio, Martin B.
    We show that the technique known as concatenated continuous dynamical decoupling (CCD) can be applied to a trapped-ion setup for a robust implementation of the quantum Rabi model in a variety of parameter regimes. These include the case where the Dirac equation emerges, and the limit in which a quantum phase transition takes place. We discuss the applicability of the CCD scheme in terms of the fidelity between different initial states evolving under an ideal quantum Rabi model and their corresponding trapped-ion realization, and demonstrate the effectiveness of noise suppression of our method.
  • Publication
    25 years of self-organized criticality: solar and astrophysics
    (Springer, 2016-01-01) Aschwanden, Markus J.; Crosby, Norma B.; Dimitropoulou, Michaila; Georgoulis, Manolis K.; Hergarten, Stefan; Mcateer, James; Milovanov, Alexander V.; Mineshige, Shin; Morales, Laura; Nishizuka, Naoto; Pruessner, Gunnar; Sánchez Fernández, Luis Raúl; Surja Sharma, A.; Strugarek, Antoine; Uritsky, Vadim
    Shortly after the seminal paper "Self-Organized Criticality: An explanation of 1/f noise" by Bak et al. (1987), the idea has been applied to solar physics, in "Avalanches and the Distribution of Solar Flares" by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.
  • Publication
    Connecting nth order generalised quantum Rabi models: Emergence of nonlinear spin-boson coupling via spin rotations
    (2018-09-28) Casanova, Jorge; Puebla Antunes, Ricardo; Moya Cessa, Hector; Plenio, Martin B.
    We establish an approximate equivalence between a generalised quantum Rabi model and its nth order counterparts, where spin-boson interactions are nonlinear as they comprise a simultaneous exchange of n bosonic excitations. Although there exists no unitary transformation between these models, we demonstrate their equivalence to a good approximation in a wide range of parameters. This shows that nonlinear spin-boson couplings, i.e., nth order quantum Rabi models, are accessible to quantum systems with only linear coupling between boson and spin modes by simply adding spin rotations and after an appropriate transformation. Furthermore, our result prompts novel approximate analytical solutions to the dynamics of the quantum Rabi model in the ultrastrong coupling regime improving previous approaches.
  • Publication
    Major results from the first plasma campaign of the Wendelstein 7-X stellarator
    (IOP Science, 2017-10) Wolf, R. C; Gogoleva, Alena; Vela Vela, Luis Ernesto; European Commission
    After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 x 1019 m-3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.
  • Publication
    Effects of a semi-infinite stratification on the Rayleigh-Taylor instability in an interface with surface tension
    (AIP Publishing, 2017-09) Andrea González, Ángel de; González Gutierrez, Leo M.
    The Rayleigh-Taylor instability (RTI) in an infinite slab where a constant density lower fluid is initially separated from an upper stratified fluid is discussed in linear regime. The upper fluid is of increasing exponential density and surface tension is considered between both of them. It was found useful to study stability by using the initial value problem approach (IVP), so that we ensure the inclusion of certain continuum modes, otherwise neglected. This methodology includes the branch cut in the complex plane, consequently, in addition to discrete modes (surface RTI modes), a set of continuum modes (internal RTI modes) also appears. As a result, the usual information given by the normal mode method is now complete. Furthermore, a new role is found for surface tension: to transform surface RTI modes (discrete spectrum) into internal RTI modes belonging to a continuous spectrum at a critical wavenumber. As a consequence, the cut-off wavenumber disappears: i.e. the growth rate of the RTI surface mode does not decay to zero at the cut-off wavenumber, as previous researchers used to believe. Finally, we found that, due to the continuum, the asymptotic behavior of the perturbation with respect to time is slower than the exponential when only the continuous spectrum exists.
  • Publication
    The impact of magnetic shear on the dynamics of a seeded 3D filament is slab geometry
    (Elsevier, 2017-08-01) Gracias, William Agnelo; Tamain, P.; Serre, E.; Pitts, R.A.; García Gonzalo, Luis; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid; European Commission
    Seeded filament simulations are used to study blob dynamics with the state-of-the-art TOKAM3X fluid code in the scrape-off layer (SOL) using a slab geometry. The filamentary dynamics recovered with the code are compared with previously predicted analytical blob velocity scalings while also studying the effect of field line pitch angle on these dynamics and are found to be similar. The effect of changing magnetic topology on filamentary motion is also investigated. Magnetic shear is introduced in the model by the sudden and localised variation of field line pitch angle for a narrow radially located region constituting effectively a shearing zone. Three such shear zones are tested to see how they affect filament motion. Filaments are initialised radially upstream from the shear zone and recorded as they convect towards the far-SOL side. The lowest intensity shear zone allows many of the higher amplitude filaments to pass through after dampening them. On the other hand, the highest intensity shear zones prevent all filaments from progressing to the wall beyond the shear zone and, in certain cases for high density amplitude filaments, is able to generate a new filament downstream from the shear zone.
  • Publication
    Extended computation of the viscous Rayleigh-Taylor instability in a horizontally confined flow
    (APS, 2021-05) Martinez-Carrascal, Jon; Calderon-Sanchez, J.; Gonzalez-Gutierrez, L. M.; Andrea González, Ángel de; European Commission; Ministerio de Ciencia, Innovación y Universidades (España)
    In this article, the classical Rayleigh-Taylor instability is extended to situations where the fluid is completely confined, in both the vertical and horizontal directions. This article starts with the two-dimensional (2D) viscous periodic case with finite height where the effect of adding surface tension to the interface is analyzed. This problem is simulated from a dual perspective: first, the linear stability analysis obtained when the Navier-Stokes equations are linearized and regularized in terms of density and viscosity; and second, looking at the weakly compressible version of a multiphase smoothed particle hydrodynamics (WCSPH) method. The evolution and growth rates of the different fluid variables during the linear regime of the SPH simulation are compared to the computation of the eigenvalues and eigenfunctions of the viscous version of the Rayleigh-Taylor stability (VRTI) analysis with and without surface tension. The most unstable mode, which has the maximal linear growth rate obtained with both approaches, as well as other less unstable modes with more complex structures are reported. The classical horizontally periodic (VRTI) case is now adapted to the case where two additional left and right walls are included in the problem, representing the cases where a two-phase flow is confined in a accelerated tank. This 2D case where no periodic assumptions are allowed is also solved using both techniques with tanks of different sizes and a wide range of Atwood numbers. The agreement with the linear stability analysis obtained by a Lagrangian method such as multiphase WCSPH is remarkable.
  • Publication
    Terahertz-induced oscillations in encapsulated graphene
    (Wiley, 2023-05) Iñarrea Las Heras, Jesús; Platero, Gloria; Comunidad de Madrid; Ministerio de Asuntos Económicos y Transformación Digital (España)
    A theoretical study on the rise of photo-oscillations in the magnetoresistance of hexagonal boron nitride (hBN)-encapsulated graphene is presented. The previous radiation-driven electron orbit model devised to study the same oscillations, well-known as MIRO, in 2D semiconductor systems (GaAs/AlGaAS heterostructure) is used. It is obtained that these graphene platforms under radiation and a static magnetic field are sensitive to terahertz and far-infrared radiation. The power, temperature, and frequency dependences of the photo-oscillations are studied. For power dependence, it is predicted that for cleaner graphene and high enough power it is possible to observe zero-resistance states and a resonance peak.
  • Publication
    Spin-boson model as a simulator of non-Markovian multiphoton Jaynes-Cummings models
    (MDPI, 2019-05) Puebla Antunes, Ricardo; Zicari, Giorgio; Arrazola, Iñigo; Solano, Enrique; Paternostro, Mauro; Casanova, Jorge; European Commission; Ministerio de Economía y Competitividad (España)
    The paradigmatic spin-boson model considers a spin degree of freedom interacting with an environment typically constituted by a continuum of bosonic modes. This ubiquitous model is of relevance in a number of physical systems where, in general, one has neither control over the bosonic modes, nor the ability to tune distinct interaction mechanisms. Despite this apparent lack of control, we present a suitable transformation that approximately maps the spin-boson dynamics into that of a tunable multiphoton Jaynes-Cummings model undergoing dissipation. Interestingly, the latter model describes the coherent interaction between a spin and a single bosonic mode via the simultaneous exchange of n bosons per spin excitation. Resorting to the so-called reaction coordinate method, we identify a relevant collective bosonic mode in the environment, which is then used to generate multiphoton interactions following the proposed theoretical framework. Moreover, we show that spin-boson models featuring structured environments can lead to non-Markovian multiphoton Jaynes-Cummings dynamics. We discuss the validity of the proposed method depending on the parameters and analyse its performance, which is supported by numerical simulations. In this manner, the spin-boson model serves as a good analogue quantum simulator for the inspection and realization of multiphoton Jaynes-Cummings models, as well as the interplay of non-Markovian effects and, thus, as a simulator of light-matter systems with tunable interaction mechanisms.
  • Publication
    Ultrafast critical ground state preparation via bang-bang protocols
    (IOP Science, 2020-09) Innocenti, Luca; Chiara, Gabriele de; Paternostro, Mauro; Puebla Antunes, Ricardo
    The fast and faithful preparation of the ground state of quantum systems is a challenging but crucial task for several applications in the realm of quantum-based technologies. Decoherence limits the maximum time-window allowed to an experiment to faithfully achieve such desired states. This is of particular significance in systems featuring a quantum phase transition, where the vanishing energy gap challenges an adiabatic ground state preparation. We show that a bang-bang protocol, consisting of a time evolution under two different values of an externally tunable parameter, allows for a high-fidelity ground state preparation in evolution times no longer than those required by the application of standard optimal control techniques, such as the chopped-random basis quantum optimization. In addition, owing to their reduced number of variables, such bang-bang protocols are very well suited to optimization tasks, reducing the high computational cost of other optimal control protocols. We benchmark the performance of such approach through two paradigmatic models, namely the Landau-Zener and the Lipkin-Meshkov-Glick model. Remarkably, we find that the critical ground state of the latter model, i.e. its ground state at the critical point, can be prepared with a high fidelity in a total evolution time that scales slower than the inverse of the vanishing energy gap.
  • Publication
    Kibble-Zurek scaling in quantum speed limits for shortcuts to adiabaticity
    (APS, 2020-07) Puebla Antunes, Ricardo; Deffner, Sebastian; Campbell, Steve
    Geometric quantum speed limits quantify the tradeoff between the rate at which quantum states can change and the resources that are expended during the evolution. Counterdiabatic driving is a unique tool from shortcuts to adiabaticity to speed up quantum dynamics while completely suppressing nonequilibrium excitations. We show that the quantum speed limit for counterdiabatically driven systems undergoing quantum phase transitions fully encodes the Kibble-Zurek mechanism by correctly predicting the transition from adiabatic to impulse regimes. Our findings are demonstrated for three scenarios, namely the transverse field Ising model, the Landau-Zener model, and the Lipkin-Meshkov-Glick model.
  • Publication
    Energetic cost of quantum control protocols
    (IOP, 2019-10-28) Abah, Obinna; Puebla Antunes, Ricardo; Kiely, Anthony; De Chiara, Gabriele; Paternostro, Mauro; Campbell, Steve
    Wequantitatively assess the energetic cost of several well-known control protocols that achieve a finite time adiabatic dynamics, namely counterdiabatic and local counterdiabatic driving, optimal control, and inverse engineering. By employing a cost measure based on the norm of the total driving Hamiltonian, we show that a hierarchy of costs emerges that is dependent on the protocol duration. As case studies we explore the Landau–Zener model, the quantum harmonic oscillator, and the Jaynes– Cummings model and establish that qualitatively similar results hold in all cases. For the analytically tractable Landau–Zener case, we further relate the effectiveness of a control protocol with the spectral features of the new driving Hamiltonians and show that in the case of counterdiabatic driving, it is possible to further minimize the cost by optimizing the ramp.
  • Publication
    High triplet energy host materials for blue TADF OLEDs—A tool box approach
    (Frontiers, 2020-07) Rodella, Francesco; Bagnich, Sergey; Duda, Eimantas; Meier, Tobias; Kahle, Julian; Athanasopoulos, Stavros; Köhler, Anna; Strohriegl, Peter; European Commission
    The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.