| 1. |
- Algaba, Juan-Carlos, et al.
(författare)
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Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
- 2021
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 911:1
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Forskningsöversikt (refereegranskat)abstract
- In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M o˙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
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| 2. |
- Ribeiro Jr, Luiz Antonio, et al.
(författare)
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Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons
- 2018
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The dynamical properties of polarons in armchair graphene nanoribbons (GNR) is numerically investigated in the framework of a two-dimensional tight-binding model that considers spin-orbit (SO) coupling and electron-lattice (e-l) interactions. Within this physical picture, novel polaron properties with no counterparts to results obtained from conventional tight-binding models are obtained. Our findings show that, depending on the systems width, the presence of SO coupling changes the polarons charge localization giving rise to different degrees of stability for the charge carrier. For instance, the joint action of SO coupling and e-l interactions could promote a slight increase on the charge concentration in the center of the lattice deformation associated to the polaron. As a straightforward consequence, this process of increasing stability would lead to a depreciation in the polarons motion by decreasing its saturation velocity. Our finds are in good agreement with recent experimental investigations for the charge localization in GNR, mostly when it comes to the influence of SO coupling. Moreover, the contributions reported here provide a reliable method for future works to evaluate spin-orbit influence on the performance of graphene nanoribbons.
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| 3. |
- Silva, Gesiel Gomes, et al.
(författare)
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Bipolaron Dynamics in Graphene Nanoribbons
- 2019
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Graphene nanoribbons (GNRs) are two-dimensional structures with a rich variety of electronic properties that derive from their semiconducting band gaps. In these materials, charge transport can occur via a hopping process mediated by carriers formed by self-interacting states between the excess charge and local lattice deformations. Here, we use a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs. Our results show that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs. Remarkably, the bipolaron dynamics can occur in acoustic and optical regimes concerning its saturation velocity. The phase transition between these two regimes takes place for a critical field strength in which the bipolaron moves roughly with the speed of sound in the material.
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| 4. |
- da Cunha, Wiliam Ferreira, et al.
(författare)
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Reactive Scattering between Excitons and Charge Carriers in Conjugated Polymers
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:41, s. 23451-23458
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Tidskriftsartikel (refereegranskat)abstract
- The temperature influence on the scattering process between an exciton and a positively charged carrier (polaron or bipolaron) is theoretically investigated using a version of the SuSchriefferHeeger (SSH) model modified to include temperature, Coulomb interactions, and an external electric field. In general, it is observed that the products of the reactive scattering are spin independent when thermal effects are taken into account. For the interaction between a polaron and an exciton, the polaron can be annihilated, when subjected to temperatures higher than a critical value, or pass through the exciton maintaining both their consistencies, if a lower temperature regime is considered. Regarding the recombination between a bipolaron and an exciton, it is observed that the bipolaron can be annihilated whereas the exciton dissociates into two trions or into one polaron and one trion. In all cases, the recombination mechanisms depend on a suitable balance between temperature and electric field. These results may extend the knowledge about electroluminescence process in conjugated polymers, thus being of potential use to improve internal quantum efficiency in polymer light-emitting diodes.
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| 5. |
- Ferreira da Cunha, Wiliam, et al.
(författare)
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Impurity effects on polaron dynamics in graphene nanoribbons
- 2015
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 91, s. 171-177
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Tidskriftsartikel (refereegranskat)abstract
- The impurity effects on the dynamics of polarons in armchair graphene nanoribbons are numerically investigated in the scope of a two-dimensional tight-binding approach with lattice relaxation. The results show that the presence of an impurity changes significantly the net charge distribution associated to the polaron structure. Moreover, the interplay between external electric field and the local impurities plays the role of drastically modifying the polaron dynamics. Interestingly, nanoribbons containing mobile polarons are noted to take place even when considering high impurity levels, which is associated with the highly conductive character of the graphene nanoribbons. This investigation may enlighten the understanding of the charge transport mechanism in carbon-based nanomaterials. (C) 2015 Elsevier Ltd. All rights reserved.
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| 6. |
- Ferreira Monteiro, Fabio, et al.
(författare)
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Encapsulated beta-carotene in ZnO nanotubes: Theoretical insight into the stabilization dynamics
- 2015
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Ingår i: Chemical Physics Letters. - : ELSEVIER SCIENCE BV. - 0009-2614 .- 1873-4448. ; 636, s. 62-66
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Tidskriftsartikel (refereegranskat)abstract
- The stabilization dynamics of a molecular dye (beta-carotene) encapsulated in single-wall zinc oxide nanotubes (ZnONTs) is theoretically investigated in the scope of molecular dynamics and density functional theory simulations. Our findings show that the beta-carotene encapsulation in ZnONTs is an energetically favorable process. Once encapsulated, this molecular dye remains close to the ZnONT wall, in accordance with recent experimental reports. Interestingly, the interaction between the beta-carotene and ZnONT can form a charge delocalization state, where an amount of charge concentrated on beta-carotene is transferred to ZnONT. (C) 2015 Elsevier B.V. All rights reserved.
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| 7. |
- Passos Abreu, Ana Virginia, et al.
(författare)
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Impact of the Electron-Phonon Interactions on the Polaron Dynamics in Graphene Nanoribbons
- 2016
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Ingår i: Journal of Physical Chemistry A. - : AMER CHEMICAL SOC. - 1089-5639 .- 1520-5215. ; 120:27, s. 4901-4906
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the electron-phonon (e-ph) interactions on the filed-included polaron dynamics in armchair graphene nanoribbons (GNRs) is theoretically investigated in the scope of a two-dimensional tight-binding model. The results show that the localization of the polaronic charge increases when the strength of e-ph coupling also increases. Consequently, the polaron saturation velocity decreases for higher e-ph coupling strengths. Interestingly, the interplay between the e-ph coupling strength and the GNR width changes substantially the polaron dynamics properties.
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| 8. |
- Ribeiro, Luiz Antonio, et al.
(författare)
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Bloch oscillations in organic and inorganic polymers
- 2017
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 146:14
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Tidskriftsartikel (refereegranskat)abstract
- The transport of polarons above the mobility threshold in organic and inorganic polymers is theoretically investigated in the framework of a one-dimensional tight-binding model that includes lattice relaxation. The computational approach is based on parameters for which the model Hamiltonian suitably describes different polymer lattices in the presence of external electric fields. Our findings show that, above critical field strengths, a dissociated polaron moves through the polymer lattice as a free electron performing Bloch oscillations. These critical electric fields are considerably smaller for inorganic lattices in comparison to organic polymers. Interestingly, for inorganic lattices, the free electron propagates preserving charge and spin densities localization which is a characteristic of a static polaron. Moreover, in the turning points of the spatial Bloch oscillations, transient polaron levels are formed inside the band gap, thus generating a fully characterized polaron structure. For the organic case, on the other hand, no polaron signature is observed: neither in the shape of the distortion-those polaron profile signatures are absent-nor in the energy levels-as no such polaron levels are formed during the simulation. These results solve controversial aspects concerning Bloch oscillations recently reported in the literature and may enlighten the understanding about the charge transport mechanism in polymers above their mobility edge. Published by AIP Publishing.
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| 9. |
- Ribeiro, Luiz Antonio, et al.
(författare)
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Concentration effects on intrachain polaron recombination in conjugated polymers
- 2015
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 17:2, s. 1299-1308
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Tidskriftsartikel (refereegranskat)abstract
- The influence of different charge carrier concentrations on the recombination dynamics between oppositely charged polarons is numerically investigated using a modified version of the Su-Schrieffer-Heeger (SSH) model that includes an external electric field and electron-electron interactions. Our findings show that the external electric field can play the role of avoiding the formation of excited states (polaron-exciton and neutral excitation) leading the system to a dimerized lattice. Interestingly, depending on a suitable balance between the polaron concentration and the electric field strength, the recombination mechanism can form stable polaron-excitons or neutral excitations. These results may provide guidance to improve the electroluminescence efficiency in Polymer Light Emitting Diodes.
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| 10. |
- Ribeiro, Luiz Antonio, et al.
(författare)
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Dynamic Formation of Bipolaron-Exciton Complexes in Conducting Polymers
- 2018
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Ingår i: Journal of Physical Chemistry A. - : AMER CHEMICAL SOC. - 1089-5639 .- 1520-5215. ; 122:15, s. 3866-3872
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Tidskriftsartikel (refereegranskat)abstract
- The recombination dynamics of two oppositely charged bipolarons within a single polymer chain is numerically studied in the scope of a one-dimensional tight-binding model that considers electron electron and electron-phonon (e-ph) interactions. By scanning among values of e-ph coupling and electric field, novel channels for the bipolaron recombination were yielded based on the interplay between these two parameters. The findings point to the formation of a compound species formed from the coupling between a bipolaron and an exciton. Depending on the electric field and e-ph coupling strengths, the recombination mechanism may yield two distinct products: a trapped (and almost neutral) or a moving (and partially charged) bipolaron-exciton. These results might enlighten the understanding of the electroluminescence processes in organic light-emitting devices.
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| 11. |
- Ribeiro, Luiz Antonio, et al.
(författare)
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Temperature effects on intrachain recombination of bipolarons in conjugated polymers
- 2014
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Ingår i: Chemical Physics Letters. - : Elsevier. - 0009-2614 .- 1873-4448. ; 614, s. 151-155
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Tidskriftsartikel (refereegranskat)abstract
- The temperature influence over the intrachain recombination dynamics between oppositely charged bipolarons has been theoretically investigated using a version of the Su-Schrieffer-Heeger model. The results shown that, below of a critical temperature regime, these charge carriers can recombine into a biexciton. This excited state is a light emissive specie that can emit one photon and turn into an exciton state. This specie can thereafter decay to the ground state by other photon emission. This knowledge can enlighten the understanding needed to improve the internal quantum efficiency of electroluminescence in Polymer Light Emitting Diodes.
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| 12. |
- Ribeiro, Luiz Antonio, et al.
(författare)
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Transport of Polarons in Graphene Nanoribbons
- 2015
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society. - 1948-7185. ; 6:3, s. 510-514
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Tidskriftsartikel (refereegranskat)abstract
- The field-induced dynamics of polarons in armchair graphene nanoribbons (GNRs) is theoretically investigated in the framework of a two-dimensional tight-binding model with lattice relaxation. Our findings show that the semiconductor behavior, fundamental to polaron transport to take place, depends upon of a suitable balance between the GNR width and the electronphonon (eph) coupling strength. In a similar way, we found that the parameter space for which the polaron is dynamically stable is limited to an even narrower region of the GNR width and the eph coupling strength. Interestingly, the interplay between the external electric field and the eph coupling plays the role to define a phase transition from subsonic to supersonic velocities for polarons in GNRs.
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| 13. |
- Silva, Gesiel Gomes, et al.
(författare)
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Influence of quasi-particle density over polaron mobility in armchair graphene nanoribbons
- 2018
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 20:24, s. 16712-16718
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Tidskriftsartikel (refereegranskat)abstract
- An important aspect concerning the performance of armchair graphene nanoribbons (AGNRs) as materials for conceiving electronic devices is related to the mobility of charge carriers in these systems. When several polarons are considered in the system, a quasi-particle wave function can be affected by that of its neighbor provided the two are close enough. As the overlap may affect the transport of the carrier, the question concerning how the density of polarons affect its mobility arises. In this work, we investigate such dependence for semiconducting AGNRs in the scope of nonadiabatic molecular dynamics. Our results unambiguously show an impact of the density on both the stability and average velocity of the quasi-particles. We have found a phase transition between regimes where increasing density stops inhibiting and starts promoting mobility; densities higher than 7 polarons per 45 angstrom present increasing mean velocity with increasing density. We have also established three different regions relating electric field and average velocity. For the lowest electric field regime, surpassing the aforementioned threshold results in overcoming the 0.3 angstrom fs(-1) limit, thus representing a transition between subsonic and supersonic regimes. For the highest of the electric fields, density effects alone are responsible for a stunning difference of 1.5 angstrom fs(-1) in the mean carrier velocity.
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