Peer-reviewed Publications |
Bergeron, H., Curado, E. M. F., Gazeau, J. P., & Rodrigues, L. M. C. S. (2016). Extensivity of Rényi entropy for the Laplace–de Finetti distribution. Physica A: Statistical Mechanics and its Applications, 441, 23–31.
Résumé: The Boltzmann Gibbs entropy is known to be asymptotically extensive for the Laplace de Finetti distribution. We prove here that the same result holds in the case of the Renyi entropy. We also show some interesting lower and upper bounds for the asymptotic limit of these entropies. (C) 2015 Elsevier B.V. All rights reserved.
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Bergeron, H., Curado, E. M. F., Gazeau, J. P., & Rodrigues, L. M. C. S. (2016). Symmetric deformed binomial distributions: An analytical example where the Boltzmann-Gibbs entropy is not extensive. Journal of Mathematical Physics, 57(2), 023301.
Résumé: Asymptotic behavior (with respect to the number of trials) of symmetric generalizations of binomial distributions and their related entropies is studied through three examples. The first one has the q-exponential as the generating function, the second one involves the modified Abel polynomials, and the third one has Hermite polynomials. We prove analytically that the Renyi entropy is extensive for these three cases, i.e., it is proportional (asymptotically) to the number n of events and that q-exponential and Hermite cases have also extensive Boltzmann-Gibbs. The Abel case is exceptional in the sense that its Boltzmann-Gibbs entropy is not extensive and behaves asymptotically as the square root of n. This result is obtained numerically and also confirmed analytically, under reasonable assumptions, by using a regularization of the beta function and its derivative. Probabilistic urn and genetic models are presented for illustrating this remarkable case. (C) 2016 AIP Publishing LLC.
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Bergeron, H., Czuchry, E., Gazeau, J. - P., & Malkiewicz, P. (2016). Nonadiabatic bounce and an inflationary phase in the quantum mixmaster universe. Physical Review D, 93(12), 043521.
Résumé: Following our previous paper, Bergeron et al., Smooth quantum dynamics of the mixmaster universe, Phys. Rev. D 92, 061302(R) (2015), concerning the quantization of the vacuum Bianchi IX model and the Born-Huang-Oppenheimer framework, we present a further analysis of the dynamical properties of the model. Consistently with the deep quantum regime, we implement the harmonic approximation of the anisotropy potential. We thus obtain manageable dynamical equations. We study the quantum anisotropic oscillations during the bouncing phase of the universe. Neglecting the backreaction from transitions between quantum anisotropy states, we obtain analytical results. In particular, we identify a parameter that is associated with dynamical properties of the quantum model and describes a sort of phase transition. Once the parameter exceeds its critical value, the Born-Huang-Oppenheimer approximation breaks down. The application of the present result to a simple model of the universe indicates that the parameter indeed exceeds its critical value and that there takes place a huge production of anisotropy at the bounce. This in turn must lead to a sustained phase of accelerated expansion, an inflationary phase. The quantitative inclusion of backreaction shall be examined in a follow-up paper based on the vibronic approach.
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Bergeron, H., Czuchry, E., Gazeau, J. - P., & Malkiewicz, P. (2016). Vibronic framework for quantum mixmaster universe. Physical Review D, 93(6), 064080.
Résumé: Following our previous papers concerning the quantization of the vacuum Bianchi-IX model within or beyond the Born-Oppenheimer and adiabatic approximation, we develop a more elaborate analysis of the dynamical properties of the model based on the vibronic approach utilized in molecular physics. As in the previous papers, we restrict our approach to the harmonic approximation of the anisotropy potential in order to obtain resoluble analytical expressions.
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Cazaux, S., Boschman, L., Rougeau, N., Reitsma, G., Hoekstra, R., Teillet-Billy, D., Morisset, S., Spaans, M., & Schlatholter, T. (2016). The sequence to hydrogenate coronene cations: A journey guided by magic numbers. Sci Rep, 6, 19835.
Résumé: The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2.
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Choi, D. - J., Robles, R., Gauyacq, J. - P., Rubio-Verdu, C., Lorente, N., & Ignacio Pascual, J. (2016). Spin-polarised edge states in atomic Mn chains supported on Cu2N/Cu (100). J. Phys. Condens. Matter., 28(23), 23lt01.
Résumé: Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu2N/Cu (100) reveal an unsuspected electronic edge state at [Formula: see text] eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at [Formula: see text] -1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences.
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Choi, D. - J., Robles, R., Gauyacq, J. - P., Ternes, M., Loth, S., & Lorente, N. (2016). Structural and magnetic properties of FeMnx chains (x=1–6) supported on Cu2N/Cu (100). Phys. Rev. B, 94(8), 085406.
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Gauyacq, J. P., & Lorente, N. (2016). Extremely long-lived magnetic excitations in supported Fe chains. Phys. Rev. B, 94(4), 045420.
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Gruber, E., Wilhelm, R. A., Petuya, R., Smejkal, V., Kozubek, R., Hierzenberger, A., Bayer, B. C., Aldazabal, I., Kazansky, A. K., Libisch, F., Krasheninnikov, A. V., Schleberger, M., Facsko, S., Borisov, A. G., Arnau, A., & Aumayr, F. (2016). Ultrafast electronic response of graphene to a strong and localized electric field. Nat. Commun., 7, 13948.
Résumé: The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 10(12) A cm(-2), the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.
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Le Moal, E., Marguet, S., Canneson, D., Rogez, B., Boer-Duchemin, E., Dujardin, G., Teperik, T. V., Marinica, D. C., & Borisov, A. G. (2016). Engineering the emission of light from a scanning tunneling microscope using the plasmonic modes of a nanoparticle. Phys. Rev. B, 93(3), 035418.
Résumé: The inelastic tunnel current in the junction formed between the tip of a scanning tunneling microscope (STM) and the sample can electrically generate optical signals. This phenomenon is potentially of great importance for nano-optoelectronic devices. In practice, however, the properties of the emitted light are difficult to control because of the strong influence of the STM tip. In this work, we show both theoretically and experimentally that the sought-after, well-controlled emission of light from an STM tunnel junction may be achieved using a nonplasmonic STM tip and a plasmonic nanoparticle on a transparent substrate. We demonstrate that the native plasmon modes of the nanoparticle may be used to engineer the light emitted in the substrate. Both the angular distribution and intensity of the emitted light may be varied in a predictable way by choosing the excitation position of the STM tip on the particle.
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Marinica, D. - C., Aizpurua, J., & Borisov, A. G. (2016). Quantum effects in the plasmon response of bimetallic core-shell nanostructures. Opt. Express, 24(21), 23941–23956.
Résumé: We report a quantum mechanical study of the plasmonic response of bimetallic spherical core/shell nanoparticles. The systems comprise up to 10<sup>4</sup> electrons and their optical response is addressed with Time Dependent Density Functional Theory calculations. These quantum results are compared with classical electromagnetic calculations for core/shell systems formed by Al/Na, Al/Au and Ag/Na, as representative examples of bimetallic systems. We show that for shell widths in the nanometer range, the system cannot be described as a simple stack of two metals. The finite size effect and the transition layer formed between the core and the shell strongly modify the optical properties of the compound nanoparticle. In particular this configuration leads to a frequency shift of the plasmon resonance with shell character and an increased plasmon decay into electron-hole pairs which eventually quenches this resonance for very thin shells. This effect is difficult to capture with a classical theory even upon adjustment of the parameters of a combination of metallic dielectric functions.
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Teperik, T. V., Kazansky, A. K., & Borisov, A. G. (2016). Electron tunneling through water layer in nanogaps probed by plasmon resonances. Phys. Rev. B, 93(15), 155431.
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Zapata Herrera, M., Aizpurua, J., Kazansky, A. K., & Borisov, A. G. (2016). Plasmon Response and Electron Dynamics in Charged Metallic Nanoparticles. Langmuir, 32(11), 2829–2840.
Résumé: Using the time-dependent density functional theory, we perform quantum calculations of the electron dynamics in small charged metallic nanoparticles (clusters) of spherical geometry. We show that the excess charge is accumulated at the surface of the nanoparticle within a narrow layer given by the typical screening distance of the electronic system. As a consequence, for nanoparticles in vacuum, the dipolar plasmon mode displays only a small frequency shift upon charging. We obtain a blue shift for positively charged clusters and a red shift for negatively charged clusters, consistent with the change of the electron spill-out from the nanoparticle boundaries. For negatively charged clusters, the Fermi level is eventually promoted above the vacuum level leading to the decay of the excess charge via resonant electron transfer into the continuum. We show that, depending on the charge, the process of electron loss can be very fast, on the femtosecond time scale. Our results are of great relevance to correctly interpret the optical response of the nanoparticles obtained in electrochemistry, and demonstrate that the measured shift of the plasmon resonances upon charging of nanoparticles cannot be explained without account for the surface chemistry and the dielectric environment.
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Zhu, W., Esteban, R., Borisov, A. G., Baumberg, J. J., Nordlander, P., Lezec, H. J., Aizpurua, J., & Crozier, K. B. (2016). Quantum mechanical effects in plasmonic structures with subnanometre gaps. Nat. Commun., 7, 11495.
Résumé: Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions.
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