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Mini Simposio de Física




Este primer mini simposio lo organizó la FCNM - ESPOL con el fin de divulgar temas de actualidad científica en el área de física de materiales, con enfoques tanto experimental como teórico. Los expositores son científicos renombrados nacionales e internacionales, que nos presentarán sus resultados más recientes.

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Scanning the electromagnetic modes in layered vdW heterostructures
Vito Despoja, Ph. D.
Recently, the electromagnetic modes in atomically thick van der Waals layered heterostructures have shown large potential for their practical application in  plasmonics, photonics and optoelectronics. For example, by vertical stacking of various TMDs, hBN  and graphene the heterostructure of varous optical properties can be achieved which can act as a photo-sensor, photovoltaic or photo-emitter [1,2,3].
Here we shall first focus to present our recently proposed theoretical approach  designed to simulate the scanning of the electromagnetic modes in layered vdW heterostructures in (Q,omega) domene [4]. The results of our formalism is applied to study the diverse and tunable plasmon resonances in layered conductive vdW heterostructures. Particularly, the results for the plasmon resonances in hBN/gr multilayers will be presented. Then the occurence of the egzotic evenescent electromagnetic modes also named traped photon, which is result of binding between s(TE) photons and excitons in semiconducting vdW heterostructures will be demonstrated [5].  The photon confinement (or exciton-photon binding strength) is characterised by the photon bending parameter Г, the difference between the trapped photon ωt  and photon at the photon-exciton crossing point (Qc=ωex). We demonstrate that the binding Г can be manipulated by increasing the number of WS2 layers in the WS2/hBN composite so that ultra strong binding is achieved  (ωex-ωt)/ωex >10%.  Finally the result of our recent study of binding between the excitons in molecular (C60)  crystaline film and s(TE) cavity photons will be presented [6]. The binding strength is here characterised by the Rabi splitting  of the cavity photon  Ω  at the crossing with the exciton (ωex). The transition from weak to ultra-strong binding (Ω/ωex >10%) with the increase of the crystalline film thickness is achieved.
[1] F. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari, Nature Photon 4, 611–622 (2010)
[2] L. Britnell, et al,  Science 2 May 2013 340, 1311  (2013)
[3] J. Wang, X. Mu, M. Sun b, T. Mu, Applied Materials Today 16, 1-20  (2019)
[4] D. Novko, K. Lyon, D. J. Mowbray, and V. Despoja, Phys. Rev. B 104, 115421  (2021)
[5] S. A. Mikhailov and K. Ziegler Phys. Rev. Lett. 99, 016803 (2007)
[6] V. Despoja and D. Novko, Phys. Rev. B 106, 205401 (2022)
Tunable valley currents in aligned bilayer graphene/BN
Rebeca Ribeiro-Palau, Ph. D
The relative angular alignment between 2D layers of a van der Waals (vdW) heterostructure can dramatically alter its fundamental properties[1]. A striking example is the recent observation of strongly correlated states and intrinsic superconductivity in twisted bilayer graphene[2]. Another remarkable effect of angular layer alignment, predicted for certain vdW heterostructures, is the emergence of phases of matter with non-trivial topological properties, where charge carriers flow without dissipation, being protected against perturbations. In graphene aligned with boron nitride (BN), such a phase has been predicted, with topological protection linked not to the spin, as commonly observed, but rather to the valley degree of freedom.
The experimental observations of these topological valley currents [3] has been largely put in question by theorist, results of numerical simulation [4] and recent scanning SQUID results[5]. In these, the observed non-local signal have been attributed mostly to localized states on the edge of graphene. In this talk, we will show how these two pictures are not incompatible and can be re-conciliated if we take the angular layer alignment into account.
[1] Ribeiro-Palau et al., Science 361 (2018), 690
[2] Cao et al., Nature 556 (2018) 43.
[3] Gorbachev et al., Science 436 (2014), 448; Komatsu et al., Science Advances 4 (2018) eaaq0194
[4] J M Marmolejo-Tejada et al., J. Phys. Mater. 1 (2018) 015006
[5] A. Aharon-Steinberg et al., Nature 593 (2021), 528
Moiré semiconductors: 2D solids at the mesoscale
Francisco Mireles, Ph.D.
After almost two decades of the discovery of 2D materials, a new revolution for the physics of condensed matter is emerging: moiré materials. The weak van der Waals forces that mediate the interaction between different layers of 2D materials allow their stable stacking despite differences between their lattice constants and orientations. When both parameters are small, a moiré pattern is obtained: a super lattice whose periodicity can reach micrometric scales. In this talk I will discuss the effects that this superlattice has on charge carriers and other electronic excitations in these materials, with particular emphasis on semiconductors, such as transition metal dichalcogenides and phosphorene. I will describe how the moiré pattern scales the fundamental excitations of the Ångstrom system down to micrometers, producing analogs of strongly correlated solids and systems at the mesoscale1,2.
I.Soltero, J. Guerrero-Sánchez, F. Mireles, and D. A. Ruiz-Tijerina, Physcal Review B 105, 235421 (2022).
Acknowledgments:  This work was supported in part through DGAPA-UNAM, project PAPIIT No. IN113920.
“In-situ” Raman Spectroscopy as a novel tool to Understand Functionalization in Carbon Nanomaterials
Julio Chacon-Torres, Ph. D.
Over the past few years, Chemistry and Physics have merged together developing a novel method for studying supramolecular chemical, electrochemical and physicochemical surface interactions between carbon nanostructures and diverse functional groups and molecules. In-situ Raman spectroscopy has become the tool of choice to analyze these supramolecular interactions, doping, and functionalization, especially in carbon-based nanostructures. In our most recent works, we have disclosed a near-universal Raman response where charge transfer governs the electrochemical activation of carbon nanomaterials when exposed to potassium undergoing an electron doping (n-type doping) process.[1]–[3] A complete picture of how electron doping alters the Raman response in carbon nanostructures and the structure of the nanomaterial is now evident, as well as the immediate changes revealed after exposure to water, gasses, or pre-selected functionalities. We have found three main important results of interest for the Raman community: i) Electron doping/intercalation in some cases inhibits the Raman response and induces a shift that can be observed and traced in most carbon nanostructures (SWCNTs,[2] MWCTs,[4] carbon nano-onions,[3] graphite,[1] graphene,[5] and carbon nanoribbons[6]). ii) There exists a trend where the RBM vibrational mode down-shifts along the doping process in single-walled carbon nanotubes depending on their diameter and chirality.[2] iii) Functional groups can be identified by means of in-situ Raman spectroscopy. When they attach to the surface of the nanomaterial, additional D-bands appear becoming the fingerprint of functionalization without damaging the crystallinity of the nanostructure.[3], [7], [8] We have extended these studies to non-carbon nanostructures, revealing unprecedented and novel results that could drive a new metrology towards the implementation of Raman spectroscopy as a tool to trace controlled synthesis of functional nanomaterials for optoelectronics, batteries, and sensing devices.
[1]    J. C. Chacón-Torres, L. Wirtz, and T. Pichler, “Raman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron-phonon coupling in graphene layers,” Phys. Status Solidi B Basic Res., vol. 251, no. 12, pp. 2337–2355, Dec. 2014.
[2]    C. Kröckel et al., “Understanding the Electron-Doping Mechanism in Potassium-Intercalated Single-Walled Carbon Nanotubes,” J. Am. Chem. Soc., vol. 142, no. 5, pp. 2327–2337, Feb. 2020.
[3]    M. E. Pérez-Ojeda et al., “Carbon nano-onions: Potassium intercalation and reductive covalent functionalization,” J. Am. Chem. Soc., vol. 143, no. 45, pp. 18997–19007, Nov. 2021.
[4]    J. C. Chacón-Torres et al., “Potassium intercalated multiwalled carbon nanotubes,” Carbon N. Y., vol. 105, no. Supplement C, pp. 90–95, Aug. 2016.
[5]    R. Podila, J. Chacón-Torres, J. T. Spear, T. Pichler, P. Ayala, and A. M. Rao, “Spectroscopic investigation of nitrogen doped graphene,” Appl. Phys. Lett., vol. 101, no. 12, p. 123108, Sep. 2012.
[6]    J. Jakovac, L. Marušić, D. Andrade-Guevara, J. C. Chacón-Torres, and V. Despoja, “Infra-Red Active Dirac Plasmon Serie in Potassium Doped-Graphene (KC8) Nanoribbons Array on Al2O3 Substrate,” Materials , vol. 14, no. 15, Jul. 2021, doi: 10.3390/ma14154256.
[7]    G. Abellán et al., “Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals,” Angew. Chem. Int. Ed Engl., Oct. 2017, doi: 10.1002/anie.201707462.
[8]    J. C. Chacón-Torres, L. Wirtz, and T. Pichler, “Manifestation of charged and strained graphene layers in the Raman response of graphite intercalation compounds,” ACS Nano, vol. 7, no. 10, pp. 9249–9259, Oct. 2013.
Electric dipole moment in the Su-Schrieffer-Heeger (SSH) model
Leonardo Basile, Ph. D.
The macroscopic polarization is the most essential concept in any phenomenological description of dielectric materials but calculating this and other electric multipole moments in crystalline insulators is not an obvious process due to the quantum mechanical behavior of the electron cloud and the periodicity of the lattice.  In this talk, the electric dipole moment in a crystalline insulator will be calculated. For this purpose, I will use the tight-binding SSH (Su-Schrieffe-Heeger) model, that represents a one-dimensional crystalline insulator. To compute the electric dipole moment, a quantum mechanical operator of the position in one dimension can be defined. It will be shown that this model presents a quantized electric dipole moment that generates charge accumulations at the ends of the crystal.