MLMichele Lazzeri

Michele Lazzeri

Institut de minéralogie, de physique des matériaux et de cosmochimie
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Michele Lazzeri has a total of 361 co-authors. Here are the top co-authors by number of shared publications:

Latest Publications

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    Vibrational properties of epitaxial graphene buffer layer on silicon carbide

    by Guillaume Radtke, Michele Lazzeri

    Cited by 1
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    Phonon Dispersion and Proton Disorder of Ice VII and VIII

    by G. Radtke, S. Klotz, M. Lazzeri, P. Loubeyre, M. Krisch, A. Bossak

    The phonon dispersion of ice VII and that of its proton-ordered analog ice VIII are investigated through a combination of inelastic x-ray scattering (IXS) measurements and first-principles calculations of the oxygen sublattice dynamic structure factor. Particular attention is devoted to hydrogen-disorder in ice VII, addressed theoretically through a statistical ensemble of fictitious ordered supercell configurations. Similar phonon densities of states are found in both phases but are significantly less structured in the case of ice VII. Our data further show that, despite a full proton disorder, the acoustic phonon branches in this phase clearly inherit the periodicity of its body-centered cubic oxygen lattice. The calculations predict, however, the presence of gap openings in the one-atom phonon dispersion. These predictions are supported by revisiting the analysis of previous single-crystal IXS measurements along the longitudinal [111] branch of ice VII.

    Cited by 3
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    Magnetoelastic interactions in SrCu2(BO3)2 studied by Raman scattering experiments and first principles calculations

    by K. Thirunavukkuarasu, G. Radtke, Z. Lu, M. Lazzeri, P. C. M. Christianen, M. V. Ballottin, H. A. Dabkowska, B. D. Gaulin, D. Smirnov, M. Jaime, et al.

    Dynamic and static crystal lattice properties of SrCu2(BO3)2 are studied by means of Raman scattering, magnetostriction, and thermal expansion measurements in magnetic fields to 45 T. Raman experiments versus temperature reveal that some phonon modes show an unusual behavior: their frequencies soften (modes at 200 and 450 cm−1) while others harden (modes at 385 and 478 cm−1) when decreasing the temperature below 15 K. Magneto-Raman experiments show that their field dependence correlates with their respective temperature dependencies; e.g., modes that are hardened with increasing temperature also harden with applied magnetic fields and modes that become softer with temperature also soften with applied fields. We use density functional theory to successfully model and compute the energies of these modes, classifying them into two types: pantograph (modes that soften when decreasing the temperature) and nonpantograph. We conclude that the former involves the modification of the intradimer exchange interaction J and the latter the interdimer J′. Finally, dilatometry is used to correlate field-dependent Raman modes to the closing of the spin gap as well as fractional-magnetization stripe states M=1/4 Ms and M=1/3 Ms, where Ms is the saturation magnetization.

    Cited by 2
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    Anisotropic Thermal Conductivity of Inkjet-Printed 2D Crystal Films: Role of the Microstructure and Interfaces

    by Mizanur Rahman, Khaled Parvez, Giorgia Fugallo, Chaochao Dun, Oliver Read, Adriana Alieva, Jeffrey J. Urban, Michele Lazzeri, Cinzia Casiraghi, Simone Pisana

    Two-dimensional (2D) materials are uniquely suited for highly anisotropic thermal transport, which is important in thermoelectrics, thermal barrier coatings, and heat spreaders. Solution-processed 2D materials are attractive for simple, low-cost, and large-scale fabrication of devices on, virtually, any substrate. However, to date, there are only few reports with contrasting results on the thermal conductivity of graphene films, while thermal transport has been hardly measured for other types of solution-processed 2D material films. In this work, inkjet-printed graphene, h-BN and MoS2 films are demonstrated with thermal conductivities of ∼10 Wm−1K−1 and ∼0.3 Wm−1K−1 along and across the basal plane, respectively, giving rise to an anisotropy of ∼30, hardly dependent on the material type and annealing treatment. First-principles calculations indicate that portion of the phonon spectrum is cut-off by the quality of the thermal contact for transport along the plane, yet the ultra-low conductivity across the plane is associated with high-transmissivity interfaces. These findings can drive the design of highly anisotropic 2D material films for heat management applications.

    Cited by 8
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    Ultrafast nonlinear phonon response of few-layer hexagonal boron nitride

    by Taehee Kang, Jia Zhang, Achintya Kundu, Klaus Reimann, Michael Woerner, Thomas Elsaesser, Bernard Gil, Guillaume Cassabois, Christos Flytzanis, Giorgia Fugallo, et al.

    Nonequilibrium dynamics of transverse-optical (TO) phonons and low-frequency interlayer shear and breathing modes are mapped in femtosecond midinfrared pump-probe experiments. Time-resolved changes of TO phonon absorption reveal a TO phonon lifetime of 1.2 ps, while low-frequency excitations decay with a time constant of 22 ps. The coupling of intralayer TO and interlayer motions manifests in a quasi-instantaneous redshift of the TO phonon resonance by some 10 cm−1. Theoretical calculations account for the coupling scenario and underline the relevance of interphonon interactions for the nonlinear phonon response.

    Cited by 11
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    Flat Bands and Giant Light-Matter Interaction in Hexagonal Boron Nitride

    by C. Elias, G. Fugallo, P. Valvin, C. L’hEnoret, J. Li, J. H. Edgar, F. Sottile, M. Lazzeri, A. Ouerghi, B. Gil, et al.

    Dispersionless energy bands in k space are a peculiar property gathering increasing attention for the emergence of novel electronic, magnetic, and photonic properties. Here, we explore the impact of electronic flat bands on the light-matter interaction. The van der Waals interaction between the atomic layers of hexagonal boron nitride induces flat bands along specific lines of the Brillouin zone. The macroscopic degeneracy along these lines leads to van Hove singularities with divergent joint density of states, resulting in outstanding optical properties of the excitonic states. For the direct exciton, we report a giant oscillator strength with a longitudinal-transverse splitting of 420 meV, a record value, confirmed by our ab initio calculations. For the fundamental indirect exciton, flat bands result in phonon-assisted processes of exceptional efficiency, that compete with direct absorption in reflectivity, and that make the internal quantum efficiency close to values typical of direct band gap semiconductors.

    Cited by 75
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    Fermi resonance in the Raman spectrum of graphene

    by Dipankar Kalita, Michele Amato, Alexandre Artaud, Laëtitia Marty, Vincent Bouchiat, Johann Coraux, Christian Brouder, Michele Lazzeri, Nedjma Bendiab

    We report the observation of an intense anomalous peak at 1608 cm−1 in the Raman spectrum of graphene associated with the presence of chromium nanoparticles in contact with graphene. Bombardment with an electron beam demonstrates that this peak is distinct from the well-studied D′ peak appearing as defects are created in graphene; this peak is found to be nondispersive. We argue that the bonding of chromium atoms with carbon atoms softens the out-of-plane optical (ZO) phonon mode, in such a way that the frequency of its overtone decreases to 2ωZO∼ωG, where ωG=1585 cm−1 is the frequency of the Raman-active E2g mode. Thus, the observed peak is attributed to the 2ZO mode which becomes Raman active following a mechanism known as Fermi resonance. First-principles calculations of the vibrational and anharmonic properties of the graphene/Cr interface support this scenario.

    Cited by 9
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    Modeling Vibrational EELS: From Bulk to Point Defects

    by Guillaume Radtke, Michele Lazzeri, Dario Taverna, Nicolas Menguy, Fredrik Hage, Demie Kepaptsoglou, Quentin Ramasse, Ondrej Krivanek, Tracy Lovejoy

    Cited by 1
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    Infrared reflectance, transmittance, and emittance spectra of MgO from first principles

    by Giorgia Fugallo, Benoit Rousseau, Michele Lazzeri

    By using density functional theory calculations we determined the influence of anharmonic effects on the infrared reflectance, transmittance, and emittance of MgO. The goal is to determine the limit of validity of a perturbative (multiphonon) approach. MgO is chosen as a test material because of the availability of different kinds of radiative properties measured experimentally. Nonanalytic terms of the three-phonon scattering coefficients are explicitly calculated and do not provide measurable effects. The agreement is overall very good to such an extent that, already at room temperature, one can clearly identify regions in which four-phonon scattering processes are dominant with respect to the three-phonon ones. The influence of isotopic disorder at cryogenic temperatures is also settled.

    Cited by 35
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    Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype

    by Silvia Pandolfi, Carlos Renero-Lecuna, Yann Le Godec, Benoit Baptiste, Nicolas Menguy, Michele Lazzeri, Christel Gervais, Kristina Spektor, Wilson A. Crichton, Oleksandr O. Kurakevych

    Hexagonal Si allotropes are expected to enhance light absorption in the visible range as compared to common cubic Si with diamond structure. Therefore, synthesis of these materials is crucial for the development of Si-based optoelectronics. In this work we combine in- situ high-pressure high-temperature synthesis and vacuum heating to obtain hexagonal Si. High pressure is one of the most promising routes to stabilize these allotropes. It allows to obtain large-volume nanostructured ingots by a sequence of direct solid-solid transformations, ensuring high-purity samples for detailed characterization. Thanks to our synthetic approach,wwe provide the first evidence of a polycrystalline bulk sample of hexagonal Si. Exhaustivestructural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking), in contrast with Si-2H (AB stacking) proposed previously. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab-initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region, for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.

    Cited by 62
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