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See detailAb initio approaches to Resonant Raman Spectroscopy of Transition Metal Dichalcogenides
Pereira Coutada Miranda, Henrique UL

Doctoral thesis (2017)

This thesis deals with the theory and simulation of resonant Raman spectroscopy in two-dimensional transition metal dichalcogenides. We present two different ab initio approaches. In the first, we ... [more ▼]

This thesis deals with the theory and simulation of resonant Raman spectroscopy in two-dimensional transition metal dichalcogenides. We present two different ab initio approaches. In the first, we calculate the Raman susceptibility tensor as a function of laser energy from the change of the dielectric susceptibility with atomic displacements. In the second, we formulate the Raman tensor in terms of time-dependent perturbation theory and calculate it using electron-light and electron-phonon coupling matrix elements obtained from density functional theory. We investigate the role of resonance, quantum interference and excitonic effects in the Raman spectra of single and triple-layer MoTe2. We compare our simulations with experimental results, explaining the dependence of the Raman intensities on the excitation energy. We demonstrate that the two approaches are formally and numerically equivalent in the adiabatic limit. In addition, the second approach allows to include the dynamical effects and captures a shift of the intensities with respect to the adiabatic case. This method is also more computationally efficient and is extended to include temperature effects using many-body perturbation theory. We have implemented both of these methods in a software package with interfaces to open source ab initio codes. Furthermore, we have developed web based tools to visualize excitonic states and phonon modes. [less ▲]

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See detailQuantum Interference Effects in Resonant Raman Spectroscopy of Single- and Triple-Layer MoTe2 from First-Principles
Pereira Coutada Miranda, Henrique UL; Reichardt, Sven UL; Froehlicher, Guillaume et al

in Nano Letters (2017), 17(4), 2381--2388

We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe2. Raman intensities are computed entirely from first-principles by calculating ... [more ▼]

We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe2. Raman intensities are computed entirely from first-principles by calculating finite differences of the dielectric susceptibility. In our analysis, we investigate the role of quantum interference effects and the electron−phonon coupling. With this method, we explain the experimentally observed intensity inversion of the A′1 vibrational modes in triple-layer MoTe2 with increasing laser photon energy. Finally, we show that a quantitative comparison with experimental data requires the proper inclusion of excitonic effects. [less ▲]

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See detailExcitons in boron nitride single layer
Galvani, Thomas; Paleari, Fulvio UL; Pereira Coutada Miranda, Henrique UL et al

in Physical Review. B : Condensed Matter (2016), 94(125303),

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed ... [more ▼]

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π) and conduction (π∗) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme. [less ▲]

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See detailElectronic and Vibrational proprieties of graphene on Ir(111) and SiC(100)
Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Poster (2015, September)

In the last years, graphene has become one of the most studied materials due to its peculiar electronic, optical, thermal, and mechanical properties. It is thus of major importance, for practical ... [more ▼]

In the last years, graphene has become one of the most studied materials due to its peculiar electronic, optical, thermal, and mechanical properties. It is thus of major importance, for practical applications, to study how the electronic and vibrational proprieties of graphene change when deposited on a substrate. The non-commensurability of the unit cell of graphene with the substrate leads to the formation of Moiré patterns with accordingly large supercell sizes. Ab-initio calculations using standard plane-wave based codes on these large systems are of high computational cost even for the ground-state calculations. We show the effect that such Moiré patterns have on the band structure by projecting the resulting electronic structure and phonon dispersion onto the unit cell of free-standing graphene with an unfolding scheme. We compare our results with HREELS measurements of the phonon dispersion of graphene on Ir(111). The accurate knowledge of the interaction graphene-substrate will provide important information for future applications of graphene on electronic devices. Work performed in collaboration with the experimental groups of J. Kroeger (TU Ilmenau, Germany) and T. Seyller (TU Chemnitz, Germany). [less ▲]

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See detailPhonon-limited carrier mobility and resistivity from carbon nanotubes to graphene
Li, Jing; Pereira Coutada Miranda, Henrique UL; Niquet, Yann-Michel et al

in Physical Review. B: Condensed Matter and Materials Physics (2015)

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon ... [more ▼]

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon-limited electrical mobility in graphene and in a wide range of CNTs of different types to address this issue. The theoretical study is based on a tight-binding method and a force-constant model from which all possible electron-phonon couplings are computed. The electrical resistivity of graphene is found in very good agreement with experiments performed at high carrier density. A common methodology is applied to study the transition from one to two dimensions by considering CNTs with diameter up to 16 nm. It is found that the mobility in CNTs of increasing diameter converges to the same value, i.e., the mobility in graphene. This convergence is much faster at high temperature and high carrier density. For small-diameter CNTs, the mobility depends strongly on chirality, diameter, and the existence of a band gap. [less ▲]

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See detailA force-constant model of graphene for conductivity calculations
Pereira Coutada Miranda, Henrique UL; Wirtz, Ludger UL

Poster (2015, January)

Transport in graphene is strongly limited by the electron-phonon interaction. Accurate description of the phonon dispersion relations is essential for the study of this interaction. Using current state-of ... [more ▼]

Transport in graphene is strongly limited by the electron-phonon interaction. Accurate description of the phonon dispersion relations is essential for the study of this interaction. Using current state-of-the-art ab initio density-functional theory plane-wave codes, we are limited to systems with few atoms. For larger systems (e.g., nanotubes, nanoribbons), accurate semi-empircal models are needed. We have developed a force constant model for the phonon dispersion of graphene. Our implementation can include a large number of neighbours, which allows us to simulate accurately long-range interaction effects. As shown in previous publications it is possible to reproduce the phonon dispersion frequencies of graphene with a 4th nearest neighbours force constant model. However, some features can only be captured using long-range interactions (Kohn-anomalies, certain phonon eigenvectors). Using an ab initio phonon dispersion calculated with DFPT as reference, we show the nature of the long-range interactions and explore different ways to include them in our semi-empirical model. We also study the dependence of the force constants on charge and strain. Work in collaboration with Jing Li, Yann-Michel Niquet, Luigi Genovese, and Ivan Duchemin from L_Sim, SP2M, UMR-E CEA/UJF-Grenoble 1, INAC, Grenoble, France and Christophe Delerue from IEMN - Dept. ISEN, UMR CNRS 8520, Lille, France [less ▲]

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See detailMoiré-induced replica of graphene phonons on Ir(111)
Endlich, Michael; Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL et al

in Annalen der Physik (2014), 526(9-10), 372-380

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure ... [more ▼]

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure. Calculations for a linear chain of C atoms attached to an infinitely heavy substrate reveal that imposing a superstructure by periodically varying the C-C interaction and the C-substrate coupling induces replicated phonons at wave vectors reflecting the supercell periodicity. Deviations between the phonon dispersion of graphene on Ir(111) and of pristine graphene are analyzed and rationalized in terms of the weak graphene-Ir(111) interaction. [less ▲]

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See detailSemi-empirical phonon calculations for graphene on different substrates
Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Poster (2014, April 02)

We investigate the graphene-substrate interaction via changes in the phonon dispersion of graphene. Ab-initio calculations on these systems are of high computational cost due to the non-commensurability ... [more ▼]

We investigate the graphene-substrate interaction via changes in the phonon dispersion of graphene. Ab-initio calculations on these systems are of high computational cost due to the non-commensurability of the unit cells of graphene and the substrate. This leads to the formation of Moiré patterns with accordingly large supercell sizes. We use a semi-empirical force constant model for the calculation of phonons of graphene on different metallic and insulating substrates. The interaction of graphene with the substrate is described via suitably chosen spring constants. The phonon dispersion in the primitive unit cell of graphene is obtained via an “unfolding procedure” similar to the ones used for the discussion of ARPES (angular resolved photo-emission spectroscopy) of graphene on incommensurate substrates. [less ▲]

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