How strong is the Second Harmonic Generation in single-layer monochalcogenides? A response from first-principles real-time simulations

Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am. Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and multilayer forms. To clarify the origin of this strong SHG signal, we perform first-principles real-time simulations of linear and non-linear optical properties of these two-dimensional semiconducting materials. The simulations, based on ab-initio many-body theory, accurately treat the electron-hole correlation and capture excitonic effects that are deemed important to correctly predict the optical properties of such systems. We find indeed that, as observed for other 2D systems, the SHG intensity is redistributed at excitonic resonances. The obtained theoretical SHG intensity is an order of magnitude smaller than that reported at the experimental level. This result is in substantial agreement with previously published simulations which neglected the electron-hole correlation, demonstrating that many-body interactions are not at the origin of the strong SHG measured. We then show that the experimental data can be reconciled with the theoretical prediction when a single layer model, rather than a bulk one, is used to extract the SHG coefficient from the experimental data.Comment: 8 pages, 4 figure

Effect of the quantistic zero-point atomic motion on the opto-electronic properties of diamond and trans-polyacetylene

The quantistic zero-point motion of the carbon atoms is shown to induce strong effects on the opto-electronic properties of diamond and trans-polyacetylene, a conjugated polymer. By using an ab initio approach, we interpret the sub-gap states experimentally observed in diamond in terms of entangled electron-phonon states. These states also appear in trans-polyacetylene causing the formation of strong structures in the band-structure that even call into question the accuracy of the band theory. This imposes a critical revision of the results obtained for carbon-based nano-structures by assuming the atoms frozen in their equilibrium positions

Many-body perturbation theory calculations using the yambo code

International audienceyambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as quantum-espresso and abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools

Zero point motion effect on the electronic properties of diamond, trans-polyacetylene and polyethylene

El pdf del artículo es el manuscrito de autor.It has been recently shown, using ab-initio methods, that bulk diamond is characterized by a large band-gap renormalization (∼0.6 eV) induced by the electron-phonon interaction. In this work we show that in polymers, compared to bulk materials, the larger amplitude of the atomic vibrations makes the real excitations of the system be composed by entangled electron-phonon states. We prove that these states carry only a fraction of the electronic charge, thus leading, inevitably, to the failure of the electronic picture. The present results cast doubts on the accuracy of purely electronic calculations. They also lead to a critical revision of the state-of-the-art description of carbon-based nanostructures, opening a wealth of potential implications.Peer Reviewe

Neutral electronic excitations: A many-body approach to the optical absorption spectra

Trabajo presentado al 9th ETSF Young Researchers' Meeting celebrado en Bruselas (Bélgica) del 21 al 25 de Mayo de 2012.Peer reviewe

Thermal evolution of silicon carbide electronic bands

6 pages, 3 figures + SupplementaryInternational audienceDirect observation of temperature dependence of individual bands of semiconductors for a wide temperature region is not straightforward, in particular, for bands farther from the Fermi-level. However, this fundamental property is a prerequisite in understanding the electron-phonon coupling of semiconductors. Here we apply \emph{ab initio} many body perturbation theory to the electron-phonon coupling on hexagonal silicon carbide (SiC) crystals and determine the temperature dependence of the bands. We find a significant electron-phonon renormalization of the band gap at 0~K. Both the conduction and valence bands shift at elevated temperatures exhibiting a different behavior. We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation

Pressure Dependence of Electronic, Vibrational and Optical Properties of wurtzite-Boron Nitride

Wurtzite Boron Nitride ($w$BN) is a wide band gap BN polymorph with peculiar mechanical properties (hardness and stiffness). After its first synthesis in 1963 as a transformation of hexagonal BN ($h$BN) under high temperature and pressure conditions, a lot of progress have been made in order to stabilize wurtzite phase at atmospheric pressure. Today the crystallization of good quality samples is finally possible. This fact motivates our first principles study of the electronic, vibrational and light absorption and emission properties of $w$BN over a wide range of pressures. Our findings are important in view of the potential use of $w$BN as a dielectric for integration in BN-based technologies in optoelectronics and harsh environment applications

Influence of anisotropy, tilt and pairing of Weyl nodes: the Weyl semimetals TaAs, TaP, NbAs and NbP

International audienceAbstract By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character. Graphical abstrac