65 research outputs found
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
Theory of phonon-assisted luminescence in solids: Application to hexagonal boron nitride
International audienceIn this manuscript we study luminescence of hexagonal boron nitride (hBN) by means of non-equilibrium Green's functions plus time-dependent perturbation theory. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk hBN. This material has attracted interest due to its strong luminescence in the ultraviolet [Watanabe et al., Nature Mat. 3, 404(2004)]. The origin of this luminescence has been widely discussed, but only recently has a clear signature of phonon mediated light emission emerged in the experiments [Cassabois et al., Nature Phot. 10, 262(2016)]. By means of our new theoretical approach we provide a clear and full explanation of light emission in hBN
Exploring approximations to the GW self-energy ionic gradients
The accuracy of the many-body perturbation theory GW formalism to calculate
electron-phonon coupling matrix elements has been recently demonstrated in the
case of a few important systems. However, the related computational costs are
high and thus represent strong limitations to its widespread application. In
the present study, we explore two less demanding alternatives for the
calculation of electron-phonon coupling matrix elements on the many-body
perturbation theory level. Namely, we test the accuracy of the static
Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the
constant screening approach, where variations of the screened Coulomb potential
W upon small changes of the atomic positions along the vibrational eigenmodes
are neglected. We find this latter approximation to be the most reliable,
whereas the static COHSEX ansatz leads to substantial errors. Our conclusions
are validated in a few paradigmatic cases: diamond, graphene and the C60
fullerene. These findings open the way for combining the present many-body
perturbation approach with efficient linear-response theories
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
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