114 research outputs found
Light--absorbed orbital angular momentum in the linear response regime
In exploring the light-induced dynamics within the linear response regime,
this study investigates the induced orbital angular momentum on a wide variety
of electronic structures. We derive a general expression for the torque induced
by light on different electronic systems based on their characteristic
dielectric tensor. We demonstrate that this phenomenon diverges from the
inverse Faraday effect as it produces an orbital magnetization persistent
post-illumination. Indeed, our results reveal that, while isotropic
non-dissipative materials do not absorb orbital angular momentum from
circularly polarized light, any symmetry-breaking arrangement of matter, be it
spatial or temporal, introduces novel channels for the absorption of orbital
angular momentum, or magnetization. Most notably, in dissipative materials,
circularly polarized light imparts a torque corresponding to a change in
orbital angular momentum of per absorbed photon. The potential of these
mechanisms to drive helicity-dependent magnetic phenomena paves the way for a
deeper understanding of light-matter interactions. Notably, the application of
pump-probe techniques in tandem with our findings allows experimentalists to
quantitatively assess the amount of orbital angular momentum transferred to
electrons in matter, thus hopefully enhancing our ability to steer ultrafast
light-induced magnetization dynamics
Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
We performed a systematic density functional study of the adsorption of
copper, silver, and gold adatoms on graphene, especially accounting for van der
Waals interactions by the vdW-DF and the PBE+D2 methods. In particular, we
analyze the preferred adsorption site (among top, bridge, and hollow positions)
together with the corresponding distortion of the graphene sheet and identify
diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to
the graphene sheet and that in some cases the buckling of the graphene can be
significant. The results for silver are at variance with those obtained with
GGA, which gives no binding in this case. However, we observe some quantitative
differences between the vdW-DF and the PBE+D2 methods. For instance the
adsorption energies calculated with the PBE+D2 method are systematically higher
than the ones obtained with vdW-DF. Moreover, the equilibrium distances
computed with PBE+D2 are shorter than those calculated with the vdW-DF method
Hybrid localized graph kernel for machine learning energy-related properties of molecules and solids
Nowadays, the coupling of electronic structure and machine learning
techniques serves as a powerful tool to predict chemical and physical
properties of a broad range of systems. With the aim of improving the accuracy
of predictions, a large number of representations for molecules and solids for
machine learning applications has been developed. In this work we propose a
novel descriptor based on the notion of molecular graph. While graphs are
largely employed in classification problems in cheminformatics or
bioinformatics, they are not often used in regression problem, especially of
energy-related properties. Our method is based on a local decomposition of
atomic environments and on the hybridization of two kernel functions: a graph
kernel contribution that describes the chemical pattern and a Coulomb label
contribution that 1encodes finer details of the local geometry. The accuracy of
this new kernel method in energy predictions of molecular and condensed phase
systems is demonstrated by considering the popular QM7 and BA10 datasets. These
examples show that the hybrid localized graph kernel outperforms traditional
approaches such as, for example, the smooth overlap of atomic positions (SOAP)
and the Coulomb matrices
Electronic Structure and Band Alignments of Various Phases of Titania Using the Self-Consistent Hybrid Density Functional and DFT+U Methods
To understand, and thereby rationally optimize photoactive interfaces, it is of great importance to elucidate the electronic structures and band alignments of these interfaces. For the first-principles investigation of these properties, conventional density functional theory (DFT) requires a solution to mitigate its well-known bandgap underestimation problem. Hybrid functional and Hubbard U correction are computationally efficient methods to overcome this limitation, however, the results are largely dependent on the choice of parameters. In this study, we employed recently developed self-consistent approaches, which enable non-empirical determination of the parameters, to investigate TiO2 interfacial systems—the most prototypical photocatalytic systems. We investigated the structural, electronic, and optical properties of rutile and anatase phases of TiO2. We found that the self-consistent hybrid functional method predicts the most reliable structural and electronic properties that are comparable to the experimental and high-level GW results. Using the validated self-consistent hybrid functional method, we further investigated the band edge positions between rutile and anatase surfaces in a vacuum and electrolyte medium, by coupling it with the Poisson-Boltzmann theory. This suggests the possibility of a transition from the straddling-type to the staggered-type band alignment between rutile and anatase phases in the electrolyte medium, manifested by the formation of a Stern-like layer at the interfaces. Our study not only confirms the efficacy of the self-consistent hybrid functional method by reliably predicting the electronic structure of photoactive interfaces, but also elucidates a potentially dramatic change in the band edge positions of TiO2 in aqueous electrolyte medium which can extensively affect its photophysical properties
Assessing the Performance of Recent Density Functionals for Bulk Solids
We assess the performance of recent density functionals for the
exchange-correlation energy of a nonmolecular solid, by applying accurate
calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid
metals and non-metals. The functionals tested are the modified
Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the
second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For
completeness, we also test more-standard functionals: the local density
approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria
(TPSS) meta-GGA. We find that the recent density functionals for solids reach a
high accuracy for bulk properties (lattice constant and bulk modulus). For the
cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is
actually better for the alkali metals and alkali halides. For fair comparison
of calculated and experimental results, we consider the zero-point phonon and
finite-temperature effects ignored by many workers. We show how Gaussian basis
sets and inaccurate experimental reference data may affect the rating of the
quality of the functionals. The results show that PBEsol and AM05 perform
somewhat differently from each other for alkali metal, alkaline earth metal and
alkali halide crystals (where the maximum value of the reduced density gradient
is about 2), but perform very similarly for most of the other solids (where it
is often about 1). Our explanation for this is consistent with the importance
of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil
Advances in Density-Functional Calculations for Materials Modeling
During the past two decades, density-functional (DF) theory has evolved from niche applications for simple solid-state materials to become a workhorse method for studying a wide range of phenomena in a variety of system classes throughout physics, chemistry, biology, and materials science. Here, we review the recent advances in DF calculations for materials modeling, giving a classification of modern DF-based methods when viewed from the materials modeling perspective. While progress has been very substantial, many challenges remain on the way to achieving consensus on a set of universally applicable DF-based methods for materials modeling. Hence, we focus on recent successes and remaining challenges in DF calculations for modeling hard solids, molecular and biological matter, low-dimensional materials, and hybrid organic-inorganic materials
Crystal and electronic structures of nitridophosphate compounds as cathode materials for Na-ion batteries
International audienceUsing density-functional theory, we have studied the electronic and magnetic properties of two promising compounds that can be used as cathode materials, namely, Na2Fe2P3O9N and Na-3 TiP3O9N. When Na is extracted, we found the volume change to be quite small, with values of similar to-0.6% for Na3TiP3O9N and -5% for Na2Fe2P3O9N. Our calculated voltageswith theHubbard-type correction (GGA+U) approximation are 2.93 V for Na3TiP3O9N/Na2TiP3O9N and 2.68 V for Na2Fe2P3O9N/NaFe2P3O9N, in good agreement with the experimental data. Our results confirm that these compounds are very promising for rechargeable Na-ion batteries
Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids
International audienceWe have performed ab initio calculations for a series of energetic solids to explore their structural and electronic properties. To evaluate the ground state Volume of these molecular solids, different dispersion correct on methods were accounted in DFT, namely the Tkatchenko-Scheffler method (With and without self-consistent screening), Grimme's methods D2, D3(BJ)), and the vdW-DF method. Our results reveal that dispersion correction methods are essential it understanding these complex structures with van der Waals interactions and hydrogen bonding. The calculated ground state volumes and bulk moduli show that the performance of each method is not unique, and therefore a careful examination is mandatory for interpreting theoretical predictions. This work also emphasizes the importance of quasiparticle calculations in predicting the band gap, which is obtained here with the GW approximation. We find that the obtained band gaps are ranging froth 4 to 7 eV for the different compounds, indicating their insulating nature. In addition, we show the essential role of quasiparticle band structure calculations to correlate the gap with the energetic properties
- …