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SUMMARY Numerical experiments of seismic wave propagation in a laterally homogeneous layered medium explore subsurface imaging at subwavelength distances for dense seismic arrays. We choose a time-reversal approach to simulate fundamental mode Rayleigh surface wavefields that are equivalent to the cross-correlation results of three-component ambient seismic field records. We demonstrate that the synthesized 2-D spatial autocorrelation fields in the time domain support local or so-called focal spot imaging. Systematic tests involving clean isotropic surface wavefields but also interfering body wave components and anisotropic incidence assess the accuracy of the phase velocity and dispersion estimates obtained from focal spot properties. The results suggest that data collected within half a wavelength around the origin is usually sufficient to constrain the used Bessel functions models. Generally, the cleaner the surface wavefield the smaller the fitting distances that can be used to accurately estimate the local Rayleigh wave speed. Using models based on isotropic surface wave propagation we find that phase velocity estimates from vertical–radial component data are less biased by P-wave energy compared to estimates obtained from vertical–vertical component data, that even strong anisotropic surface wave incidence yields phase velocity estimates with an accuracy of 1 per cent or better, and that dispersion can be studied in the presence of noise. Estimates using a model to resolve potential medium anisotropy are significantly biased by anisotropic surface wave incidence. The overall accurate results obtained from near-field measurements using isotropic medium assumptions imply that dense array seismic Rayleigh wave focal spot imaging can increase the depth sensitivity compared to ambient noise surface wave tomography. The analogy to elastography focal spot medical imaging implies that a high station density and clean surface wavefields support subwavelength resolution of lateral medium variations.

We review one of the most versatile theoretical approaches to the study of time-dependent correlated quantum transport in nano-systems: the non-equilibrium Green's function (NEGF) formalism. Within this formalism, one can treat, on the same footing, inter-particle interactions, external drives and/or perturbations, and coupling to baths with a (piece-wise) continuum set of degrees of freedom. After a historical overview on the theory of transport in quantum systems, we present a modern introduction of the NEGF approach to quantum transport. We discuss the inclusion of inter-particle interactions using diagrammatic techniques, and the use of the so-called embedding and inbedding techniques which take the bath couplings into account non-perturbatively. In various limits, such as the non-interacting limit and the steady-state limit, we then show how the NEGF formalism elegantly reduces to well-known formulae in quantum transport as special cases. We then discuss non-equilibrium transport in general, for both particle and energy currents. Under the presence of a time-dependent drive - encompassing pump-probe scenarios as well as driven quantum systems - we discuss the transient as well as asymptotic behavior, and also how to use NEGF to infer information on the out-of-equilibrium system. As illustrative examples, we consider model systems general enough to pave the way to realistic systems. These examples encompass one- and two-dimensional electronic systems, systems with electron-phonon couplings, topological superconductors, and optically responsive molecular junctions where electron-photon couplings are relevant., Comment: Topical review, 97 pages, 15 figures

A prerequisite to characterize magnetic materials is the capability to describe systems containing unpaired electrons. In this study, we benchmark the one-shot GW (G0W0) on top of different unrestricted mean-field solutions for open-shell molecules using Dunning's correlation-consistent basis sets expanded in terms of Gaussian functions. We find that the G0W0 correction to hybrid functionals provides reasonably accurate results for the ionization energies of open-shell systems when compared to those obtained from high-level ab initio methods. Moreover, the quality of the G0W0 exchange–correlation approximation is evaluated by the discrepancy between the ionization energy of the neutral molecules and the electron affinity of the corresponding cations. Furthermore, we assess the capability of the GW to reproduce the correct energy ordering of molecular spin–orbitals. To such an aim, we thoroughly discuss three open-shell molecules CN, NH2, and O2, for which approximate functionals fail to correctly capture the single-electron spectrum. Particularly, we demonstrate that the overestimation of the exchange energy in the studied spin–orbitals is reduced by the GW dynamic correlation term, restoring the molecular orbital ordering. Interestingly, we find that deviations of the exchange and correlation energies, in comparison with our ab initio reference, can be very different for molecular orbitals with different symmetry, e.g. σ and π-type orbitals., Authors acknowledge support from Spanish Agencia Estatal de Investigación (Grant Nos. PID2019-107338RB-C66, PID2019-109555GB-I00 and RTC-2016-5681-7), and the Eusko Jaurlaritza and UPV/EHU (Grant Nos. PIBA19-0004 and IT1246-19). DSP also acknowledges support from the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant No. 863098).

Mars is the first extraterrestrial planet with seismometers (Seismic Experiment for Interior Structure, SEIS) deployed directly on its surface in the framework of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. The lack of strong Marsquakes, however, strengthens the need of seismic noise studies to additionally constrain the Martian structure. Seismic noise autocorrelations of single-station recordings permit the determination of the zero-offset reflection response underneath SEIS. We present a new autocorrelation study which employs state-of-the-art approaches to determine a robust reflection response by avoiding bias from aseismic signals which are recorded together with seismic waves due to unfavorable deployment and environmental conditions. Data selection and segmentation is performed in a data-adaptive manner which takes the data root-mean-square amplitude variability into account. We further use the amplitude-unbiased phase cross-correlation and work in the 1.2-8.9 Hz frequency band. The main target are crustal scale reflections, their robustness and convergence. The strongest signal appears at 10.6 s, and, if interpreted as a P-wave reflection, would correspond to a discontinuity at about 21 km depth. This signal is a likely candidate for a reflection from the base of the Martian crust due to its strength, polarity, and stability. Additionally we identify, among the stable signals, a signal at about 6.15 s that can be interpreted as the P-wave reflection from the mid-crust at about 9.5 km depth., This is the InSight contribution number 144 and IPGP contribution number 4215. French authors are supported by ANR MAGIS (ANR-19-CE31-0008-08) and by CNES SEIS/InSight Phase E2. MS thanks SANIMS (RTI2018-095594-B-I00), Generalitat de Catalunya (2017SGR1022), and invitations for research stays at IPGP.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

Seismic interferometry by ambient noise autocorrelations is a special case of Green's function retrieval for single-station analysis. Although high-frequency noise autocorrelations are now used to extract the reflectivity beneath seismic stations, low-frequency autocorrelations are hardly applied. Here, we present the observation of the Earth orbiting surface waves from low-frequency noise autocorrelations which are used to extract normal-mode frequencies for the Hum. The performances of the classical and phase autocorrelations are analyzed using seismic data from GEOSCOPE station TAM in Algeria. Both approaches are independent and perform differently for data with large amplitude variability. We show that the phase autocorrelation can robustly extract Rayleigh waves and normal modes because it is not biased by large amplitude signals (e.g., earthquakes). This is convenient because no data preprocessing (data selection or amplitude clipping) is required as usually employed for the classical approaches. This implies that the phase correlation takes advantage of the full data set and waveform information to achieve a high signal extraction convergence. Single-station phase autocorrelations may become an important tool in planetary seismology where data are limited due to the expensive and difficult data acquisition and can consist of high-amplitude variability due to unknown conditions. The upcoming INSIGHT (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) Mars mission plans the deployment of one broadband seismometer and the successful measurement of normal-mode frequencies and surface-wave dispersion curves will constrain its reference structure. Although we present low-frequency autocorrelations, our findings remain valid for cross correlations, other applications, and other frequency bands., This work was supported by the projects CGL2013-48601-C2-1-R and ANR-14-CE01-0012.

The possibility that an unconventional depletion in the center of the charge density distribution of certain nuclei occurs due to a purely quantum mechanical effect has attracted theoretical and experimental attention in recent years. We report on ab initio self-consistent Green's function calculations of one of such candidates, $^{34}$Si, together with its Z+2 neighbour $^{36}$S. Binding energies, rms radii and density distributions of the two nuclei as well as low-lying spectroscopy of $^{35}$Si, $^{37}$S, $^{33}$Al and $^{35}$P are discussed. The interpretation of one-nucleon removal and addition spectra in terms of the evolution of the underlying shell structure is also provided. The study is repeated using several chiral effective field theory Hamiltonians as a way to test the robustness of the results with respect to input inter-nucleon interactions. The prediction regarding the (non-)existence of the bubble structure in $^{34}$Si varies significantly with the nuclear Hamiltonian used. However, demanding that the experimental charge density distribution and the root mean square radius of $^{36}$S are well reproduced, along with $^{34}$Si and $^{36}$S binding energies, only leaves the NNLO$_{\text{sat}}$ Hamiltonian as a serious candidate to perform this prediction. In this context, a bubble structure, whose fingerprint should be visible in an electron scattering experiment of $^{34}$Si, is predicted. Furthermore, a clear correlation is established between the occurrence of the bubble structure and the weakening of the 1/2$^-$-3/2$^-$ splitting in the spectrum of $^{35}$Si as compared to $^{37}$S., 19 pages, 21 figures

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyse in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the {\it ab initio} solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials., Accepted for Phys. Rev. B

We use, for the first time, ab initio coupled-cluster theory to compute the spectral function of the uniform electron gas at a Wigner-Seitz radius of $r_\mathrm{s}=4$. The coupled-cluster approximations we employ go significantly beyond the diagrammatic content of state-of-the-art $GW$ theory. We compare our calculations extensively to $GW$ and $GW$-plus-cumulant theory, illustrating the strengths and weaknesses of these methods in capturing the quasiparticle and satellite features of the electron gas. Our accurate calculations further allow us to address the long-standing debate over the occupied bandwidth of metallic sodium. Our findings indicate that the future application of coupled-cluster theory to condensed phase material spectra is highly promising., Comment: 6 pages, 2 figures

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., We report first-principles GW results on the dispersion of the bulk band-gap edges in the three-dimensional topological insulator Sb2Te3. We find that, independently of the reference density-functional-theory band structure and the crystal-lattice parameters used, the one-shot GW corrections enlarge the fundamental band gap, bringing its value in close agreement with experiment. We conclude that the GW corrections cause the displacement of the valence-band maximum (VBM) to the Γ point, ensuring that the surface-state Dirac point lies above the VBM. We extend our study to the analysis of the electron-energy-loss spectrum (EELS) of bulk Sb2Te3. In particular, we perform energy-filtered transmission electron microscopy and reflection EELS measurements. We show that the random-phase approximation with the GW quasiparticle energies and taking into account virtual excitations from the semicore states leads to good agreement with our experimental data., We acknowledge funding from the University of Basque Country UPV/EHU (IT-756-13), the Departamento de Educacion del Gobierno Vasco, the Tomsk State University Competitiveness Improvement Program, the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), the Science Development Foundation under the President of the Republic of Azerbaijan [Grant No. EIF-2011-1(3)-82/69/4-M-50], and Saint Petersburg State University (Project No. 11.50.202.2015). The energy-filtered TEM experiment was performed at Beyond-Nano (PON a3-00363). The EELS experiment was performed at UNIMORE, and acknowledges partial support from PRIN GRAF 20105ZZTSE-008. The work was also supported by the Alexander von Humboldt Foundation through a postdoctoral fellowship, and by the Helmholtz Association through the Virtual Institute for Topological Insulators (VITI).

The nonresonant tunneling regime for charge transfer across nanojunctions is critically dependent on the so-called $\ensuremath{\beta}$ parameter, governing the exponential decay of the current as the length of the junction increases. For periodic materials, this parameter can be theoretically evaluated by computing the complex band structure (CBS)---or evanescent states---of the material forming the tunneling junction. In this work we present the calculation of the CBS for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the $\mathit{GW}$ approximation. We compare the description of localization and $\ensuremath{\beta}$ parameters among the adopted methods and with experimental data. We show that local and semilocal density functionals systematically underestimate the $\ensuremath{\beta}$ parameter, while hybrid-exchange schemes partially correct for this discrepancy, resulting in a much better agreement with $\mathit{GW}$ calculations and experiments. Self-consistency effects and self-energy representation issues of the $\mathit{GW}$ corrections are discussed together with the use of Wannier functions to interpolate the electronic band structure.

During the past decades, much research has been done towards the efficient calculation of impedance integrals in the Method of Moments. However, these results were almost always uniquely concerned with penetrable media. In this contribution, it will be shown how the integrals can be treated in highly conductive media as well. The rapid exponential decline of the Green's function, due to the losses, is the root of all additional complexities. The method as presented here takes care of these problems in a scalable way, i.e. the computation time becomes independent of the conductivity of the material. It is not meant as a replacement for techniques in penetrable media, due to some additional costs, but is - to our knowledge - the only approach that currently exists to efficiently handle conductive media. This paper presents the ideas and techniques in a fairly condensed manner. More information can be found in [1].

We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a ${\text{SiO}}_{2}$ crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We find that the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.

It is commonly accepted that the GW approximation for the electron self-energy is successful for the description of the band structure of weakly to moderately correlated systems, whereas it will fail for strongly correlated materials. In the present work, we discuss two important aspects of this approximation: first, the "self-screening error," which is due to an incorrect treatment of induced exchange, and second, the atomic limit, in which, instead, correlation is directly responsible for the observed problem. Using the example of the removal of a particle from a box, we show that the self-screening error stems from the use of test charge-test charge screening and that it can be corrected by a two-point vertex contribution to the self-energy derived from time-dependent density functional theory (TDDFT). We explain why the addition of a particle, instead, requires the use of a different approximate vertex. This illustrates why the general vertex function, valid both for valence and conduction states, must be a three-point function. Moreover, we show that also the bad performance of GW in the atomic limit is due to the neglect of the vertex in the self-energy; in that case, the TDDFT-derived vertex correction is not sufficient in order to remove the error even qualitatively. We discuss the effects of the self-screening error as well as the atomic limit using GW for the exactly solvable two-site Hubbard model.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We present a modification of the Δ self-consistent field (ΔSCF) method of calculating energies of excited states in order to make it applicable to resonance calculations of molecules adsorbed on metal surfaces, where the molecular orbitals are highly hybridized. The ΔSCF approximation is a density-functional method closely resembling standard density-functional theory (DFT), the only difference being that in ΔSCF one or more electrons are placed in higher lying Kohn-Sham orbitals instead of placing all electrons in the lowest possible orbitals as one does when calculating the ground-state energy within standard DFT. We extend the ΔSCF method by allowing excited electrons to occupy orbitals which are linear combinations of Kohn-Sham orbitals. With this extra freedom it is possible to place charge locally on adsorbed molecules in the calculations, such that resonance energies can be estimated, which is not possible in traditional ΔSCF because of very delocalized Kohn-Sham orbitals. The method is applied to N2, CO, and NO adsorbed on different metallic surfaces and compared to ordinary ΔSCF without our modification, spatially constrained DFT, and inverse-photoemission spectroscopy measurements. This comparison shows that the modified ΔSCF method gives results in close agreement with experiment, significantly closer than the comparable methods. For N2 adsorbed on ruthenium (0001) we map out a two-dimensional part of the potential energy surfaces in the ground state and the 2π resonance. From this we conclude that an electron hitting the resonance can induce molecular motion, optimally with 1.5 eV transferred to atomic movement. Finally we present some performance test of the ΔSCF approach on gas-phase N2 and CO in order to compare the results to higher accuracy methods. Here we find that excitation energies are approximated with accuracy close to that of time-dependent density-functional theory. Especially we see very good agreement in the minimum shift of the potential energy surfaces in the excited state compared to the ground state., The Center for Individual Nanoparticle Functionality (CINF) is sponsored by the Danish National Research Foundation. This work was supported by the Danish Center for Scientific Computing.

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometry optimization and the many-body effects induced by the creation of an electron-hole pair have been calculated within a first-principles framework. Starting from hydrogenated silicon clusters of different size, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond originates significative excitonic luminescence features in the visible range that are in fair agreement with the experimental outcomes., Comment: 12 pages 4 figures

We show that the plasma frequency in wire photonic crystals depends upon the Bloch vector. An accurate formula is given., to be published in Phys. Rev. E

We have calculated the self-consistent Green's function for a number of atoms and diatomic molecules. This Green's function is obtained from a conserving self-energy approximation, which implies that the observables calculated from the Green's functions agree with the macroscopic conservation laws for p;article number, momentum, and energy. As a further consequence, the kinetic and potential energies agree with the virial theorem, and the many possible methods for calculating the total energy all give the same result. In these calculations we use the finite temperature formalism and calculate the Green's function on the imaginary time axis. This allows for a simple extension to nonequilibrium systems. We have compared the energies from self-consistent Green's functions to those of nonselfconsistent schemes and also calculated ionization potentials from the Green's functions by using the extended Koopmans' theorem. (c) 2005 American Institute of Physics.

Within a first-principles framework we show how many-body effects crucially modify the electronic and optical properties of free-standing Germanium nanowires. The electron-hole binding energy and probability distribution are found to depend on both wire size and orientation. Moreover, we observe an almost complete compensation of self-energy and excitonic effects for some of the analyzed quantum wires, which we explain as being due to their clusterlike atomic structure.

In recent years there have been some rather successful applications of a new variational technique for calculating the total energies of electronic systems. The new method is based on many-body perturbation theory and uses the one-electron Green function as the basic "variable" rather than the wave function of traditional variational calculations. It is the purpose of the present work to promote the new methods within the realm of traditional theoretical chemistry by demonstrating their utility for calculating the correlation energies of a number of atoms at a level corresponding to second-order Moller-Plesset perturbation theory. The generalization to any desired order of perturbation theory is not hard to accomplish. (C) 2004 American Institute of Physics.

It was recently proposed to use variational functionals based on many-body perturbation theory for the calculation of the total energies of many-electron systems. The accuracy of such functionals depends on the degree of sophistication of the underlying perturbation expansions. An older such functional and a recently constructed functional, both at the level of the GW approximation (GWA), were tested on the electron gas with indeed very encouraging results. In the present work we test the older of these functionals on atoms and find correlation energies much better than those of the random-phase approximation but still definitely worse as compared to the case of the gas. Using the recent functional of two independent variables it becomes relatively easy to include second-order exchange effects not present in the GWA. In the atomic limit we find this to be very important and the correlation energies improve to an accuracy of 10-20 % when obtained from calculations much less demanding than those of, e.g., configuration-interaction expansions.

We present a first-principles approach to the calculation of the electron-phonon interaction. This approach solves some theoretical difficulties in the standard derivation of the electron-phonon interaction. We do not make a Born-Oppenheimer approximation from the outset but transform the electronic coordinates to a frame attached to the nuclear framework. Subsequently coupled equations are derived which connect the nuclear density-density correlation function to the electron Green function, the screened interaction, and the vertex. This set of equations is completely equivalent to the full problem and therefore higher-order effects are systematically included. The derived equations are further compared to those obtained from the Frohlich Hamiltonian. It is shown that careless use of this Hamiltonian leads to double counting but also insight is given why use of this Hamiltonian has led to many useful results. Finally a simple method is presented that allows for the inclusion of electron-phonon coupling within a density-functional context.