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The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this work, we focus on the exciton properties of molybdenum sisulfide (MoS2) by solving the ab initio many-body Bethe–Salpeter equation, as a function of momentum, to obtain the excitation spectra of both monolayer and bulk MoS2. We analyse the spectrum and the exciton dispersion on the basis of a model excitonic Hamiltonian capable of providing an efficient description of the excitations in the bulk crystal, starting from the knowledge of the excitons of a single layer. In this way, we obtain a general characterization of both bright and darks excitons in terms of the interplay between the electronic band dispersion (i.e. interlayer hopping) and the electron–hole exchange interaction. We identify for both the 2D and the 3D limiting cases the character of the lowest-energy excitons in MoS2, we explain the effects and relative weights of both band dispersion and electron–hole exchange interaction and finally we interpret the differences observed when changing the dimensionality of the system.

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derived a cumulant formulation for neutral excitation spectra that contains excitonic effects and the coupling between excitons and phonons. The cumulant approach allows us to include dynamical effects arising from the electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art $GW$-plus-Bethe-Salpeter calculation of excitonic states and a density functional perturbation theory calculation of phonons and electron-phonon coupling. We demonstrate that, in order to obtain a consistent treatment of exciton-phonon coupling, diagrams have to be taken into account that can be neglected when the effect of lattice vibrations is treated in a static or quasistatic approximation. From the application of this approach to a model system, we analyzed the main features of the exciton-phonon interaction and provided a general picture of their link with the properties of materials such as exciton mass and exciton Bohr radius.

Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, , with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications.

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

International audience; Low-dimensional materials differ from their bulk counterpart in many respects. In particular, thescreening of the Coulomb interaction is strongly reduced, which can have important consequencessuch as the significant increase of exciton binding energies. In bulk materials the binding energyis used as an indicator in optical spectra to distinguish different kinds of excitons, but this is notpossible in low-dimensional materials, where the binding energy is large and comparable in size forexcitons of very different localization. Here we demonstrate that the exciton band structure, whichcan be accessed experimentally, instead provides a powerful way to identify the exciton character.By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane andsingle-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensionalmaterials, highlighting the different role played by the exchange electron-hole interaction and by theelectronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators., This research was supported by a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme. Computational time was granted by GENCI (Project No. 544). We also acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010- AdG-267374), Spanish Grant (FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and European Community FP7 project CRONOS (Grant number 280879-2) and COST Actions CM1204 (XLIC) and MP1306 (EUSpec).

Transition-metal dichalcogenides (TMD) share the same global layered structure, but distinct polymorphs are characterized by different local coordinations of the transition-metal atoms. Here we compared the 1T and 2H families of metallic TMD, both in the bulk and in the two-dimensional forms. By means of first-principles time-dependent density functional calculations of the loss function, we established the direct connection between the low-energy plasmon properties and the crystal-structure symmetry. The different atomic environments affect the d − d electron-hole excitations, which are prominent at low energies, resulting in distinct in-plane plasmon dispersions in the two families. Conversely, the different periodicity of the plasmon reappearance along the c axis perpendicular to the layers can be used to distinguish the various crystal structures of TMD.

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic ${\mathrm{VO}}_{2}$ quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of ${\mathrm{VO}}_{2}$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the ${\mathrm{VO}}_{2}$ band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V $3d$ valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., We present the derivation of an ab initio and parameter-free effective electron-electron interaction that goes beyond the screened random phase approximation and accounts for superconducting pairing driven by spin fluctuations. The construction is based on many-body perturbation theory and relies on the approximation of the exchange-correlation part of the electronic self-energy within time-dependent density functional theory. This effective interaction is included in an exchange-correlation kernel for superconducting density functional theory in order to achieve a completely parameter free superconducting gap equation. First results from applying the new functional to a simplified two-band electron gas model are consistent with experiments.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., We characterize experimentally and theoretically the collective electronic excitations in two prototypical layered transition-metal dichalcogenides, NbSe2 and Cu0.2NbS2. The energy- and momentum-dependent dynamical structure factor was measured by inelastic x-ray scattering (IXS) spectroscopy and simulated by time-dependent density-functional theory. We find good agreement between theory and experiment, provided that Nb semicore states are taken into account together with crystal local-field effects. Both materials have very similar spectra, characterized by two main plasmons at 9 and 23 eV, which we show to both have π+σ character on the basis of a detailed analysis of the band structure. Finally, we discuss the role of the layer anisotropy in the dispersion of these plasmons., We acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish grant (2010-21282-C02-01), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), European Commission project CRONOS (Grant No. 280879-2). This research was also supported by a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme, and by Generalidad Valenciana (ISIC Nano program). S.H., K.O.R. and C.J.S. were supported by the Academy of Finland (projects 1256211, 1254065, 1259526) and University of Helsinki Research Funds (project 490076).

By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excitons. This analysis is general and can be applied to other systems in which the electron wave functions are strongly localized, as in strongly correlated insulators., The authors acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish grants (2010-21282-C02-01 and PIB2010US-00652), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), European Commission projects CRONOS (Grant No. 280879-2 CRONOS CP-FP7), and the Maison de la simulation for technical support. Computational time was granted by GENCI (Project No. 544) and by the CNANO-SIMULEE-2009 project. This work was carried out under the HPC-EUROPA2 project, with the support of the European Community–Research Infrastructure Action of the FP7

Motivated by recent experimental observations of Tongay et al. [Nano Lett. 12, 5576 (2012)] we show how the electronic properties and Raman characteristics of single layer MoSe2 are affected by elastic biaxial strain. We found that with increasing strain: (1) the E′ and E′′ Raman peaks (E2g and E1g in bulk) exhibit significant redshifts (up to ∼30 cm−1), (2) the position of the A1′ peak remains at ∼180 cm−1 (A1g in bulk) and does not change considerably with further strain, (3) the dispersion of low energy flexural phonons crosses over from quadratic to linear, and (4) the electronic band structure undergoes a direct to indirect band gap crossover under ∼3% biaxial tensile strain. Thus the application of strain appears to be a promising approach for a rapid and reversible tuning of the electronic, vibrational, and optical properties of single layer MoSe2 and similar MX2 dichalcogenides., This work was supported by the Flemish Science Foundation (FWO-Vl) and the Methusalem programme of the Flemish government. Computational resources were partially provided by TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure). H.S. is supported by a FWO Pegasus Marie Curie Long Fellowship.

By solving the first-principles many-body Bethe-Salpeter equation, we compare the optical properties of two prototype and technological relevant organic molecular crystals: picene and pentacene. Albeit very similar for the structural and electronic properties, picene and pentacene show remarkable differences in their optical spectra. While for pentacene the absorption onset is due to a charge-transfer exciton, in picene it is related to a strongly localized Frenkel exciton. The detailed comparison between the two materials allows us to discuss, on general grounds, how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. It represents a clear example of the relevance of the competition between localization and delocalization in the description of two-particle electronic correlation. © 2012 American Physical Society., We acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG, Proposal No. 267374), Spanish Grants No. FIS2011- 65702-C02-01 and No. PIB2010US-00652, ACI-Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), Consolider nanoTHERM (Grant No. CSD2010-00044), and European Commission project CRONOS (280879-2 CRONOS CP-FP7).

Motivated by recent experiments, we perform a microscopic analysis of the dynamical charge response of layered transition-metal dichalcogenides that display a low-temperature charge-density wave (CDW) order. In agreement with measurements, our parameter-free results show a negative in-plane plasmon dispersion that switches to positive slope upon electron (or hole) doping. This finding is explained by the peculiar behavior of the intraband transitions, which are partially suppressed under doping, and it is not linked to the CDW order. Finally, in the direction perpendicular to the layers, we predict the reappearance around the Bragg reflections of the spectra of the first Brillouin zone, a clear effect of the crystal local-field impact. Our results give a general picture of the collective excitations in these materials suggesting a simpler reinterpretation of the experiments. © 2012 American Physical Society., Financial support was provided by Spanish (FIS2011-65702-C02-01 and PIB2010US-00652), ACI-Promociona (ACI2009-1036), and Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07) grants and the European Research Council Advanced Grant DYNamo (ERC-2010-AdG, Proposal No. 267374).

We investigate the dynamic response of pristine and potassium-doped picene, the first example of a new family of organic molecular superconductors, by combining first-principles calculations and state-of-the-art experimental tools. We find that charge-carrier plasmons in K3 picene have a negative or almost negligible dispersion, which deviates from the traditional picture of metals based on the homogeneous electron gas. We show how this finding is the result of the competition between metallicity and electronic localization on the molecular units. Conduction electrons alone give rise to the negative dispersion, which is reduced by molecular polarization and crystal local-field effects. This analysis allows us to obtain a general picture of the plasmon dispersion in metallic molecular crystals., This work has been supported by the Deutsche Forschungsgemeinschaft (Grant Nos. KN393/13 and KN393/14). We acknowledge financial support also from Spanish MEC (FIS2011-65702-C02-01), ACI-Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), and the European Research Council Advanced Grant DYNamo (ERC-2010-AdG Proposal No. 267374), and computational time from Barcelona Supercomputing Center, “Red Espanola de Supercomputacion”, SGIker ARINA (UPV/EHU).

For atomic thin layer insulating materials we provide an exact analytic form of the two-dimensional (2D) screened potential. In contrast to three-dimensional systems where the macroscopic screening can be described by a static dielectric constant, in 2D systems the macroscopic screening is nonlocal (q dependent) showing a logarithmic divergence for small distances and reaching the unscreened Coulomb potential for large distances. The crossover of these two regimes is dictated by 2D layer polarizability that can be easily computed by standard first-principles techniques. The present results have strong implications for describing gap-impurity levels and also exciton binding energies. The simple model derived here captures the main physical effects and reproduces well, for the case of graphane, the full many-body GW plus Bethe-Salpeter calculations. As an additional outcome we show that the impurity hole-doping in graphane leads to strongly localized states, which hampers applications in electronic devices. In spite of the inefficient and nonlocal two-dimensional macroscopic screening we demonstrate that a simple k·p approach is capable to describe the electronic and transport properties of confined 2D systems., We acknowledge funding by the Spanish Ministry of Science and Innovation (MICINN), Grant No. FIS2010-21282-C02-01, Alianza Cooperativa Internacional (ACI-promociona) Project No. ACI2009-1036, “Grupos Consolidados Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (UPV/EHU) del Gobierno Vasco” Project No. IT-319-07, and the European Community through e-I3 European Theoretical Spectroscopy Facility (ETSF) project, Contract No. 211956, and THEMA, Contract No. 228539.

4 páginas, 3 figuras.-- PACS numbers: 78.67.Wj, 71.35.Cc, 71.35.Lk, 73.22.Pr, Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally., This work was supported by the Spanish MEC (FIS2007-65702-C02-01), ACI-Promociona (ACI2009- 1036) Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), and the European Union through e-I3 ETSF project (Contract No. 211956).

8 páginas, 3 figuras.-- et al., Recently, a new organic superconductor, K-intercalated picene, with high transition temperatures Tc (up to 18 K) has been discovered. We have investigated the electronic properties of an undoped relative of this superconductor, solid picene, using a combination of experimental and theoretical methods. Our results provide deep insights into the occupied and unoccupied electronic states., We are grateful to the Deutsche Forschungsgemeinschaft for financial support (KN393/5 and KN393/9). This work was also supported by the Spanish MEC (FIS2007-65702-C02-01), ACIPromociona (ACI2009-1036), ‘Grupos Consolidados UPV/EHU del Gobierno Vasco’ (IT-319- 07), ETORTEK and by the European Union through e-I3 ETSF (contract 211956) and THEMA (contract 228539) projects. We acknowledge support from the Barcelona Supercomputing Center (‘Red Espanola de Supercomputacion’).