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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). ; Peer Reviewed

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. ; We acknowledge financial support from the European Research Council Advanced grant DYNamo (Grant No. ERC- 2010-AdG-267374), Spanish grant (Grant No. 2010-21282-C02-01), Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13), and European Commission project CRONOS (Grant No. 280879-2). This research was also supported by a Marie Curie FP7 Integration Grant within the Seventh European Union Framework Programme. ; Peer Reviewed

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'). ; Peer reviewed

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). ; Peer reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al. ; Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO2 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. ; R. E. M., C. L. M., and R. F. H. were supported by the National Science Foundation (Grant No. DMR-1207507). A. R., P. C., and L. X. acknowledge support by the European Research Council Advanced Grant DYNamo (Grant No. ERC-2010-AdG-267374) and Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-578-13). A. R., P. C., L. X., M. W., and J. S. received support from the European Commission project CRONOS (Grant No. 280879-2) and M. G. from a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme. ; Peer Reviewed

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. ; Peer Reviewed

We examine the experimental and theoretical electron-energy loss spectra in 2H-${\mathrm{Cu}}_{0.2}$NbS2 and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range q < 0.35 Å−1. We argue that for a stoichiometric pure 2H-NbS2 the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides. ; SH was supported by the Academy of Finland (projects 1254065, 1283136, 1259526 and 1295696), MG by a Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme, PC by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 660695. AR acknowledges financial support from the European Research Council(ERC-2010-AdG-267374), Spanish grant (FIS2013-46159-C3-1-P), Grupos Consolidados(IT578-13), and AFOSR Grant No. FA2386-15-1-0006 AOARD 144088, H2020-NMP-2014 project MOSTOPHOS, GA no. SEP-210187476 and COST Action MP1306 (EUSpec). Computational time was granted by GENCI(Project No. 544). ; Peer reviewed