Search

Oxygen vacancies are common to most metal oxides and usually play a crucial role in determining the properties of the host material. In this work, we perform ab initio calculations to study the influence of vacancies in doped manganites La(1−x)SrxMnO3, varying both the vacancy concentration and the chemical composition within the ferromagnetic-metallic range (0.2, Work at ICN2 was funded by FEDER/Ministerio de Ciencia e Innovación – Agencia Estatal de Investigación (grant PGC2018-096955-B-C43), Generalitat de Catalunya (2017SGR1506), and the EU-H2020 programme (654360 and 824143) under the NFFA-Europe Transnational Access Activity and the EU MaX Center of Excellence. ICN2 is also supported by the Severo Ochoa Program (SEV--2013--0295) and the CERCA Program of Generalitat de Catalunya. D. J. and V. F. acknowledge support from CONICET (PIP 00020-2019) and ANPCyT (PICT1857), Argentina. D. J. acknowledges CONICET for a doctoral fellowship.

The origin of the charge density wave in TaTe4 is discussed on the basis of a first-principles density functional theory analysis of the Fermi surface, electron-hole response function, phonon band structure of the average structure, and structural optimization of the modulated phase. Analysis of the band structure and Fermi surface of the average structure clearly proves that despite the presence of TaTe4 chains in the crystal structure, TaTe4 is in fact a 3D material as far as the electronic structure near the Fermi level is concerned. A Fermi surface nesting mechanism is dismissed as the origin of the 2a × 2a × 3c structural modulation. The optimized 2a × 2a × 3c structure, which is found to be the more stable modulation in agreement with the experimental observations, can be obtained directly from a soft-phonon mode computed for the undistorted structure. Our results suggest that the driving force for the distortion is the maximization of Ta-Ta metal-metal bonding subject to inducing the minimum bonding decrease in the Te sublattice., This paper was supported by Spanish MINECO (the Severo Ochoa Centers of Excellence Program under Grants No. SEV-2017-0706 and No. FUNFUTURE CEX2019- 000917-S), Spanish MICIU, AEI, and EU FEDER (Grants No. PGC2018-096955-B-C43 and No. PGC2018-096955-BC44), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143). Computational resources have been provided partially by the supercomputing facilities of the Université Catholique de Louvain (CISM/UCL) and the Consortium des Équipements de Calcul Intensif en Fédération Wallonie Bruxelles (CÉCI) funded by the Fond de la Recherche Scientifique de Belgique (F.R.S.-FNRS) under Convention No. 2.5020.11 and by the Walloon Region.

Engineering oxygen octahedra rotation patterns in ABO3 perovskites is a powerful route to design functional materials. Here we propose a strategy that exploits point defects that create local electric dipoles and couple to the oxygen sublattice, enabling direct actuation on the rotational degrees of freedom. This approach, which relies on substituting an A site with a smaller ion, paves a way to couple dynamically octahedra rotations to external electric fields. A common antisite defect, AlLa, in rhombohedral LaAlO3 is taken as a prototype to validate the idea, with atomistic density functional theory calculations supported with an effective lattice model to simulate the dynamics of switching of the local rotational degrees of freedom to long distances. Our simulations provide an insight of the main parameters that govern the operation of the proposed mechanism, and allow to define guidelines for screening other systems where this approach could be used for tuning the properties of the host material., J.J., X.H., A.A., and M.P. acknowledge financial support from Grant No. PGC2018-096955-B-C43, funded by MCIN/AEI/10.13039/501100011033 and by ERDF, A way of making Europe, and Generalitat de Catalunya (Grant No. 2017SGR1506). We also acknowledge the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143), and INTERSECT (H2020-NMBP-TO-IND Project GA No. 814487). ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV- 2017-0706) and the CERCA Program of Generalitat de Catalunya. J.J. and G.H. acknowledge the financial support from PID2020-118479RB-I00 and Severo Ochoa FUNFUTURE (CEX2019-000917-S) projects of the Spanish Ministry of Science and Innovation (Grant No. MCIN/AEI/10.13039/501100011033). X.H. acknowledges financial support from F.R.S.-FNRS through the PDR Grants PROMOSPAN (Grant No. T.0107.20). J.J. acknowledges financial support from China Scholarship Council (CSC) with Grant No. 201904910557.

Spin-orbit coupling (SOC) is generally understood as a highly localized interaction within each atom, whereby core electrons holding large J splittings transfer the SOC to the valence electrons of the same atom, while their direct impact on neighbor valence orbitals is usually small. Seivane and Ferrer [Phys. Rev. Lett. 99, 183401 (2007)PRLTAO0031-900710.1103/PhysRevLett.99.183401] proposed an approach within a tight-binding type ab initio framework assuming that the transfer of SOC from core to valence orbitals only takes place when both are on the same atom, leading to the so-called on-site approximation, which then has been successfully applied to a variety of systems. In this work we thoroughly test its general validity by confronting SOC related properties such as spin splittings, spin textures, or magnetic anisotropies calculated under the on-site approximation versus the more general approach where all the contributions to the SOC, including three-center integrals, are explicitly included. After considering a variety of systems with different dimensionalities, all presenting a strong SOC, we conclude that although the on-site approximation often provides accurate results, it breaks down in some systems where 5d electrons are close to the Fermi level due to their strong SOC and moderately large spatial extension. Furthermore, there are a few examples where subtle inaccuracies lead to qualitatively wrong conclusions, the most clear case being the doping of the topological surface state in Bi2Se3(0001). Finally, magnetic anisotropy energies calculated under this approximation tend to be underestimated., This work was financially supported by the Spanish MINECO, MICIU, AEI, and EU FEDER (Grants No. MAT2015-66888-C3-1R, No. RTI2018-097895-B-C41, No. PGC2018-094783, No. PGC2018-096955-C43, and No. PGC2018-096955-C44), Generalitat de Catalunya (Grant No. 2017SGR1506), and the EU MaX Center of Excellence (EU-H2020 Grant No. 824143). ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and the CERCA Program of Generalitat de Catalunya. ICMAB is supported by the Spanish MICINN through the Severo Ochoa Centers of Excellence Program (Grant No. CEX2019-000917-S). R.C. acknowledges the funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodoswka-Curie Grant Agreement No. 665919., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

NbSe

ABOperovskites host a huge range of symmetry lowering structural distortions, each of which can tune, or even switch on or off, different functional properties due to the strong coupling between the lattice, spin and charge degrees of freedom in these materials. The sheer number of different meta-stable structures present in perovskites creates a challenge for materials design via theory and simulation. Here, we tackle this issue using a first principles structure searching method on a prototypical half-metallic perovskite, LaSrMnO, to predict how epitaxial strain can engineer structural and magnetic properties.We reveal a rich structural phase diagram through strain engineering in which the octahedral tilt pattern, and hence the crystal symmetry, is altered from the bulk.We show how the low-symmetry of the various phases in turn induces new structural modes, an increase in the magnetic anisotropy energy, and weak antiferromagnetic spin-canting., The work has been partly performed under the Project HPC-EUROPA3 (INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme. This work was supported by Spanish MINECO (the Severo Ochoa Center of Excellence Program under Grant No. SEV-2017-0706), Spanish MICIU, AEI and EU FEDER (Grant No. PGC2018-096955-B-C43), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143), J Pilo acknowledges the scholarship granted for a postdoctoral stay at ICN2 by CONACYT.

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron‐correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine‐scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi‐dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale., The authors thank Prof. Enric Canadell for enlightening discussions. The authors also thank Prof. Etienne Snoeck and Dr. Christophe Gatel for obtaining HRTEM images of the 80 nm‐thick film at CEMES (Toulouse), and Dr. Belén Ballesteros for assistance during preliminary TEM observations. F.S. acknowledges the Spanish Ministry of Industry, Economy and Competitiveness (MINECO) for financial support through the “Severo Ochoa” programme for Centers of Excellence in R&D (SEV‐2015‐0496) and Project No. RTI2018‐098537‐B‐C21. G.C., J.S., and L.R. acknowledge financial support from MINECO (Grant No. MAT2016‐77100‐C2‐1‐P) and the Catalan Government (Generalitat de Catalunya, Grant No. 2017 SGR 579). M.P. acknowledges financial support from Spanish MICIU, AEI and EU FEDER (Grant No. PGC2018‐096955‐B‐C43) and Generalitat de Catalunya (Grant No. 2017SGR1506). Research at ICN2 was also supported by the CERCA programme (Generalitat de Catalunya) and by the “Severo Ochoa” programme for Centers of Excellence in R&D of MINECO (Grant No. SEV‐2017‐0706). C.M. acknowledges funding from MINECO under Project No. MAT2017‐82970‐C2‐R and from the European's Union Horizon 2020 research and innovation programme under Grant No. 823717‐ESTEEM3.

The first-principles electron–hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2X (X = ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron–hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2kF response of the quenched samples of both salts display a low temperature curved and tilted triangularcontinuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron–hole fluctuations. This broad maxima region is around 1% of the Brillouin zone area for the X = ClO4 salt (and X = PF6) but only 0.1% for the X = NO3 salt. It is suggested that the strong reduction of associated SDW fluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron–hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches., This work was supported by Spanish MINECO (the Severo Ochoa Centers of Excellence Program under Grants No. SEV2017-0706 and SEV-2015-0496), Spanish MICIU, AEI and EU FEDER (Grants No. PGC2018-096955-B-C43 and No. PGC2018-096955-B-C44), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143).

This article is part of the JCP Special Topic on Electronic Structure Software., A review of the present status, recent enhancements, and applicability of the SIESTA program is presented. Since its debut in the mid-1990s, SIESTA’s flexibility, efficiency, and free distribution have given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of SIESTA combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a realspace grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here, we describe the more recent implementations on top of that core scheme, which include full spin–orbit interaction, non-repeated and multiple-contact ballistic electron transport, density functional theory (DFT)+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as WANNIER90 and the second-principles modeling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering SIESTA runs, and various post-processing utilities. SIESTA has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low-level libraries, as well as data standards and support for them, particularly the PSeudopotential Markup Language definition and library for transferable pseudopotentials, and the interface to the ELectronic Structure Infrastructure library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments., Siesta development was historically supported by different Spanish National Plan projects (Project Nos. MEC-DGES-PB95-0202, MCyT-BFM2000-1312, MEC-BFM2003-03372, FIS2006-12117, FIS2009-12721, FIS2012-37549, FIS2015-64886-P, and RTC-2016-5681-7), the latter one together with Simune Atomistics Ltd. We are thankful for financial support from the Spanish Ministry of Science, Innovation and Universities through Grant No. PGC2018-096955-B. We acknowledge the Severo Ochoa Center of Excellence Program [Grant Nos. SEV-2015-0496 (ICMAB) and SEV-2017-0706 (ICN2)], the GenCat (Grant No. 2017SGR1506), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143). P.G.-F. acknowledges support from Ramón y Cajal (Grant No. RyC-2013-12515). J.I.C. acknowledges Grant No. RTI2018-097895-B-C41. R.C. acknowledges the European Union’s Horizon 2020 Research and Innovation Program under Marie Skłodoswka-Curie Grant Agreement No. 665919. D.S.P, P.K., and P.B. acknowledge Grant No. MAT2016-78293-C6, FET-Open No. 863098, and UPV-EHU Grant No. IT1246-19. V. W. Yu was supported by a MolSSI Fellowship (U.S. NSF Award No. 1547580), and V.B. and V.W.Y. were supported by the ELSI Development by the NSF (Award No. 1450280).

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions at ${T}_{{\mathrm{P}}_{1}}$ and ${T}_{{\mathrm{P}}_{2}}$ associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron–hole response function. Because of the stronger inter-chain interactions, the FS and electron–hole response function are considerably more complex for NbSe3 than m-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron–hole response for NbSe3 are found with the (0a*, 0c*) and (1/2a*, 1/2c*) transverse components at ${T}_{{\mathrm{P}}_{1}}$ and ${T}_{{\mathrm{P}}_{2}}$, respectively, whereas the second maximum is not observed for m-TaS3 at ${T}_{{\mathrm{P}}_{2}}$. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse a* components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling along c* in NbSe3 and along both a* and c* for m-TaS3. Phonon spectrum calculations reveal the formation of a giant 2kF Kohn anomaly for m-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides., This work was supported by Spanish MINECO (the Severo Ochoa Centers of Excellence Program under Grants SEV 2017-0706 and FUNFUTURE CEX2019-000917-S), Span ish MICIU, AEI and EU FEDER (Grants No. PGC2018- 096955-B-C43 and No. PGC2018-096955-B-C44), General itat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme), and the European Union MaX Center of Excel lence (EU-H2020 Grant No. 824143). Phonons computational resources have been provided by the supercomputing facil ities of the Universit´e catholique de Louvain (CISM/UCL) and the Consortium des Équipements de Calcul Intensif en F´ed´eration Wallonie Bruxelles (CÉCI) funded by the Fond de la Recherche Scientifique de Belgique (FRS-FNRS) under convention 2.5020.11 and by the Walloon Region.

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: (i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and (ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first-principles calculations point toward an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface. (Figure Presented). Fil: Sacanell, Joaquin Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Hernández Sánchez, Joaquín. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rubio Lopez, Adrian Ezequiel. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Martinelli, Hernan. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Siepe, Jimena Vanesa. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Leyva de Guglielmino, Ana Gabriela. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Ferrari, Valeria Paola. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Juan, Dilson. Universidad Nacional de San Martín; Argentina Fil: Alonso Pruneda, José Miguel. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Mejía Gómez, Augusto Enrique. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Lamas, Diego Germán. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We report peculiar transport fingerprints at the secondary Dirac points created by the interaction between graphene and boron nitride layers. By performing ab initio calculations, the electronic characteristics of the moiré patterns produced by the interaction between layers are first shown to be in good agreement with experimental data, and further used to calibrate the tight-binding model implemented for the transport study. By means of a real-space order-N quantum transport (Kubo) methodology, low-energy (Dirac point) transport properties are contrasted with those of high-energy (secondary) Dirac points, including both Anderson disorder and Gaussian impurities to respectively mimic short-range and long-range scattering potentials. Mean free paths at the secondary Dirac points are found to range from 10 nm to a few hundreds of nm depending on the static disorder, while the observation of satellite resistivity peaks depends on the strength of quantum interferences and localization effects., This work was funded by Spanish FEDER-MINECO (Grants No. FIS2009-12721-C04-01 and No. CSD2007-00041) and AGAUR (Grant No. FI-DGR 2011FI B 00993). S.R. acknowledges support from the Spanish Ministry of Economy and Competitiveness (under Contract No. MAT2012-33911). The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 “Graphene Flagship”.

Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis., This research was funded by the CERCA Programme/Generalitat de Catalunya and supported by the Spanish Ministry of Economy and Competitiveness, MINECO (Grant No. SEV-2017-0706), Grants PID2019-107338RB-C65, and PID2019-107338RB-C62, funded by MCIN/AEI/ 10.13039/501100011033, Grant PGC2018-096955-B-C43 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”, FLAG-ERA grant LEGOCHIP Projects PCI2019-111890-2 and PCI2019-111933-2 funded by MCIN/AEI /10.13039/501100011033 and cofounded by the European Union, the GenCat (Grant No. 2017SGR1506), the European Union MaX Center of Excellence (EU-H2020 Grant No. 924143), the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Grant No. EFA 194/16 TNSI), and Xunta de Galicia (Centro Singular de Investigación de Galicia accreditation 2019–2022, ED431G 2019/03). C.M. was supported by Grant RYC2019-028110-I funded by MICIN/AEI/ 10.13039/501100011033 and by “ESF Investing in your future”. M.T. has been supported by Spanish State Research Agency/FSE (ref. BES-2017-08078, project ref. SEV-2013-0295-17-2).

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal-insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale.

We report the first-principles DFT calculation of the electron–hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2kF spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2kF broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron–hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2kF bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron–hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2kF SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors, This work was supported by Spanish MINECO (the Severo Ochoa Centers of Excellence Program under Grants No. SEV-2017-0706 and SEV-2015-0496), Spanish MICIU, AEI and EU FEDER (Grants No. PGC2018-096955-B-C43 and No. PGC2018-096955-B-C44), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Program), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143). C Bourbonnais and G Montambaux are thanked for a critical reading of the manuscript and D Jérome for useful discussions before this work.

We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)

Inventing an effective method to store large amounts of hydrogen at room temperature is one of the key challenges in developing a hydrogen-based economy. Metal hydrides have attracted attention owing to their promising hydrogen storage capabilities. We have systematically studied the structural and electronic properties of mechanically synthesized Ti0.5V1.5-xCrx (0 ≤ x ≤ 0.3) alloys and investigated the influence of the addition of Cr atoms on the hydrogen storage properties of vanadium-rich body-centered-cubic (V-BCC) alloys. X-ray diffraction (XRD) results indicate that all alloys are composed of BCC main phase, with the lattice parameters exhibiting no change following chemical modification. The kinetic measurements have revealed that Cr-containing alloys exhibit improved hydrogen uptake. X-ray photoelectron spectroscopy (XPS) measurements have shown that the addition of Cr has a significant effect on the anti-oxidation properties of V-BCC alloys, increasing their chemical activity and thus enhancing the hydrogen storage properties. Moreover, XPS results elucidate the role of activation of the studied materials. Additionally, the electrochemical properties of the negative electrodes (as part of Ni-MHx secondary batteries) made of Ti0.5V1.4-xNi0.1Crx (0 ≤ x ≤ 0.3) system have been studied by cyclic charge-discharge and demonstrate that doping of the V-BCC alloys with Cr can significantly improve the cycle-life stability of anode that exhibits similar discharge performance up to 50 cycles. First principles simulations are used to analyse the changes in the electronic density of states close to the Fermi level, as a function of Cr concentration, as well as binding energies and structural changes upon hydrogen absorption. Furthermore, ab initio studies confirmed that H absorption is favoured with increasing Cr-content. Our study highlights the importance of the addition of Cr to V-BCC alloys on both solid-gas and electrochemical hydrogenation reactions., Financial assistance from National Science Centre, Poland (no. 2015/17/N/ST8/00271). This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 having benefitted from the access provided by Mateusz Balcerzak in DESY NanoLab (Hamburg, Germany) and ICN2 (Barcelona, Spain) within the framework of the NFFA-Europe Transnational Access Activity. M. Pruneda acknowledges financial support from Spanish MICIU, AEI, and EU FEDER (Grant No. PGC2018-096955-B-C43) and Generalitat de Catalunya (Grant No. 2017SGR1506).

We present a general unfolding method for the electronic bands of systems with double-periodicity. Within density functional theory with atomic orbitals as basis-set, our method takes into account two symmetry operations of the primitive cell: a standard expansion and a single rotation, letting to elucidate the physical effects associated to the mutual interactions between systems with more than one periodicity. As a result, our unfolding method allows studying the electronic properties of vertically stacked two-dimensional homo- or heterostructures. We apply our method to study 3 x 3 single-layer graphene, √13 × √13 twisted single-layer graphene, and 2√3 x 2√3 graphene- √7 x √7 tungsten disulfide heterostructure with an interlayer angle of 10.9º. Our unfolding method allows observing typical mini gaps reported in heterostructures, as well as other electronic deviations from pristine structures, impossible to distinguish without an unfolding method. We anticipate that this unfolding method can be useful to compare with experiments to elucidate the electronic properties of two-dimensional homo- or heterostructures, where the interlayer angle can be considered as an additional parameter., Authors acknowledge the partial support from DGAPA-UNAM project PAPIIT IN109618 and CONACyT project A1-S-39242. FSO acknowledges to DGAPA- UNAM for postdoctoral fellowship.

Bulk and single-layer 2H-NbSe2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2H-NbSe2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2H-NbSe2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2H-NbSe2 may provide further understanding of its low temperature state and the competition between different instabilities., Work at Bellaterra was supported by Spanish MINECO (Grants FIS2015-64886-C5-3-P and FIS2015-64886-C5-4-P, and the Severo Ochoa Centers of Excellence Program under Grants SEV-2017-0706 and SEV-2015-0496), Generalitat de Catalunya (Grant 2017SGR1506 and the CERCA Program) and by the European Union MaX Center of Excellence (Grant 824143). The work by B.G. has been performed in the context of the Physics Ph.D. program of the Universitat Autònoma de Barcelona. M.M.U acknowledges support by the Spanish MINECO under grant no. MAT2017-82074-ERC and by the ERC Starting grant “LINKSPM” (Grant 758558).

Bulk and single-layer 2 H-NbSe

Following the recent synthesis of graphene&ndash, based antiferromagnetic ultrathin heterostructures made of Co and Fe, we analyse the effect of the spacer between both ferromagnetic materials. Using density functional calculations, we carried out an exhaustive study of the geometric, electronic and magnetic properties for intercalated single Co MLs on top of Ir(111) coupled to monolayered Fe through n graphene layers (n = 1, 2, 3) or monolayered h-BN. Different local atomic arrangements have been considered to model the Moir&eacute, patterns expected in these heterostructures. The magnetic exchange interactions between both ferromagnets ( J C o - F e ) are computed from explicit calculations of parallel and anti-parallel Fe/Co inter&ndash, layer alignments, and discussed in the context of recent experiments. Our analysis confirms that the robust antiferromagnetic superexchange&ndash, coupling between Fe and Co layers is mediated by the graphene spacer through the hybridization of C&rsquo, s p z orbitals with Fe and Co&rsquo, s 3d states. The hybridization is substantially suppressed for multilayered graphene spacers, for which the magnetic coupling between ferromagnets is critically reduced, suggesting the need for ultrathin (monolayer) spacers in the design of synthetic graphene-based antiferromagnets. In the case of h&ndash, BN, p z orbitals also mediate d(Fe/Co) coupling. However, there is a larger contribution of local ferromagnetic interactions. Magnetic anisotropy energies were also calculated using a fully relativistic description, and show out&ndash, of&ndash, plane easy axis for all the configurations, with remarkable net values in the range from 1 to 4 meV.

On the basis of first-principles DFT calculations the wave-vector and temperature dependencies of the Lindhard response function of the blue bronze K0.3MoO3 have been calculated. The kIF+ kIIF interband component of the response, which is responsible for the Peierls instability, has been quantitatively analyzed. It is found that (i) the electron-hole coherence length of this response determines the length scale of the experimental intrachain CDW correlations, and (ii) the intrachain q dependence of such a response also determines the shape of the Kohn anomaly experimentally measured. These findings provide compelling evidence that the Peierls transition of the blue bronze K0.3MoO3 follows the weak electron-phonon coupling scenario in the adiabatic approximation, something that had not yet been proved on the basis of first-principles calculations for a real material. It is proposed that the CDW interchain coupling occurs through a Coulomb coupling between dipolar CDWs. The nature of the phonon mode leading to the dipolar nature of the CDWs is also discussed, and the relevance of these results to rationalize the CDW instabilities in other oxides and bronzes is pointed out. These findings are also contrasted with recent results for other CDW materials like chalcogenides and tellurides., This work was supported by Spanish MINECO (the Severo Ochoa Centers of Excellence Program under Grants No. SEV-2017-0706 and No. SEV-2015-0496), Spanish MICIU, AEI and EU FEDER (Grants No. PGC2018-096955-B-C43 and No. PGC2018-096955-B-C44), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme), and the European Union MaX Center of Excellence (EU-H2020 Grant No. 824143).

We present a density functional theory study of the electronic structure of single-layer TiSe2, and focus on the charge density wave (CDW) instability present on this 2D material. We explain the $2\times 2$ periodicity of the CDW from the phonon band structure of the undistorted crystal, which is unstable under one of the phonon modes at the M point. This can be understood in terms of a partial band gap opening at the Fermi level, which we describe on the basis of the symmetry of the involved crystal orbitals, leading to an energy gain upon the displacement of the atoms following the phonon mode in a 2 × 1 structure. Furthermore, the combination of the corresponding phonons for the three inequivalent M points of the Brillouin zone leads to the 2 × 2 distortion characteristic of the CDW state. This leads to a further opening of a full gap, which reduces the energy of the 2 × 2 structure compared to the 2 × 1 one of a single M point phonon, and makes the CDW structure the most stable one. We also analyze the effect of charge injection into the layer on the structural instability. We predict that the 2 × 2 structure only survives for a certain range of doping levels, both for electrons and for holes, as doping reduces the energy gain due to the gap opening. We predict the transition from the commensurate 2 × 2 distortion to an incommensurate one with increasing wavelength upon increasing the doping level, followed by the appearance of the undistorted 1 × 1 structure for larger carrier concentrations., This work was supported by Spanish MINECO (Grants FIS2015-64886-C5-3-P and FIS2015-64886-C5-4-P, and the Severo Ochoa Centers of Excellence Program under Grants SEV-2013-0295 and SEV-2015-0496), Generalitat de Catalunya (Grant 2017SGR1506 and the CERCA Programme) and by the European Union H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598). The calculations were performed using the resources granted by Red Española de Supercomputación (RES).

An accurate and efficient general method to constrain the magnetization of individual atoms or groups of atoms within a fully relativistic non-collinear spin density functional theory formalism is presented and implemented within the SIESTA code. This approach can be applied to study a variety of complex magnetic configurations and to build effective magnetic Hamiltonians for multiscaling micromagnetic simulations. As an example, the method is applied to obtain constrained magnetic states for a Fe3 structure, and for a S = 1/2 kagome layer (vanadium oxyfluoride V7O6F18). Of paramount importance in spintronics is the control and manipulation of magnetic interactions between constituent species, characterized mainly by the pair-wise magnetic exchange tensor ${{ \mathcal J }}_{{ij}}$. By constraining the atomic magnetizations of an infinite Fe linear chain, the total selfconsistent energy values are mapped to a generalized Heisenberg model, obtaining not only the diagonal terms of ${{ \mathcal J }}_{{ij}}$ but also the off-diagonal contributions due to the explicit presence of the spin–orbit coupling in the formalism. The diagonal values of ${{ \mathcal J }}_{{ij}}$ promote short ranged ferromagnetic alignment whilst the non-zero off-diagonal values can lead to the formation of the spiral states in the chain, as expected from theory., ICN2 and ICMAB are supported by the Severo Ochoa Centers of Excellence Program (Grant No. SEV–2013–0295 and SEV–2015–0496), Spanish MINECO program (Grants No.FIS2015-64886-C5-3-P and FIS2015-64886-C5-4-P) and by the Generalitat de Catalunya (Grant No. 2017SGR1506). RC acknowledges the funding from the European Union’s Horizon 2020 research and innovation program under Marie SklodoswkaCurie grant agreement no. 665919 and EC H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598).

A density functional theory study concerning the origin of the recently reported 2 2 charge density wave (CDW) instability in single-layer TiTe2 is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a 2 2 distortion. The study suggests that the magnitude of the semi-metallic overlap between the valence band top at and the conduction band bottom at M is a key factor controlling the tendency towards the distortion. It is also shown that tensile strain stabilizes a 2 2 CDW, and we suggest that this fact could be further used to induce the instability in double-layers of TiTe2, which in the absence of strain remain undistorted in the experiment. The driving force for the CDW instability seems to be the same phonon mediated mechanism acting for single-layer TiSe2, although in single-layer TiTe2 the driving force is smaller, and the semimetallic character is kept below the transition temperature., This work was supported by Spanish MINECO (Grants FIS2015-64886-C5-3-P and FIS2015- 64886-C5-4-P, and the Severo Ochoa Centers of Excellence Program under Grants SEV-2017-0706 and SEV-2015-0496), Generalitat de Catalunya (Grant 2017SGR1506 and the CERCA Programme) and by the European Union H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598). The work by BG has been performed in the context of the Physics PhD programme of the Universitat Autònoma de Barcelona.

While the dispersion of nanomaterials is known to be effective in enhancing the thermal conductivity and specific heat capacity of fluids, the mechanisms behind this enhancement remain to be elucidated. Herein, we report on highly stable, surfactant-free graphene nanofluids, based on N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), with enhanced thermal properties. An increase of up to 48% in thermal conductivity and 18% in specific heat capacity was measured. The blue shift of several Raman bands with increasing graphene concentration in DMF indicates that there is a modification in the vibrational energy of the bonds associated with these modes, affecting all the molecules in the liquid. This result indicates that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. Density functional theory and molecular dynamics simulations were used to gather data on the interaction between graphene and solvent, and to investigate a possible order induced by graphene on the solvent. The simulations showed a parallel orientation of DMF towards graphene, favoring π–π stacking. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the fluid's thermal properties., The Catalan Institute of Nanoscience and Nanotechnology (ICN2) acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295) and funding from the CERCA Programme/Generalitat de Catalunya. Funding from the Spanish Ministry (MINECO/FEDER: MAT2015-68394-R NaCarFLOW, FIS2015-70862-P PHENTOM and FIS2015-64886-C5-3-P SIESTA) is also acknowledged. FC, BE, MP and PO acknowledge support from the EU Center of Excellence MaX-Materials Design at the Exascale (Grant No. 676598), Generalitat de Catalunya (Grant No. 2014SGR301) and supercomputing resources from the Red Española de Supercomputación (RES). ACA acknowledges Fundació Cellex de Barcelona for financial support.

Interfaces are the birthplace of a multitude of fascinating discoveries in fundamental science, and have enabled modern electronic devices, from transistors, to lasers, capacitors or solar cells. These interfaces between bulk materials are always bi-dimensional (2D) 'surfaces'. However the advent of graphene and other 2D crystals opened up a world of possibilities, as in this case the interfaces become one-dimensional (1D) lines. Although the properties of 1D nanoribbons have been extensively discussed in the last few years, 1D interfaces within infinite 2D systems had remained mostly unexplored until very recently. These include grain boundaries in polycrystalline samples, or interfaces in hybrid 2D sheets composed by segregated domains of different materials (as for example graphene/BN hybrids, or chemically different transition metal dichalcogenides). As for their 2D counterparts, some of these 1D interfaces exhibit polar characteristics, and can give rise to fascinating new physical properties. Here, recent experimental discoveries and theoretical predictions on the polar discontinuities that arise at these 1D interfaces will be reviewed, and the perspectives of this new research topic, discussed., The authors enjoyed financial support from the Spanish FEDER-MINECO (Grant No. FIS2012-37549-C05-02) and Generalitat de Catalunya (2014 SGR 301). MP is grateful to Nicholas Bristowe and Emilio Artacho for fruitful discussions. ICN2 acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295.

We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm K. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials., J.E.B.-V. acknowledges support from CONACyT (Mexico, D.F.). This work was supported by European Union Seventh Framework Programme under grant agreement 604391 Graphene Flagship (R.M.G.). S.R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluna, and the Severo Ochoa Program (MINECO SEV-2013-0295). M.P. and L.C. acknowledge Spanish MINECO (FIS2015-64886-C5-3-P) and Generalitat de Catalunya (2014SGR301). B.M. and T.R. greatly acknowledge the financial support by European Research Council for COMBAT project (grant no. 615132).

Molecular dynamics is used in combination with density functional theory to determine the thermal transport properties of the single-layer hexagonal boron nitride (SL h-BN) from ab initio calculations. Within this approach, the possible anisotropy in the thermal conductivity of SL h-BN was studied. For samples with finite length (of the order of 20 nm), we find a significant dependence of the conductivity on the transport direction. We make a direct comparison of the results obtained for two-dimensional (2D) layers and for nanoribbons with similar size, and show that, as a consequence of edge scattering, the ribbon geometry induces a significant decrease in the conductivity, and produces a strong change in the anisotropy. For the zigzag and armchair transport directions, the dependence of the thermal conductivity on the system length was also obtained, as well as its value in the 2D bulk limit case. A very small anisotropy was found for the limit of long samples, in contrast with the finite length ones. This is explained analyzing the dependence of the average square group velocities on the transport direction and the phonon frequency.

La0.8Sr0.2MnO3 (LSM) is one of the most commonly used cathodes in Solid Oxide Fuel Cells (SOFC). In spite of the fact that nanostructured cathodes are expected to display improved performance, the high operating temperature (∼ 1000°C) of LSM-based SOFCs hinders their stability. In the present work, we have developed nanostructured cathodes prepared from LSM nanotubes of enhanced performance, allowing its use at lower temperatures (∼ 800°C). We observed that our cathodes have qualitative improvements compared with microstructured materials: firstly, the diffusion in the gas phase is optimized to a negligible level and secondly, evidence of ionic conduction is found, which is extremely rare in LSM cathodes. We propose that this important change in the electrochemical properties is due to the nanostructuration of the cathode and deserves further attention, including the exploration of other materials. Fil: Sacanell, Joaquin Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Hernández Sánchez, Joaquin. Comisión Nacional de Energía Atómica; Argentina Fil: Rubio Lopez, Adrian Ezequiel. Comisión Nacional de Energía Atómica; Argentina Fil: Martinelli, Hernan. Comisión Nacional de Energía Atómica; Argentina Fil: Siepe, Jimena. Comisión Nacional de Energía Atómica; Argentina Fil: Leyva de Guglielmino, Ana Gabriela. Comisión Nacional de Energía Atómica; Argentina Fil: Ferrari, Valeria Paola. Comisión Nacional de Energía Atómica; Argentina Fil: Pruneda, Miguel. Institut Català de Nanociència i Nanotecnologia; España Fil: Juan, Dilson. Comisión Nacional de Energía Atómica; Argentina Fil: Lamas, Diego Germán. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications.

The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation–anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation–anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

10 páginas, 11 figuras, 2 tablas.-- et al., The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO2)n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient., This work was funded by the Spanish Ministerio de Ciencia e Innovación, MICINN, projects MAT2008-4918, CSD2008-0023, FIS2009-12721- C04-01, and CSD2007-00050. R.S.A. thanks the Junta de Andalucía for a predoctoral grant (P08-FQM-3661 and EXC/ 2005/FQM-1126).

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., First-principles calculations are used to demonstrate electromechanical control of charge and spin at zigzag-edged interfaces between graphene and boron-nitride domains in hybrid monolayers. We show how, through a direct piezoelectric effect, the interfacial bound charges and associated electric fields can be tuned by application of an external mechanical force (stress) on the system. This results in mechanical control of the edge magnetization (piezomagnetic effect), and the possibility to transform a semiconducting heterostructure into a half-metal. The inverse effect (application of an external electric field to induce a mechanical deformation) goes together with a magnetoelectric response, which under ideal conditions we estimate to be comparable to that of prototypical Cr2O3. These effects originate from the magnetic properties of graphene's zigzag edges and the dielectric properties of the boron-nitride domain, and can also be expected in any other coplanar heterostructures with polar discontinuities., We acknowledge support from the Spanish FEDER-MINECO (Grant No. FIS2012-37549-C05-02) and Generalitat de Catalunya (2014 SGR 301 and AGAUR Grant No. FI-DGR 2011FI B 00993). We would like to thank the Spanish Supercomputer Network for computing resources. ICN2 acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2013-0295.

The UK environmental e-science initiative supports the development and modification of simulation tools used to study radiation damage effects. We discuss the development and modification to the DL_POLY molecular dynamics (MD) code. Using the newly developed tools, we study the effects of radiation damage related to the safe encapsulation of highly radioactive materials, including nuclear waste. We address the possible differences between the radiation damage in the bulk and at the surface of a material, and perform MD simulations of energetic events in zircon structure. We find that in the case of readily amorphizable material, the formation of a stable alternative covalent network reduces the possible effect of the surface on the damaged structure.

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first principles calculations point towards an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface. Figure 1

The dynamics of one atom thick h-BN suspended nanoribbons have been obtained by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine type equation to explore their use as energy harvesting devices. Similarly to our previous proposal for a graphene-based harvester1, an applied compressive strain is used to drive the clamped-clamped nanoribbon structure into a bistable regime, where quasi-harmonic vibrations are combined with low frequency swings between the minima of a double-well potential. h-BN, graphene and MoS2 similar structures have been compared in terms of the static response to a compressive strain and of the dynamic evolution induced by an external noisy vibration. Due to its intrinsic piezoelectric response, the mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced non-linearity, the proposed device is predicted to harvest an electrical root mean square (rms) power of more than 180 fW when it is excited by a noisy external force characterized by a white Gaussian frequency distribution with an intensity in the order of Frms=5pN.

We study the dynamics of h-BN monolayers by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine-type equation to explore their use in nonlinear vibration energy harvesting devices. An applied compressive strain is used to drive the system into a nonlinear bistable regime, where quasi-harmonic vibrations are combined with low-frequency swings between the minima of a double-well potential. Due to its intrinsic piezoelectric response, the nonlinear mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced nonlinearity, a 20 nm device is predicted to harvest an electrical power of up to 0.18 pW for a noisy vibration of 5 pN., Funding from the Ministerio de Ciencia e Innovación under contract No ENE2009-14340-CO2-02 (OPACMEMS) and FIS2012-37549, and from the EU-FP7 FET-Proactive Coordination Action, Project no. 270005 (ZEROPOWER), are greatly acknowledged.

Resumen del trabajo presentado a la International Graphene Innovation Conference, celebrada en Qingdao (China) del 22 al 24 de septiembre de 2016.

10 páginas, 7 figuras, 2 tablas.-- El pdf del artículo es la versión pre-print: arXiv:0910.1748v2, We have adapted the maximally localized Wannier function approach of Souza et al (2002 Phys. Rev. B 65 035109) to the density functional theory based SIESTA code (Soler et al 2002 J. Phys.: Condens. Mater. 14 2745) and applied it to the study of Co substitutional impurities in bulk copper as well as to the Cu(111) surface. In the Co impurity case, we have reduced the problem to the Co d-electrons and the Cu sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined density functional parameters in a fully orthonormal basis set. In order to test the quality of the Wannier approach to surfaces, we have studied the electronic structure of the Cu(111) surface by again transforming the density functional problem into the Wannier representation. An excellent description of the Shockley surface state is attained, permitting us to be confident in the application of this method to future studies of magnetic adsorbates in the presence of an extended surface state., Financial support from the Spanish MICINN through grant FIS2009-12721-C04-01 is gratefully acknowledged. RK is supported by the CSIC-JAE predoctoral fellowship. PO acknowledges support from the MICINN CONSOLIDER grant CSD2007-00059.

The radiation damage properties of synthetic pyrochlore-defect fluorite compounds containing lanthanides on the A-site and Ti, Zr, Sn, and Hf on the B-site have been studied extensively using Kr ion irradiation. Using statistical analysis, we show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and defect energies. The best current model is able to predict the critical temperature to within about 80 degrees Kelvin. The model indicates that radiation tolerance is correlated with an increase in the X anion coordinate toward the value characteristic of the defect fluorite topology, a smaller unit cell dimension, and lower defect energies. Our analysis also demonstrates that radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. Of the two possible cation sites in ideal pyrochlore, the B-site cation appears to play the major role in bonding. This result is supported, for a subset of pyrochlore compounds, by ab initio calculations, which reveal a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

We combine classical and quantum-mechanical simulations to study the structural changes in $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_{3}$ under irradiation. We identify common defects, and suggest that their stability is related to the covalent character of Ti-O bonding. We address the issue of resistance to amorphization by radiation damage, and propose that a complex material is amorphizable by radiation damage if it is able to form a covalent network. On a more detailed level, we suggest that resistance to amorphization is defined by the competition between the short-range covalent and long-range ionic forces.

We combine simulation, theoretical and experimental results to study radiation damage effects in complex oxides under irradiation. In zircon, we study large density variations in the damaged structure, and show how damage percolation results in the enhanced transport extending to macroscopic lengthscale, and suggest that percolation threshold serve as a benchmark for acceptable waste load in a waste form. In perovskite, we identify common defects in the damaged structure, and relate their stability to chemical bonding. Finally, we formulate the criterion for resistance to amorphization by radiation damage: a material is amorphizable if it is able to form a covalent network.