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The electronic structure of semiconductors and insulators is affected by ionic motion through electron-phonon interaction, yielding temperature-dependent band gap energies and zero-point renormalization (ZPR) at absolute zero temperature. For polar materials, the most significant contribution to the band gap ZPR can be understood in terms of the Fr\"ohlich model, which focuses on the nonadiabatic interaction between an electron and the macroscopic electrical polarization created by a long-wavelength optical longitudinal phonon mode. On the other hand, spin-orbit interaction (SOC) modifies the bare electronic structure, which will, in turn, affect the electron-phonon interaction and the ZPR. We present a comparative investigation of the effect of SOC on the band gap ZPR of twenty semiconductors and insulators with cubic symmetry using first-principles calculations. We observe a SOC-induced decrease of the ZPR, up to 30%, driven by the valence band edge, which almost entirely originates from the modification of the bare electronic eigenenergies and the decrease of the hole effective masses near the $\Gamma$ point. We also incorporate SOC into a generalized Fr\"ohlich model, addressing the Dresselhaus splitting which occurs in noncentrosymmetric materials, and confirm that the predominance of nonadiabatic effects on the band gap ZPR of polar materials is unchanged when including SOC. Our generalized Fr\"ohlich model with SOC provides a reliable estimate of the SOC-induced decrease of the polaron formation energy obtained from first principles and brings to light some fundamental subtleties in the numerical evaluation of the effective masses with SOC for noncentrosymmetric materials. We finally warn about a possible breakdown of the parabolic approximation within the physically relevant energy range of the Fr\"ohlich interaction for materials with high phonon frequencies treated with SOC., Comment: 34 pages including Supplemental Material; 10 figures, 1 table, 3 Supplementary figures, 9 Supplementary tables

Lorsque l’Université du Québec reçoit la collection de Robert-Lionel Séguin, le recteur Louis-Edmond Hamelin mesure tout le potentiel scientifique et pédagogique de cette acquisition unique, mais aussi l’ensemble des défis qui se posent désormais à l’institution. Le projet scientifique et culturel de Séguin s’inscrit dans un environnement universitaire et avec une responsabilité publique ; comment collaborer avec le réseau muséal ? Comment assurer la pérennité de la collection et de sa mise en valeur ? Comment mettre en place une gouvernance appropriée ? La collection Séguin nous rappelle l’importance de l’affirmation d’un projet culturel inspirant et nous questionne sur le rôle des collectionneurs privés et sur les fonctions et les opérations d’un centre de conservation et d’études sur les collections., When the Université du Québec obtained the Robert-Lionel Séguin collection, President Louis-Edmond Hamelin evaluated the scholarly potential represented by this unique acquisition, while reflecting also on the challenges it created for the institution. Séguin’s intellectual and cultural endeavour was then integrated into a university environment with a unique responsibility, leading to questions about collaboration with museum networks, about long-term conservation and uses and also about how to appropriately administer the collection. The Séguin collection reaffirms the need for an inspirational cultural framework and leads us to consider the role of private collectors, as well as the function and mode of operation of a collections-based research and conservation centre.

Black Phosphorus (BP) is unique among layered materials owing to its homonuclear lattice and strong structural anisotropy. While recent investigations on few layers BP have extensively explored the in-plane (a,c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE) and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. PL reveals an unexpected b-polarized emission occurring in the visible at 1.75 eV, far above the fundamental gap (0.3 eV). PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman scans, where the scattering efficiency of both Ag phonon modes is enhanced. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

The zero-point lattice expansion (ZPLE) is a small variation of the lattice parameters induced by the presence of phonons in a material compared to the static lattice picture. It contributes significantly to the zero-point renormalization (ZPR) of the band gap energy, but its consequences have not been investigated as thoroughly as those stemming from electron-phonon interactions. In the usual first-principles approach, one evaluates the ZPLE by minimizing the $T=0$ K Helmholtz free energy. In this work, we show that the formalism based on the Gr\"uneisen parameters, which commonly neglects zero-point effects, can be efficiently used to compute ZPLE for both isotropic and anisotropic materials at much lower computational cost. We systematically test this formalism on 22 cubic and wurtzite materials and obtain excellent agreement with free energy minimization results for both the ZPLE and the resulting band gap ZPR. We use our results to validate an empirical expression estimating the ZPLE-induced ZPR and unveil its sensitivity to the temperature range involved in estimating the ZPLE from experimental data. Our findings finally reveal that the ZPLE contribution to the band gap ZPR can reach 20% to more than 80% of the electron-phonon interaction contribution for heavier or more ionic materials, including materials containing light atoms. Considering both contributions on an equal footing is thus essential should one attempt to compare theoretical ZPR results with experimental data., Comment: 30 pages, 5 figures, 14 tables. Sec. II D expanded for internal consistency, typo corrected in Fig.4., clarified Tables captions, minor corrections to the text; results and conclusions unchanged

Polarons, that is, charge carriers correlated with lattice deformations, are ubiquitous quasiparticles in semiconductors, and play an important role in electrical conductivity. To date most theoretical studies of so-called large polarons, in which the lattice can be considered as a continuum, have focused on the original Fr\"ohlich model: a simple (non-degenerate) parabolic isotropic electronic band coupled to one dispersionless longitudinal optical phonon branch. The Fr\"ohlich model allows one to understand characteristics such as polaron formation energy, radius, effective mass and mobility. Real cubic materials, instead, have electronic band extrema that are often degenerate or anisotropic and present several phonon modes. In the present work, we address such issues. We keep the continuum hypothesis inherent to the large polaron Fr\"ohlich model, but waive the isotropic and non-degeneracy hypotheses, and also include multiple phonon branches. For polaron effective masses, working at the lowest order of perturbation theory, we provide analytical results for the case of anisotropic electronic energy dispersion, with two distinct effective masses (uniaxial) and numerical simulations for the degenerate 3-band case, typical of III-V and II-VI semiconductor valence bands. We also deal with the strong-coupling limit, using a variational treatment: we propose trial wavefunctions for the above-mentioned cases, providing polaron radii and energies. Then, we evaluate the polaron formation energies, effective masses and localisation lengths using parameters representative of a dozen II-VI, III-V and oxide semiconductors, for both electron and hole polarons...In the non-degenerate case, we compare the perturbative approach with the Feynman path integral approach in characterisizing polarons in the weak coupling limit..., Comment: 18 pages, 10 figures in main text; 11 pages, 1 figure in supplementary material

Electronic and optical properties of materials are affected by atomic motion through the electron-phonon interaction: not only band gaps change with temperature, but even at absolute zero temperature, zero-point motion causes band-gap renormalization. We present a large-scale first-principles evaluation of the zero-point renormalization of band edges beyond the adiabatic approximation. For materials with light elements, the band gap renormalization is often larger than 0.3 eV, and up to 0.7 eV. This effect cannot be ignored if accurate band gaps are sought. For infrared-active materials, global agreement with available experimental data is obtained only when non-adiabatic effects are taken into account. They even dominate zero-point renormalization for many materials, as shown by a generalized Fr\"ohlich model that includes multiple phonon branches, anisotropic and degenerate electronic extrema, whose range of validity is established by comparison with first-principles results., Comment: Note that the supplementary material, included in the main file, is structured differently than in the published version, with a different numbering scheme

The topological order of a material is intrinsically related to its bulk electronic structure. Topological phase transitions require that the bulk band gap vanishes. However, the gap is affected by atomic motion through electron–phonon interaction, even at T = 0 K. As a consequence, electron–phonon interaction can either promote or suppress topologically non-trivial phases. In this work, the temperature dependence of the pressure-induced topological phase transition in Rashba semiconductor BiTeI is investigated through first-principles methods. We first present an overview of electron–phonon interaction within the framework of density-functional perturbation theory (DFPT) and derive a qualitative argument to understand how it will affect the band gap for both typical semiconductors and topological insulators. Then, by tracking both the pressure and temperature dependence of the bulk band gap, we show how the Weyl semimetal and topological insulator phases of BiTeI evolve with temperature, thus providing a guideline for experimental detection.

Paper published as part of the special topic on Electronic Structure Software, ABINIT is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe–Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the “temperaturedependent effective potential” approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which ABINIT relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The ABINIT DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library LIBPAW. ABINIT has strong links with many other software projects that are briefly mentioned., This work (A.H.R.) was supported by the DMREF-NSF Grant No. 1434897, National Science Foundation OAC-1740111, and U.S. Department of Energy DE-SC0016176 and DE-SC0019491 projects. N.A.P. and M.J.V. gratefully acknowledge funding from the Belgian Fonds National de la Recherche Scientifique (FNRS) under Grant No. PDR T.1077.15-1/7. M.J.V. also acknowledges a sabbatical “OUT” grant at ICN2 Barcelona as well as ULiège and the Communauté Française de Belgique (Grant No. ARC AIMED G.A. 15/19-09). X.G. and M.J.V. acknowledge funding from the FNRS under Grant No. T.0103.19-ALPS. X.G. and G.-M. R. acknowledge support from the Communauté française de Belgique through the SURFASCOPE Project (No. ARC 19/24-057). X.G. acknowledges the hospitality of the CEA DAM-DIF during the year 2017. G.H. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (Materials Project Program No. KC23MP). The Belgian authors acknowledge computational resources from supercomputing facilities of the University of Liège, the Consortium des Equipements de Calcul Intensif (Grant No. FRS-FNRS G.A. 2.5020.11), and Zenobe/CENAERO funded by the Walloon Region under Grant No. G.A. 1117545. M.C. and O.G. acknowledge support from the Fonds de Recherche du Québec Nature et Technologie (FRQ-NT), Canada, and the Natural Sciences and Engineering Research Council of Canada (NSERC) under Grant No. RGPIN-2016-06666. The implementation of the libpaw library (M.T., T.R., and D.C.) was supported by the ANR NEWCASTLE project (Grant No. ANR-2010-COSI-005-01) of the French National Research Agency. M.R. and M.S. acknowledge funding from Ministerio de Economia, Industria y Competitividad (MINECO-Spain) (Grants Nos. MAT2016-77100-C2-2-P and SEV-2015-0496) and Generalitat de Catalunya (Grant No. 2017 SGR1506). This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation program (Grant Agreement No. 724529). P.G. acknowledges support from FNRS Belgium through PDR (Grant No. HiT4FiT), ULiège and the Communauté française de Belgique through the ARC project AIMED, the EU and FNRS through M.ERA.NET project SIOX, and the European Funds for Regional Developments (FEDER) and the Walloon Region in the framework of the operational program “Wallonie-2020.EU” through the project Multifunctional thin films/LoCoTED. The Flatiron Institute is a division of the Simons Foundation. A large part of the data presented in this paper is available directly from the Abinit Web page www.abinit.org. Any other data not appearing in this web page can be provided by the corresponding author upon reasonable request.

Abinit is a material- and nanostructure-oriented package that implements density-functional theory (DFT) and many-body perturbation theory (MBPT) to find, from first principles, numerous properties including total energy, electronic structure, vibrational and thermodynamic properties, different dielectric and non-linear optical properties, and related spectra. In the special issue to celebrate the 40th anniversary of CPC, published in 2009, a detailed account of Abinit was included [Gonze et al. (2009)], and has been amply cited. The present article comes as a follow-up to this 2009 publication. It includes an analysis of the impact that Abinit has had, through for example the bibliometric indicators of the 2009 publication. Links with several other computational materials science projects are described. This article also covers the new capabilities of Abinit that have been implemented during the last three years, complementing a recent update of the 2009 article published in 2016. Physical and technical developments inside the abinit application are covered, as well as developments provided with the Abinit package, such as the multibinit and a-tdep projects, and related Abinit organization developments such as AbiPy . The new developments are described with relevant references, input variables, tests, and tutorials. Program summary Program Title: Abinit Program Files doi: http://dx.doi.org/10.17632/csvdrr4d68.1 Licensing provisions: GPLv3 Programming language: Fortran2003, Python Journal reference of previous version: X .Gonze et al, Comput. Phys. Commun. 205 (2016) 106–131 Does the new version supersede the previous version?: Yes. The present 8.10.3 version is now the up-to-date stable version of abinit , and supercedes the 7.10.5 version. Reasons for the new version: New developments Summary of revisions: • Many new capabilities of the main abinit application, related to density-functional theory, density-functional perturbation theory, GW, the Bethe-Salpeter equation, dynamical mean-field theory, etc. • New applications in the package: multibinit (second-principles calculations)and tdep (temperature-dependent properties) Nature of problem: Computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and vibrational lifetimes, etc. For large-scale systems, second-principles calculations, building upon the first-principles results, are also possible. Solution method: Software application based on density-functional theory and many-body perturbation theory, pseudopotentials, with plane waves or wavelets as basis functions. Different real-time algorithms are implemented for second-principles calculations.

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp2-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D1, D1′, D2 and D2′, in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the Ag1 and Ag2 modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D′ and D mo...

The effects of biaxial in-plane stress on the elastic, dielectric, and piezoelectric (PE) properties of c-axis textured thin film wurtzite phase scandium aluminum nitride (w-ScxAl1−xN) alloys have been calculated with density functional perturbation theory. The in-plane stress σR was kept below 1 GPa covering compressive and tensile values and applied to alloy supercells represented with special quasi-random structures. An increasingly tensile biaxial stress (σR > 0) produces higher displacement-response internal-strain coefficients for the constituent atoms of the alloy and the related PE properties are more sensitive to σR when the fraction x increases. A significant rise of the relative dielectric permittivity ϵr,33η and softening of the stiffness coefficient c33E are also reported with σR > 0. The effective thin film PE strain coefficient d33,f and coupling coefficient k33,f2 show a relative increase of 22% and 26%, respectively, at σR = 1 GPa and x = 0.438. Both tensile σR and x tend to decrease the c/a cell parameter ratio of the wurtzite structure with a significant impact on the PE coefficients. Based on the decomposition of the stiffness, dielectric, and PE coefficients as well as the structural data, it is suggested that tensile biaxial stress enhances the hexagonal character of w-ScxAl1−xN in a qualitatively similar manner as the scandium nitride fraction x does. The manufacture and PE characterization of a beneficially stressed thin film of w-ScxAl1−xN on a substrate of w-InyAl1−yN with adjusted x, y values are suggested to confirm the calculated values of d33,f.

Although correlated electronic-structure calculations explain very well the normal state of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, its superconducting symmetry is still unknown. Here we construct the spin and charge fluctuation pairing interactions based on its correlated normal state. Correlations significantly reduce ferromagnetic in favor of antiferromagnetic fluctuations and increase interorbital pairing. From the normal-state Eliashberg equations, we find spin-singlet $d$-wave pairing close to magnetic instabilities. Away from these instabilities, where charge fluctuations increase, we find two time-reversal symmetry-breaking spin triplets: an odd-frequency $s$ wave, and a doubly degenerate interorbital pairing between ${d}_{xy}$ and (${d}_{yz},{d}_{xz}$).

International audience; Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin layer BP are photo oxidized in ambient conditions and the energy of their electronic states shift in different dielectric environment. We prepared free-standing samples of few layer BP in glovebox conditions and probed the dielectric response in vacuum using Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). Thresholds of the excitation energy are measured at 1.9 eV, 1.4 eV and 1.1 eV for the mono-bi-and tri-layer BP, respectively and these values are used to estimate the corresponding optical band gaps. A comparison of our results with electronic structure calculations indicates that the variation of the quasi-particle gap is larger than that of the exciton binding energy. The dispersion of the plasmons versus momentum for 1-3 layer BP and bulk BP highlights a deviation from parabolic to linear dispersion and strong anisotropic fingerprints.

Raman spectrum is a common spectroscopic tool used in materials synthesis and characterization. The width and shift of Raman peaks are commonly used in characterization of graphene samples. In this presentation, I will highlight our efforts in using machine learned models to develop an efficient and accurate methodology for simulating accurate spectroscopic signatures in graphene. We are using deep neural networks trained with density functional theory calculations to take advantage of the accuracy of this approach while being able to simulate large atomic systems corresponding to doped graphene layers. We are then able to evaluate the empirical models presently used to assess the defect levels in graphene and compare them to state-of-the-art calculation on such systems.

A novel sulfate material, Na 2 Fe 2 (SO 4 ) 3 , was recently proposed as a high-density cathode material for sodium-based batteries (Barpanda et al. , 2014). This study presents ab initio simulations describing the effect of partial oxygen-to-nitrogen substitution in this material with the aim of increasing the sodium capacity of the structure as well as its energy density. The considered structure with the most likely substitution configuration increases the theoretical energy density to 650 W h kg −1 , improving over the theoretical value of 480 W h kg −1 for the original, nitrogen-free structure.

Atomic layers of Black Phosphorus (BP) have been recently isolated, ten years after graphene. BP stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known. To start with, we have investigated photoluminescence (PL) and absorption properties of high quality BP single crystals at 2K [FTIR]. The PL intensity appears comparable to high quality InAs crystals confirming the suitability of BP for infrared applications. Two peaks are evidenced at 0.275 eV and 0.26 eV. The highest energy peak is particularly narrow (FWHM = 3.8meV) and is tentatively attributed to exciton recombination, thanks to the temperature dependence of PL spectra from 2K to 300K. Further, we have measured the gap related energy shift of the PL spectra as a function of the BP thickness in a series of mechanically exfoliated samples with thicknesses down to 10 nm, complementary to the previous measures done on thinner samples [3]. As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5]. [1] G. Zhang et al., Nat. Com., 8, 14071, (2017) [2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826. [3] C. Chen et al., NanoLett., (2019) [4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017) [5] E. Gaufres et al, Nanoletters (2019); DOI: 10.1021/acs.nanolett.9b03928

For graphene to be used efficiently in scientific and technological applications, its electronic properties need to be tailored to suit precise demands. The formation of a forbidden energy gap or the modification of the Fermi energy is achievable with functionalization techniques that simultaneously leave most of the atomic structure of graphene unchanged. Nitrogen functionalization is one of the most promising and studied types of treatment, in large part due to the proximity between carbon and nitrogen in the periodic table. It has already produced promising experimental results in the fields of high-sensibility biodetectors, field-effect transistors, solar cells, and supercapacitors, among others. To further understand this functionalization, we have performed an ab initio electronic structure study of different doping configurations and functionalization dynamics of nitrogen treatments of graphene. In particular, we have study the effect of simulation cell size used to carry our these calculations. We found that the inclusion of the special point K of the first Brillouin Zone in the momentum space sampling lowers formation energy results and should not be overlooked. Nudged Elastic Band calculations were also completed to study different incorporation mechanisms from an adsorbed state to in-plane doping. In the presence of native defects, low barriers of 0.55 eV and 0.46 eV were obtained. When no defects are present, much larger barriers between 3.70 eV to 4.38 eV were found, which suggests an external source of energy is required to complete the incorporation. In the second part of this presentation, we will report our result of using machine learning models to predict higher-level quantities. The goal of this work is to study the dependence of the Raman response of nitrogen functionalization of graphene. To simulate such a response, very large simulation cells need to be considered that would be out of reach for ab initio techniques, hence the use of machine learning methods to assist in the evaluation of the properties. Machine learning methods are now used more and more as a substitute for density functional theory calculations due to their low computational costs. However, in more advanced cases, relevant datasets are not always available, and the effort that would be necessary to generate the needed data suppress the advantages of using machine learning to speed up the calculations. Furthermore, the process of training a reliable model is not trivial and can also be expensive which should encourage the usage and diffusion of portable trained models. Figure 1

Identification of the electronic state that appears upon doping a Mott insulator is important to understand the physics of cuprate high-temperature superconductors. Recent scanning tunneling microscopy of cuprates provides evidence that a charge-ordered state emerges before the superconducting state upon doping the parent compound. We study this phenomenon by computing the charge response function of the Hubbard model including frequency-dependent local vertex corrections that satisfy the compressibility sum-rule. We find that upon approaching the Mott phase from the overdoped side, the charge fluctuations at wave vectors connecting hot spots are suppressed much faster than at the other wave-vectors. It leads to a momentum dependence of the dressed charge susceptibility that is very different from either the bare susceptibility or from the susceptibility obtained from the random phase approximation. We also find that the paramagnetic lightly hole-doped Mott phase at finite-temperature is unstable to charge ordering only at zero wave-vector, confirming the results previously obtained from the compressibility. Charge order is driven by the frequency-dependent scattering processes that induce an attractive particle-hole interaction at large interaction strength and small doping., LaTex, 13 pages, 7 figures

We report experimental and theoretical studies on the magnetic and thermodynamic properties of NIT-2Py, a free radical-based organic magnet. From magnetization and specific heat measurements we establish the temperature versus magnetic field phase diagram which includes two Bose-Einstein condensates (BEC) and an infrequent half magnetization plateau. Calculations based on density functional theory demonstrates that magnetically this system can be mapped to a quasi-two-dimensional structure of weakly coupled tetramers. Density matrix renormalization group calculations show the unusual characteristics of the BECs where the spins forming the low-field condensate are different than those participating in the high-field one., Comment: 12 pages, 12 figures

Au Québec, l’urbanisation continue à gagner sur la campagne. Le phénomène se poursuit en dépit de la condamnation dont il fait l’objet et des mesures prises à son encontre. Divers arguments sont avancés pour expliquer ce paradoxe : complaisance de la classe politique, laxisme des administrations territoriales, efficacité en sous-main de divers groupes de pression. L’affaire est toutefois d’un autre ressort. En fait, lois et règlements ménagent clairement des possibilités de poursuivre l’urbanisation diffuse, y encouragent même parfois, par-delà les dénégations de principe. Faut-il y voir une regrettable inadvertance ? L’analyse montre que cette pratique obéit plutôt à un désir dont l’assise culturelle est tellement forte qu’il transcende toute norme devant, en principe, s’y opposer., Urban sprawl continues to encroach on rural land in Québec, despite widespread condemnation of the process and policies designed to curb its spread under Québec’s agricultural land preservation program. A variety of explanations are often put forward to explain the paradox: a weak political class structure, laxity on the part of regional administrations or the effectiveness of different behind-the-scenes pressure groups. In reality, none of these interpretations provides the analytical framework needed to understand the phenomenon. The issue actually lies at an entirely different level since, in fact, legislation and regulations clearly allow for continuing dispersed urban growth, sometimes even encouraging it, in spite of denials in principle. The interpretation of Québec’s agricultural land preservation program that we propose in this article refuses to give credence to arguments centered on unfortunate oversights of one form or another, and is based instead on our conviction that societies function in a world conditioned by cultural considerations. Understood in this light, the different exigencies surrounding the management of urbanization can be seen to stem from an underlying conception of land use in a given area, driven by far more powerful motivations than any learned discourse or clever calculations., La urbanización sigue difundiéndose en los territorios rurales quebequenses y eso, a pesar de ser objeto de desaprobación y de políticas de contención propuestas por el régimen quebequense para proteger el territorio agrícola. Se pueden considerar varias explicaciones para resolver esa paradoja: fragilidad de la clase política, laxismo de las administraciones territoriales, eficacia oculta de diversos grupos de presión. Y bien, ninguna de estas está a la altura del fenómeno. El asunto parece de otra índole. En realidad; leyes y reglamentos preservan claramente las posibilidades de continuar la urbanización difusa y hasta las alienta, pese a las denegaciones de principio. Negándonos de ver en ello lamentables inadvertencias, convencidos que las sociedades viven en un mundo condicionado por motivos culturales, nuestra interpretación del régimen quebequense de protección del territorio agrícola asume que las diversas normas relacionadas con la gestión de la urbanización resultan de una visión subyacente del territorio, cuya coherencia interna está afirmada por un deseo más fuerte que todos esos hermosos discursos y sus cálculos rebuscados.

We propose NaFeZr2F11 and NaFeTi2F11 as novel materials for Na intercalation cathodes. The average sodium intercalation voltage for these compounds is about 4.0 V from first principles computations. The origin of this large voltage for a nominal Fe2+/Fe3+ redox couple is proposed to be a combination of the fluorine “booster” effect, the inductive effect and the stiffness of the proposed structure upon desodiation. Keywords: Sodium battery, Ab initio simulations, Cathode material

We report anomalous antiresonances in the infrared spectra of doped and disordered single layer graphene. Measurements in both reflection microscopy and transmission configurations of samples grafted with halogenophenyl moieties are presented. Asymmetric transparency windows at energies corresponding to phonon modes near the Γ and K points are observed, in contrast to the featureless spectrum of pristine graphene. These asymmetric antiresonances are demonstrated to vary as a function of the chemical potential and defect density. We propose a model that involves coherent intraband scattering with defects and phonons, thus relaxing the optical selection rule forbidding access to q ≠ Γ phonons. This interpretation of the new phenomenon is supported by our numerical simulations that reproduce the experimental features.

ABINIT is a package whose main program allows one to find the total energy, charge density, electronic structure and many other properties of systems made of electrons and nuclei, (molecules and periodic solids) within Density Functional Theory (DFT), Many-Body Perturbation Theory (GW approximation and Bethe-Salpeter equation) and Dynamical Mean Field Theory (DMFT). ABINIT also allows to optimize the geometry according to the DFT forces and stresses, to perform molecular dynamics simulations using these forces, and to generate dynamical matrices, Born effective charges and dielectric tensors. The present paper aims to describe the new capabilities of ABINIT that have been developed since 2009. It covers both physical and technical developments inside the ABINIT code, as well as developments provided within the ABINIT package. The developments are described with relevant references, input variables, tests and tutorials. Program summary: . Program title: ABINIT. . Catalogue identifier: AEEU_v2_0. . Program summary URL: . http://cpc.cs.qub.ac.uk/summaries/AEEU_v2_0.html . . Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland. . Licensing provisions: GNU General Public License, version 3. . No. of lines in distributed program, including test data, etc.: 4845789. . No. of bytes in distributed program, including test data, etc.: 71340403. . Distribution format: tar.gz. . Programming language: Fortran2003, PERL scripts, Python scripts. . Classification: 7.3, 7.8. . External routines: (all optional) BigDFT [2], ETSF_IO [3], libxc [4], NetCDF [5], MPI [6], Wannier90 [7], FFTW [8]. . Catalogue identifier of previous version: AEEU_v1_0. . Journal reference of previous version: Comput. Phys. Comm. 180 (2009) 2582. . Does the new version supersede the previous version?: Yes. The abinit-7.10.5 version is now the up to date stable version of ABINIT. . Nature of problem: . This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and vibrational life-times, and others. . Solution method: . Software application based on Density Functional Theory, Many-Body Perturbation Theory and Dynamical Mean Field Theory, pseudopotentials, with plane waves or wavelets as basis functions. . Reasons for new version: . Since 2009, the abinit-5.7.4 version of the code has considerably evolved and is not yet up to date. The abinit- 7.10.5 version contains new physical and technical features that allow electronic structure calculations impossible to carry out in the previous versions. . Summary of revisions: . •new physical features: quantum effects for the nuclei treated by the Path-integral Molecular Dynamics; finding transition states using image dynamics (NEB or string methods); two component DFT for electron-positron annihilation; linear response in a Projector Augmented-Wave approach -PAW-, electron-phonon interactions and temperature dependence of the gap; Bethe Salpeter Equation -BSE-; Dynamical Mean Field Theory (DMFT).•new technical features: development of a PAW approach for a wavelet basis; parallelisation of the code on more than 10,000 processors; new build system.•new features in the ABINIT package: tests; test farm; new tutorials; new pseudopotentials and PAW atomic data tables; GUI and postprocessing tools like the AbiPy and APPA libraries. . Running time: . It is difficult to answer to the question as the use of ABINIT is very large. On one hand, ABINIT can run on 10,000 processors for hours to perform quantum molecular dynamics on large systems. On the other hand, tutorials for students can be performed on a laptop within a few minutes. . References: . 1 http://www.gnu.org/copyleft/gpl.txt 2 http://bigdft.org 3 http://www.etsf.eu/fileformats 4 http://www.tddft.org/programs/octopus/wiki/index.php/Libxc 5 http://www.unidata.ucar.edu/software/netcdf 6 https://en.wikipedia.org/wiki/Message_Passing_Interface 7 http://www.wannier.org 8M. Frigo and S.G. Johnson, Proceedings of the IEEE, 93, 216-231 (2005). . . (Less)

We report on de Haas--van Alphen (dHvA) and band-structure studies of the iridium-pnictide superconductor ${\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$ (${T}_{c}=2.1$ K). The observed dHvA frequencies can be well understood by our band-structure calculations with two bands crossing the Fermi energy leading to a strongly corrugated Fermi-surface cylinder around the X point and a highly evolved, multiconnected Fermi surface extending over the whole Brillouin zone. The experimental effective masses are found to be considerably larger than the calculated band masses suggesting strong many-body interactions. Nevertheless, ${T}_{c}$ remains only moderate in ${\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$ contrary to isostructural iron pnictides which probably is related to the largely different Fermi-surface topologies in these materials.

We present a $G_0W_0$ implementation that assesses the two major bottlenecks of traditional plane-waves implementations, the summations over conduction states and the inversion of the dielectric matrix, without introducing new approximations in the formalism. The first bottleneck is circumvented by converting the summations into Sternheimer equations. Then, the novel avenue of expressing the dielectric matrix in a Lanczos basis is developed, which reduces the matrix size by orders of magnitude while being computationally efficient. We also develop a model dielectric operator that allows us to further reduce the size of the dielectric matrix without accuracy loss. Furthermore, we develop a scheme that reduces the numerical cost of the contour deformation technique to the level of the lightest plasmon pole model. Finally, the use of the simplified quasi-minimal residual scheme in replacement of the conjugate gradients algorithm allows a direct evaluation of the $G_0W_0$ corrections at the desired real frequencies, without need for analytical continuation. The performance of the resulting $G_0W_0$ implementation is demonstrated by comparison with a traditional plane-waves implementation, which reveals a 500-fold speedup for the silane molecule. Finally, the accuracy of our $G_0W_0$ implementation is demonstrated by comparison with other $G_0W_0$ calculations and experimental results.

Covalent addition of functional groups onto carbon nanotubes is known to generate lattice point defects that disrupt the electronic wave function, resulting namely in a reduction of their optical response and electrical conductance. Here, conductance measurements combined with numerical simulations are used to unambiguously identify the presence of graft-induced midgap states in the electronic structure of covalently functionalized semiconducting carbon nanotubes. The main experimental evidence is an increase of the conductance in the OFF-state after covalent addition of 4-bromophenyl grafts on many single- and double-walled individual nanotubes, the effect of which is fully suppressed after thermodesorption of the adducts. The graft-induced current leakage is thermally activated and can reach several orders of magnitude above its highly insulating pristine-state level. Ab initio simulations of various configurations of functionalized nanotubes corroborate the presence of these midgap states and show their localization around the addends. Moreover, the electronic density of these localized states exhibits an extended hydrogenoid profile along the nanotube axis, providing access for long-range coupling between the grafts. We argue that covalent nanotube chemistry is a powerful tool to prepare and control midgap electronic states on nanotubes for enabling further studies of the intriguing properties of interacting 1D localized states.

The renormalization of the band structure at zero temperature due to electron-phonon coupling is explored in diamond, BN, LiF, and MgO crystals. We implement a dynamical scheme to compute the frequency-dependent self-energy and the resulting quasiparticle electronic structure. Our calculations reveal the presence of a satellite band below the Fermi level of LiF and MgO. We show that the renormalization factor $(Z)$, which is neglected in the adiabatic approximation, can reduce the zero-point renormalization (ZPR) by as much as $40%$. Anharmonic effects in the renormalized eigenvalues at finite atomic displacements are explored with the frozen-phonon method. We use a nonperturbative expression for the ZPR, going beyond the Allen-Heine-Cardona theory. Our results indicate that high-order electron-phonon coupling terms contribute significantly to the zero-point renormalization for certain materials.

Ultra-thin Black Phosphorus (BP) is a 2D semiconductor characterized by a direct and tunable band gap associated to high carriers mobility (1-3). Studying suspended thin layers of pristine BP is however challenging due to its strong degradation through a thickness dependent and photo-assisted oxidation reaction by adsorbed moisture oxygen (4). Using a protective transfer procedure from glovebox to our TEM-EELS operating at 40kV, we probe the dielectric response of suspended BP down to the monolayer in the range 0.5-40 eV, that include band gaps thresholds and surface/volume plasmons energies for BP. The dispersion of the plasmons as a function of the momentum is also measured and simulated for both in-plane crystallographic directions. (1)Liu et al, ACS Nano (2014) (2)Qiao et al, Nature Comm. (2014) (3) Long et al, Nano Letter (2016) (4)Favron et al, Nature Mater. (2015)

The Drude-like response of graphene in the terahertz and infrared region of the spectrum has made it attractive for optoelectronic applications in this range, because the response can be controlled by gating and doping. [1] Graphene infrared response can further be tailored for photonics and plasmonics, as the patterned material harbors low energy plasmon modes. [2] However, the infrared spectrum of pristine single layer graphene (SLG) is monotonous; in contrast to Raman, there are no infrared-active phonon modes, while bilayer graphene displays a Fano resonance in the infrared at ~1600 cm-1. [3] In a first time, we show experimentally that grafting SLG with halogenophenyl moieties induces optical transparencies at two specific energies: 1250 and 1600 cm-1 [4], in close similarity to the bands that can also be observed in carbon nanotubes as Fano resonances. [5] Unlike bands caused by the absorption of light by vibrational modes, these antiresonances show a decrease of the absorbance, an optical transparency effect. Moreover, we show that the amplitude of the transparencies can be modulated by changing the charge carrier density through doping, and by the defect density through controlled grafting. In as second time, we will present a theory based on quantum mechanics to calculate the optical conductivity of grafted SLG. [4] The model puts into play phonon modes with momenta different from Γ that can be addressed through scattering on defects. Numerical simulations reproduce the experimental data with good agreement. The theory also captures the dependence of the signal on charge carrier density and defect density. Our findings bring a new understanding for the physics behind the infrared activity of nanostructures, while opening new capabilities for tailoring the optical spectrum of nanomaterials. References [1] Horng et al. (2011) Phys Rev B 83:165113 [2] Low & Avouris (2014) ACS Nano 8:1086 [3] Kuzmenko et al. (2009) Phys Rev Lett 103:116804 [4] Rousseau et al. (2014) arXiv preprint arxiv:1407.8141 [5] Lapointe et al. (2012) Phys Rev Lett 109:097402