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The family of Van der Waals dichalcogenides (VdWDs) includes a large number of compositions and phases, exhibiting varied properties and functionalities. They have opened up a novel electronics of two-dimensional materials, characterized by higher integration and interfaces which are atomically sharper and cleaner than conventional electronics. Among these functionalities, some VdWDs possess remarkable thermoelectric properties. SnSe2 has been identified as a promising thermoelectric material on the basis of its estimated electronic and transport properties. In this work we carry out experimental meas-urements of the electric and thermoelectric properties of SnSe2 flakes. For a 30 micron thick SnSe2 flake at room temperature, we measure electron mobility of 40 cm^2 V^-1 s^-1, a carrier density of 4 x 10^18 cm^-3, a Seebeck coefficient S around -400 microV/K and thermoelectric power factor around 0.35 mW m^-1 K^-2. The comparison of experimental results with theoretical calculations shows fair agreement and indicates that the dominant carrier scattering mechanisms are polar optical phonons at room temperature and ionized im-purities below 50 K. In order to explore possible improvement of the thermoelectric properties, we carry out reversible electrostatic doping on a thinner flake, in a field effect setup. On this 75 nm thick SnSe2 flake, we measure a field effect variation of the Seebeck coefficient of up to 290 % at low temperature, and a corresponding variation of the thermoelectric power factor of up to 1050 %. We find that the power factor increases with the depletion of n-type charge carriers. Field effect control of thermoelectric transport opens perspectives for boosting energy harvesting and novel switching technologies based on two-dimensional materials., in press on Physical Review Materials

Polarons are quasi-particles made from electrons interacting with vibrations in crystal lattices. They derive their name from the strong electron-vibration polar interaction in ionic systems, that induces associated spectroscopic and optical signatures of such quasi-particles in these materials. In this paper, we focus on diamond, a non-polar crystal with inversion symmetry which nevertheless shows characteristic signatures of polarons, better denoted "nonpolarons" in this case. The polaronic effects are produced by short-range crystal fields with only a small influence of long-range quadrupoles. The many-body spectral function has a characteristic energy dependence, showing a plateau structure that is similar to but distinct from the satellites observed in the polar Fr\"{o}hlich case. The temperature-dependent spectral function of diamond is determined by two methods: the standard Dyson-Migdal approach, which calculates electron-phonon interactions within the lowest-order expansion of the self-energy, and the cumulant expansion, which includes higher orders of electron-phonon interactions. The latter corrects the nonpolaron energies and broadening, providing a more realistic spectral function, which we examine in detail for both conduction and valence band edges., Comment: 10 pages 16 Figures + Subfigures 2 Tables 82 References Supplementary Information (SI) at the end: SI 10 pages SI 14 Figures + Subfigures SI 1 Table

Crossed magneto-thermo-electric coefficients are central to novel sensors and spin(calori)tronic devices. Within the framework of Boltzmann’s transport theory, we calculate the resistivity and Seebeck coefficients of the most common 3d ferromagnetic metals: Fe, Co, and Ni. We use a fully first-principles variational approach, explicitly taking electron-phonon scattering into account. The electronic band structures, phonon dispersion curves, phonon linewidths, and transport spectral functions are reported, comparing with experimental data. Successive levels of approximation are discussed: constant relaxation time approximation, scattering for a non-magnetic configuration, then spin polarized calculations with and without spin–orbit coupling (enabling spin-flips). Spin polarization and explicit electron–phonon coupling are found to be necessary to reach a correct qualitative picture: the effect of spin flipping is substantial for resistivity and very delicate for the Seebeck coefficient. The spin-dependent Seebeck effect is also predicted.

Nickel titanium, also know as nitinol, is a prototypical shape memory alloy, a property intimately linked to a phase transition in the microstructure, which allows the meso/macroscopic sample shape to be recovered after thermal cycling. Not much is known about the other alloys in this binary system, which prompted our computational investigation of other compositions. In this work, structures are found by probing the potential energy surfaces of NiTi binary systems using a minima hopping method, in combination with ab initio electronic structure calculations. We find stable structures in 34 different stoichiometries and calculate derived physical properties of the low energy phases. From the results of this analysis a new convex hull is formed that is lower in energy than those in the Materials Project and Open Quantum Materials Databases. Two previously unreported phases are discovered for the NiTi2 and Ni5Ti compositions, and two metastable states in NiTi and NiTi2 shows signs of negative linear compression and negative Poisson ratio, respectively., We acknowledge the computational resources awarded by XSEDE, a project supported by National Science Foundation Grant No. ACI-1053575. We acknowledge the support from the Texas Advances Computer Center (with the Stampede2 and Bridges supercomputers). This work was supported by the DMREF-NSF 1434897, NSF OAC-1740111, and DOE DE-SC0021375 projects. M.J.V. acknowledges funding by the Belgian FNRS (PDR G.A. T.1077.15-1/7, T.0103.19, and a sabbatical “OUT” grant at ICN2), ULiege, and the Communauté Française de Belgique (ARC AIMED G.A. 15/19-09) and computational resources from the Consortium des Equipements de Calcul Intensif (FRS-FNRS G.A. 2.5020.11) and Zenobe/CENAERO funded by the Walloon Region under G.A. 1117545.

We present an unexpectedly strong influence of the proximity effect between the bulk Ru(0001) superconductor and atomically thin layers of Co on the crystal structure of the latter. The Co monolayer grows in two different modifications, such as hcp stacking and a reconstructed ε-like phase. While hcp islands show a weak proximity effect on Co and a little suppression of superconductivity in the substrate next to it, the more complex ε-like stacking becomes almost fully superconducting. We explain the weak proximity effect between Ru and hcp Co and the rather abrupt jump of the superconducting order parameter by a low transparency of the interface. In contrast, the strong proximity effect without a jump of the order parameter in the ε-like phase indicates a highly transparent interface. This work highlights that the proximity effect between a superconductor and a normal metal strongly depends on the crystal structure of the interface, which allows to engineer the proximity effect in hybrid structures.

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe

We carry out density functional theory (DFT) calculations to explore the antiferromagnetic (AFM) spin cycloid in multiferroic ${\mathrm{BiFeO}}_{3}$ of the $R3c$ ground state structure. We calculate the energy dispersion $E(\mathbf{q})$ of cycloidal spin spirals along the high symmetry directions of the pseudo-cubic unit cell and find a flat AFM spin spiral (or cycloid) ground state with a periodicity of $\ensuremath{\sim}80$ nm, which is in good agreement with experiments. To investigate which structural distortion of the $R3c$ phase is the driving mechanism for the stabilization of this cycloid, we further study three artificial phases: cubic, $R\overline{3}c$, and $R3m$. In all cases, we find a large exchange frustration. The comparison between these phases provides detailed insight about how polarization and octahedral antiphase tilting affect the different magnetic interactions and the magnetic ground state in ${\mathrm{BiFeO}}_{3}$. In $R3c\phantom{\rule{4pt}{0ex}}{\mathrm{BiFeO}}_{3}$, the magnetic ground state is driven by a competition between the frustrated exchange stemming from the hybridization between the elements Bi, Fe, O and the Dzyaloshinskii-Moriya (DM) interaction due to the Fe-Bi ferroelectric displacement. The cycloid appears to be stable because the anisotropy energy in $R3c\phantom{\rule{4pt}{0ex}}{\mathrm{BiFeO}}_{3}$ is relatively small to enforce a collinear order.

Author(s): Rattanachata, A; Nicolai, LC; Martins, HP; Conti, G; Verstraete, MJ; Gehlmann, M; Ueda, S; Kobayashi, K; Vishik, I; Schneider, CM; Fadley, CS; Gray, AX; Minar, J; Nemsak, S | Abstract: In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials.

Van der Waals heterostructures provide a versatile tool to not only protect or control, but also enhance the properties of a 2D material. We use ab initio calculations and semianalytical models to find strategies which boost the mobility of a current-carrying two-dimensional (2D) semiconductor within a heterostructure. Free-carrier screening from a metallic ``screener'' layer remotely suppresses electron-phonon interactions in the current-carrying layer. This concept is most effective in 2D semiconductors whose scattering is dominated by screenable electron-phonon interactions, and in particular, the Fr\"ohlich coupling to polar-optical phonons. Such materials are common and characterized by overall low mobilities in the small doping limit and much higher ones when the 2D material is doped enough for electron-phonon interactions to be screened by its own free carriers. We use GaSe as a prototype and place it in a heterostructure with doped graphene as the ``screener'' layer and boron nitride as a separator. We develop an approach to determine the electrostatic response of any heterostructure by combining the responses of the individual layers computed within density functional perturbation theory. Remote screening from graphene can suppress the long-wavelength Fr\"ohlich interaction, leading to a consistently high mobility around $500--600\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$/V s for carrier densities in GaSe from ${10}^{11}$ to ${10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Notably, the low-doping mobility is enhanced by a factor 2.5. This remote free-carrier screening is more efficient than more conventional manipulation of the dielectric environment, and it is most effective when the separator (boron nitride) is thin.

Stable structures of layered SnS and SnSe and their associated electronic and vibrational spectra are predicted using first-principles DFT calculations. The calculations show that both materials undergo a phase transformation upon thinning whereby the in-plane lattice parameters ratio a/b converges towards 1, similar to the high-temperature behaviour observed for their bulk counterparts. The electronic properties of layered SnS and SnSe evolve to an almost symmetric dispersion whilst the gap changes from indirect to direct. Characteristic signatures in the phonon dispersion curves and surface phonon states where only atoms belonging to surface layers vibrate should be observable experimentally., The authors gratefully acknowledge funding from the Belgian Fonds National de la Recherche Scientifique FNRS (PDR T.1 077.15-1/7) and the Communauté Française de Belgique (ARC AIMED 15/19-09). Computational resources have been provided by the Consortium des Equipements de Calcul Intensif (CECI), funded by FRS-FNRS G.A. 2.502 0.11; the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, funded by the Walloon Region under G.A. 1117545; and by PRACE-IP DECI grants, on ARCHER, Beskow and Salomon (ThermoSpin, ACEID, OPTOGEN, and INTERPHON 3IP G.A. FP7 RI-312763 and 13 G.A. 653838 of H2020). ZZ acknowledges financial support by the Ramon y Cajal program (RYC-2016-19344), the Spanish program PGC2018-096955-B-C43 (MICIU/AEI/DEDER,UE) the CERCA programme of the Generalitat de Catalunya (Grant 2017SGR1506), and by the Severo Ochoa programme (MINECO, SEV-2017-0706). We are also grateful to the Royal Society, the European Research Council (ERC-2009-StG-240500-DEDIGROWTH), and the Engineering and Physical Sciences Research Council block grants for their financial support.

We perform first-principles calculations to study the structural, mechanical, thermal, electronic, and magnetic properties of CrMoB for x = 0.25, 0.33, 0.50, 0.67 and 0.75. Based on structural search methods, we determine the ground-state structure for each concentration. The ternaries are either monoclinic (x = 0.25, 0.75) or trigonal (x = 0.33, 0.50, 0.67). The calculated mechanical properties reveal that the strength of CrMoB is maximized for x = 0.50. CrMoB exhibits excellent mechanical properties (B = 298 GPa, Y = 558 GPa, G = 235 Gpa, ν = 0.19, H =27 GPa), surpassing those of β-MoB at a lower cost. All of these ternaries are hard alloys with Vickers hardness greater than 24 GPa. Chemical bonding analysis demonstrates that the strength of the new compounds is related to the alternating planar and buckled B-B layers, as well as the strong TM-B bonds. The enhanced strength of CrMoB is a consequence of the high density of strong interlayer Cr-Mo metallic bonds around the Fermi level., This work used the XSEDE which is supported by the USA National Science Foundation (NSF) grant number ACI-1053575. The authors also acknowledge the support from the Texas Advances Computer Center (with the Stampede2 and Bridges supercomputers). We also acknowledge the use of the Super Computing System (Thorny Flat) at WVU, which is funded in part by the NSF Major Research Instrumentation Program (MRI) Award #1726534. This work was supported by the DMREF-NSF 1434897, NSF OAC-1740111 and DOE DE-SC0021375 projects.

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10000 cm2 V-1 s-1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices., We would like to thank Andrea Tomadin for discussions. The authors acknowledge funding from the European Union Horizon 2020 Programme under Grant Agreement No. 881603 Graphene Core 3. ICN2 was supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). A.P. acknowledges support from the European Commission under the EU Horizon 2020 MSCA-RISE-2019 programme (project 873028 HYDROTRONICS) and from the Leverhulme Trust under grant RPG-2019-363. K.J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL), RyC fellowship No. RYC-2017-22330, and IAE project PID2019-111673GB-I00 and financial support through the MAINZ Visiting Professorship. X.J. acknowledges the support from the Max Planck Graduate Center with the Johannes Gutenberg-Universität Mainz (MPGC). J.Z. acknowledges the support from National Natural Science Foundation of China (No. 52072042). Z.L. acknowledges the support from National Natural Science Foundation of China (No. 51520105003). T.S. acknowledges support from the University of Liege under Special Funds for Research, IPD-STEMA Programme. M.J.V. gratefully acknowledges funding from the Belgian Fonds National de la Recherche Scientifique (FNRS) under PDR grant T.0103.19-ALPS. Computational resources were provided by CECI (FRS-FNRS G.A. 2.5020.11) and the Zenobe Tier-1 supercomputer (Gouvernement Wallon G.A. 1117545) and by a PRACE-3IP DECI grant 2DSpin and Pylight on Beskow (G.A. 653838 of H2020). ICFO was supported by the Severo Ochoa program for Centers of Excellence in R&D (CEX2019-000910-S), Fundació Privada Cellex, Fundació Privada Mir-Puig, and the Generalitat de Catalunya through the CERCA program. N.v.H. acknowledges funding by the European Commission (ERC AdG 670949-LightNet), the Spanish Plan Nacional (PGC2018-096875-BI00), and the Catalan AGAUR (2017SGR1369).

Interfaces are ubiquitous in materials science, and in devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications, here for spintronics. Using first-principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor BaMnO3. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from antiferromagnetic to ferromagnetic. The graphene- BaMnO3 system presents a Rashba gap 300 times larger than in pristine graphene, leading to a flavor of quantum anomalous Hall effect (QAHE), a hybrid QAHE, characterized by the coexistence of metallic and topological insulating states. These findings could be exploited to fabricate devices that use graphene to control the magnetic configuration of a substrate., Z.Z. acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG) Grant No. ZA 780/3-1, the Ramon y Cajal MINECO program (Grant No. RYC-2016-19344), the EC H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598), the Spanish MINECO (Grant No. FIS2015-64886-C5-3-P), the CERCA programme of the Generalitat de Catalunya (Grant No. 2017SGR1506), and by the Severo Ochoa programme (MINECO, Grant No. SEV-2013-0295). M.J.V. acknowledges funding from the Communauté française de Belgique ARC grant (Grant No. AIMED 15/19-09). Y.M. acknowledges funding from the German Research Foundation (Deutsche Forschungsgemeinschaft), Grant No. MO 1731/5-1. This work has been also financially supported by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center SPP 1666. The authors acknowledge computer time from the PRACE-3IP and 4IP on resources Lindgren, Archer, and Salomon (EU Grants No. RI-312763 and No. 653838), CECI, SEGI-ULg and Zenobe hosted by CENAERO (Grant No. GA 1117545), the JARA-HPC projects (No. jara0088, No. JIAS16, and No. JHPC39), the JARA-HPC Vergabegremium and VSR commission on the supercomputer JURECA at Forschungszentrum Jülich.

Interest in layered two dimensional materials, particularly stacked heterostructures of transition metal dichalcogenides, has led to the need for a better understanding of the structural and electronic changes induced by stacking. Here, we investigate the effects of idealized heterostructuring, with periodic commensurate stacking, on the structural, electronic, and vibrational properties, when compared to the counterpart bulk transition metal dichalcogenide. We find that in heterostructures with dissimilar chalcogen species there is a strong compression of the inter-layer spacing, compared to the bulk compounds. This compression of the heterostructure is caused by an increase in the strength of the induced polarization interaction between the layers, but not a full charge transfer. We argue that this effect is real, not due to the imposed commensurability, and should be observable in heterostructures combining different chalcogens. Interestingly, we find that incommensurate stacking of Ti-based dichalcogenides leads to the stabilization of the charge density wave phonon mode, which is unstable in the 1T phase at low temperature. Mixed Ti- and Zr-heterostructures are still unstable, but with a charge density wave or a ferroelectric instability., 10 pages, 3 tables, 4 figures

The temperature dependences of both heat capacity and thermal conductivity in nanolamellar Cr2AlC single crystals are measured using modulated photothermal radiometry and compared to first-principles calculations. The electronic contribution to the thermal conductivity of Cr2AlC single crystals is computed ab initio by determining the electronic transport coefficients using density functional theory and by solving the Bloch−Boltzmann transport equation with a temperaturedependent relaxation time. The lattice thermal conductivity is predicted by going beyond the quasi-harmonic approximation and considering renormalized second- and third-order force constant matrices, with anharmonic three-phonon scattering, isotopic scattering, and scattering by carbon vacancies. Isotopic scattering does not modify the lattice thermal conductivity. In contrast, even a small concentration of carbon vacancies induces a substantial decrease of the in-plane lattice thermal conductivity. The anisotropy measured in the thermal conductivity, with a ratio of ∼2 over the whole temperature range, is confirmed theoretically. This anisotropy seems to mainly arise from lattice contributions. A similar anisotropy is expected for other MAX phases with identical layered structures.

The Seebeck coefficient is one of the key ingredients in thermoelectric properties, and it is often calculated based simply on the electronic band structure, within the frame of Boltzmann's transport theory and the constant relaxation time approximation. Despite the simplicity and popularity of this approximation, its validity is not fully justified even in lightly doped semiconductors, and it breaks down completely in metals. On the other hand, more sophisticated first-principles approaches are available but require the computation of the full electron-phonon coupling. Here, we demonstrate with several simple (alkali and noble) metals viz., Li, Na, K, Cu, Ag, Au, and Pt, that the variational approach based on ab initio couplings can reproduce experimental Seebeck coefficients quantitatively, whereas the constant relaxation time approximation yields significant quantitative discrepancies and often fails to predict the correct sign. Calculations of the electrical resistivity of these metals via the variational approach are also reported.

Physical review / B covering condensed matter and materials physics 102(4), 041101 (2020). doi:10.1103/PhysRevB.102.041101, We report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. For the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. These results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay times measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems., Published by Inst., Woodbury, NY

We include the treatment of quadrupolar fields beyond the Fröhlich interaction in the first-principles electron-phonon vertex in semiconductors. Such quadrupolar fields induce long-range interactions that have to be taken into account for accurate physical results. We apply our formalism to Si (nonpolar), GaAs, and GaP (polar) and demonstrate that electron mobilities show large errors if dynamical quadrupoles are not properly treated., G. B., G.-M. R., and G. H. acknowledge financial support from F.R.S.-FNRS. H. P. C. M. acknowledges financial support from F.R.S.-FNRS through the PDR Grants HTBaSE (T.1071.15). M. G., M. J. V., and X. G. acknowledge financial support from F.R.S.-FNRS through the PDR Grants AIXPHO (T.0238.13) and ALPS (T.0103.19). M. J. V. thanks FNRS and ULiege for a sabbatical grant in ICN2. M. S. and M. R. acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grants No. MAT2016-77100-C2-2-P and No. SEV-2015-0496; of Generalitat de Catalunya (Grant No. 2017 SGR1506); and of the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). The present research benefited from computational resources made available on the Tier-1 supercomputer of the Federation WallonieBruxelles, infrastructure funded by the Walloon Region under Grant Agreement No. 1117545.

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.

We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense q meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology., G.B., G.-M.R., and G.H. acknowledge financial support from F.R.S.-FNRS. H.P.C.M. acknowledges financial support from F.R.S.-FNRS through the PDR Grants HTBaSE (T.1071.15). M.G., M.J.V., and X.G. acknowledge financial support from F.R.S.-FNRS through the PDR Grants AIXPHO (T.0238.13) and ALPS (T.0103.19). M.J.V. thanks FNRS and ULiege for a sabbatical grant in ICN2. M.S. and M.R. acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grants No. MAT2016-77100-C2-2-P and No. SEV-2015-0496; of Generalitat de Catalunya (Grant No. 2017 SGR1506); and of the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529).

The theoretical methods currently in use for the calculation of surface phononsurface phonon dispersion curves and how they have evolved from the phenomenological force-constant models to the present day first principles theories are discussed. Aselection of paradigmatic examples for the different classes of crystal surfaces is presented with comparisons to the experimental data obtained from helium atom scattering or electron energy-loss spectroscopy.

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.

We present TB2J, a Python package for the automatic computation of magnetic interactions, including exchange and Dzyaloshinskii–Moriya, between atoms of magnetic crystals from the results of density functional calculations. The program is based on the Green's function method with the local rigid spin rotation treated as a perturbation. As input, the package uses the output of either Wannier90, which is interfaced with many density functional theory packages, or of codes based on localized orbitals. One of the main interests of the code is that it requires only one first-principles electronic structure calculation in the non-relativistic case (or three in the relativistic case) and from the primitive cell only to obtain the magnetic interactions up to long distances, instead of first-principles calculations of many different magnetic configurations and large supercells. The output of TB2J can be used directly for the adiabatic magnon band structure and spin dynamics calculations. A minimal user input is needed, which allows for easy integration into high-throughput workflows. Program summary: Program Title: TB2J CPC Library link to program files: https://doi.org/10.17632/dm45fcn69d.1 Developer's repository link: https://github.com/mailhexu/TB2J Code Ocean capsule: https://codeocean.com/capsule/6486145 Licensing provisions: BSD 2-clause Programming language: Python Nature of problem: TB2J is a package for the computing of parameters in the extended Heisenberg model of the magnetic interaction, including the isotropic exchange, anisotropic exchange and Dzyaloshinskii–Moriya interactions from first principles result. It can make use of the Wannier function Hamiltonian, which can be constructed from many first principles codes, or localized orbital based codes. Solution method: It uses the magnetic force theorem and takes the rigid spin rotation as a perturbation to the electronic structure. The energy variation is calculated from the Green's functions from tight-binding like Hamiltonian based on Wannier functions or localized orbitals. Additional comments including restrictions and unusual features: Isotropic exchange, anisotropic exchange, and Dzyaloshinskii–Moriya interactions can all be computed with the input of many DFT codes through the interface of Wannier90, or directly from localized orbital codes. The magnetic interaction parameters up to any distance can be computed from one DFT calculation. A minimum user-input is required which provides a black-box like experience. It generates output for several spin dynamics codes and thus bridges the first principles electronic structure simulation with the large scale spin dynamics simulation., This work has been funded by the Communauté Française de Belgique (ARC AIMED G.A. 15/19-09). XH thanks the support by the EU H2020-NMBP-TO-IND-2018 project “INTERSECT” (Grant No. 814487). EB thanks the FRS-FNRS for support, as does MJV for an “out” sabbatical grant to ICN2 Barcelona in 2018–2019. The authors acknowledge the CECI supercomputer facilities funded by the F.R.S-FNRS (Grant No. 2.5020.1) and the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles funded by the Walloon Region (Grant No. 1117545). Computing time was also provided by PRACE-3IP DECI grants 2DSpin and Pylight on Beskow (G.A. 653838 of H2020).

We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe

We present density functional theory (DFT) calculations of the electronic and magnetic properties of fluorine adatoms on a single side of a graphene monolayer. By extrapolating the results, the binding energy of a single fluorine adatom on graphene in the dilute limit is calculated. Our results confirm that the finite-size error in the binding energy scales inversely with the cube of the linear size of the simulation cell. We establish relationships between stability and C F bond nature, diffusion of fluorine adatoms and total magnetization in different configurations of adatoms. For single-side fluorination, sp2.33 is the maximum p-content re-hybridization found in the C F bond. We show that semilocal DFT cannot predict correctly the magnetic properties of fluorinated graphene and a higher level theory, such as DFT + U is needed. The results indicate a tendency of graphene to reduce the imbalance between adsorption on the two sublattices, and therefore total magnetization, through low-energy-barrier pathways on a time scale of ∼10 ps at room temperature. The thermodynamically favored arrangements are those with the smallest total magnetization. Indeed, the electronic structure is intimately related to the magnetic properties and changes from semi-metallic to p-type half-metallic or semiconducting features, depending on the adatoms arrangement.

We extend the density functional perturbation theory formalism to the case of non-collinear magnetism. The main problem comes with the exchange-correlation (XC) potential derivatives, which are the only ones that are affected by the non-collinearity of the system. Most of the present XC functionals are constructed at the collinear level, such that the off-diagonal (containing magnetization densities along $x$ and $y$ directions) derivatives cannot be calculated simply in the non-collinear framework. To solve this problem, we consider here possibilities to transform the non-collinear XC derivatives to a local collinear basis, where the $z$ axis is aligned with the local magnetization at each point. The two methods we explore are i) expanding the spin rotation matrix as a Taylor series, ii) evaluating explicitly the XC for the local density approximation through an analytical expression of the expansion terms. We compare the two methods and describe their practical implementation. We show their application for atomic displacement and electric field perturbations at the second order, within the norm-conserving pseudopotential methods.

arXiv:1901.05060v1, We employ an ab-initio structure search algorithm to explore the configurational space of bismuth in quasi-two dimensions. A confinement potential is introduced to restrict the movement of atoms within a predefined thickness to find the stable and metastable forms of monolayer Bi. In addition to the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new phases: α, β, and γ. Each phase exhibits peculiar electronic properties, ranging from metallic (α and γ) to semiconducting (puckered monoclinic, buckled hexagonal, and β) monolayers. Topologically nontrivial features are predicted for buckled hexagonal and γ phases. We also remark on the role of 5d electrons on the electronic properties of Bi monolayer. We conclude that Bi provides a rich playground to study distortion-mediated metal–insulator phase transitions in quasi-2D., This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant Number OCI-1053575. Additionally, the authors acknowledge support from Texas Advanced Computer Center (TACC), the Bridges supercomputer at the Pittsburgh Supercomputer Center, and Super Computing Systems (Spruce and Mountaineer) at West Virginia University (WVU). A.H.R. and S.S. acknowledge support from the National Science Foundation (NSF) DMREF-NSF 1434897, NSF OAC-1740111, and DOE DE-SC0016176 projects. S.S. acknowledges support from the Dr. Mohindar S. Seehra Research Award and the Distinguished Doctoral Scholarship at West Virginia University. Z.Z. acknowledges financial support by the Ramon y Cajal program (RYC-2016-19344), the CERCA programme of the Generalitat de Catalunya (Grant 2017SGR1506), the Severo Ochoa programme (MINECO, SEV-2017-0706), and the Deutsche Forschungsgemeinschaft (DFG) Grant No. ZA 780/3-1. M.A. acknowledges support from the Novartis Universitat Basel Excellence Scholarship for ̈ Life Sciences and the Swiss National Science Foundation (Project No. P300P2-158407, P300P2-174475). M.J.V. acknowledges funding by the Belgian FNRS (PDR G.A. T.1077.15-1/7 and a sabbatical “OUT” grant at ICN2), ULiege and the Communauté Francaise de Belgique (ARC ̧ AIMED G.A. 15/19-09), and computational resources from the Consortium des Equipements de Calcul Intensif (FRSFNRS G.A. 2.5020.11) and Zenobe/CENAERO funded by the Walloon Region under G.A. 1117545.

The high pressure lattice dynamics of rhombohedral antimony have been studied by a combination of diffuse scattering and inelastic x-ray scattering. The evolution of the phonon behavior as function of pressure was analyzed by means of two theoretical approaches: density functional perturbation theory and symmetry-based phenomenological phase transition analysis. This paper focuses on the first structural phase transition, SbI-SbIV, and the role of vibrations in leading the transition. The phonon dispersion exhibits complex behaviour as one approaches the structural transition, with the branches, corresponding to the two transitions happening at high pressure in the Va elements (A7-to-BCC and A7-to-PC) both showing softening., Comment: 8 pages and 8 figures

We report electron transport measurements in dual-gated monolayer WS2 encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts that operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. Intrinsic electron mobilities of 100 cm2/(V s) at room temperature and 2000 cm2/(V s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First-principles calculations of phonon scattering in monolayer WS2 are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these two-dimensional layers.

et al., Materials that exhibit ferromagnetism, interfacial stability, and tunability are highly desired for the realization of emerging magnetoelectronic phenomena in heterostructures. Here we present the GdAg monolayer alloy, which possesses all such qualities. By combining X-ray absorption, Kerr effect, and angle-resolved photoemission with ab initio calculations, we have investigated the ferromagnetic nature of this class of Gd-based alloys. The Curie temperature can increase from 19 K in GdAu to a remarkably high 85 K in GdAg. We find that the exchange coupling between Gd atoms is barely affected by their full coordination with noble metal atoms, and instead, magnetic coupling is effectively mediated by noble metal-Gd hybrid s,p-d bands. The direct comparison between isostructural GdAu and GdAg monolayers explains how the higher degree of surface confinement and electron occupation of such hybrid s,p-d bands promote the high Curie temperature in the latter. Finally, the chemical composition and structural robustness of the GdAg alloy has been demonstrated by interfacing them with organic semiconductors or magnetic nanodots. These results encourage systematic investigations of rare-earth/noble metal surface alloys and interfaces, in order to exploit them in magnetoelectronic applications., This work was supported by the Spanish Government (MAT2013-46593-C6-4-P, MAT2013-46593-C6-5-P, FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P), the Basque Government (IT621-13,IT627-13,IT756-13), the Diputación de Gipuzkoa (RF-0061/2015), and the Deutsche Forschungsgemeinschaft (SFB 1083 “Structure and Dynamics of Internal Interfaces”). A.Mu., M.A.V., and M.G. acknowledge the Severo Ochoa program SEV-2013-0295. B.X. and M.J.V. acknowledge two A.R.C. grants (TheMoTherm 10/15-03 and AIMED 15/19-09) from the Communauté Francaise de ̧ Belgique. Computer time was made available by PRACE-2IP and 3IP (EU FP7 Grant No. RI-283493 and RI-312763), CECI, SEGI-ULg, and the DIPC.

Interest in the properties of transition metal dichalcogenides (TMDs) has increased due to the discovery of the coupling between spin and valley degrees of freedom, which can be seen experimentally using a circularly polarised laser. After excitation the newly formed carrier populations must move towards the other valley until balance is reached. However, this relaxation process is not entirely understood in the literature, where the relative importance of the electron-electron (e-e) or electron-phonon (e-p) interactions is still a subject of debate. Previous works on WSe2 [A. Molina- Sánchez, et al - Nano letters, 2017] have shown that the e-p interaction is a good candidate to describe the relaxation process. Using a fully ab-initio framework based on the Baym-Kadanoff equations [P. M. M. C. de Melo and A. Marini, Phys. Rev. B 93, 155102 (2016)] we study the influence of the e-p interaction on MoSe2 after its excitation by a laser field. We show how phonons allow carrier relaxation and how the Kerr signal and total magnetisation are affected at different temperatures, with the latter exhibiting a non-monotonic behaviour as the temperature increases [M Ersfeld et al - Nano Letters 2019 19 (6), 4083-4090]. An important conclusion is that long lived spin states probably reside within defects. We calculate the spectral signatures of point defects in TMDs, finding two main classes based on the presence of in-gap states, and estimating the experimental resolution needed to provide quantification of the defect concentration [P de Melo et al. https://arxiv.org/abs/2010.10222]. The localization of excitonic states around the defect provides a benchmark for scanning probe characterization (figure shows a S vacancy exciton wave function, the green ball is the hole position). Figure 1

Magnetic systems represent an important challenge for electronic structure methods, in particular Density Functional Theory (DFT), which uses a single determinant wavefunction. To assess the predictions obtained by DFT in this type of materials, we benchmark different exchange correlation functionals with respect to each other, and with respect to available experimental data, on two families of binary iron alloys which are metallic and magnetic. We climb three rungs in Jacob’s ladder of DFT (i) the local density approximation, (ii) the industry standard approximation due to Perdew, Burke and Ernzerhof, and the revised version for solids, PBEsol (iii) and finally a very accurate meta-GGA functional SCAN, which corresponds to the third rung. More than 350 structures in ferromagnetic and antiferromagnetic configurations were considered. We compare the Convex Hull, the calculated magnetic moment, crystal structure, formation energy and electronic gap if present. We conclude that none of the functionals work in all conditions: whereas PBE and PBEsol can give a fair description of the crystal structure and the energetics, SCAN strongly overestimates the formation energy – giving values which are at least twice as large as PBE (and experiment). Magnetic moments are better predicted by PBE as well. Our results show that magnetic and strongly correlated materials are a tough litmus test for DFT, and that they represent the next frontier in the development of a truly universal exchange correlation functionals.

The interplay of magnons and phonons can induce strong temperature variations in the magnetic exchange interactions, leading to changes in the magnetothermal response. This is a central mechanism in many magnetic phenomena, and in the new field of Spin Caloritronics, which focuses on the combination of heat and spin currents. Boson model systems have previously been developed to describe the magnon phonon coupling, but until recently studies rely on empirical parameters. In this work we propose a first principles approach to describe the dependence of the magnetic exchange integrals on phonon renormalization, leading to changes in the magnon dispersion as a function of temperature. The temperature enters into the spin dynamics (by introducing fluctuations) as well as in the magnetic exchange itself. Depending on the strength of the coupling, these two temperatures may or may not be equilibrated, yielding different regimes. We test our approach in typical and well known ferromagnetic materials: Ni, Fe, and Permalloy. We compare our results to recent experiments on the spin-wave stiffness, and discuss departures from Bloch’s law and parabolic dispersion.

Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties, using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range e$^-$- e$^-$ correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased and data trends on the full set of fifteen existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications., 11 pages, 6 figures

D’Avino, Gabriele et al.

We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the $\pi$-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides., Comment: 3 figures, 6 pages

The quantitative analysis of electron-optical phase images recorded using off-axis electron holography often relies on the use of computer simulations of electron propagation through a sample. However, simulations that make use of the independent atom approximation are known to overestimate experimental phase shifts by approximately 10%, as they neglect bonding effects. Here, we compare experimental and simulated phase images for few-layer WSe_{2}. We show that a combination of pseudopotentials and all-electron density functional theory calculations can be used to obtain accurate mean electron phases, as well as improved atomic-resolution spatial distribution of the electron phase. The comparison demonstrates a perfect contrast match between experimental and simulated atomic-resolution phase images for a sample of precisely known thickness. The low computational cost of this approach makes it suitable for the analysis of large electronic systems, including defects, substitutional atoms, and material interfaces.

We report on the structural, electronic and magnetic properties of a monolayer of GdAg2, forming a moiré pattern on Ag(111). Combining scanning tunneling microscopy and ab initio spin-polarized calculations, we show that the electronic band structure can be shifted linearly via thermal controlled strain of the intra-layer atomic distance in the range of 1-7%, leading to lateral hetero-structuring. Furthermore, the coupling of the incommensurable GdAg2 alloy layer to the Ag(111) substrate leads to spatially varying atomic relaxation causing subsurface layer buckling, texturing of the electronic and spin properties, and inhomogeneity of the magnetic anisotropy energy across the layer. These results provide perspectives for control of electronic properties and magnetic ordering in atomically-thin layers., AC and LV acknowledge the financial support of the Spanish ministry of economy (MAT2013-46593-C6-4-P). MJhhV and BX acknowledge two ARC grants (TheMoTherm #10/15-03 and AIMED # 15/19-09) from the Communauté Française de Belgique, a PDR project (GA T.1077.15) from the Fonds National pour la Recherche Scientifique (Belgium). Computer time was provided by CECI, SEGI, Zenobe/CENAERO (Walloon region GA 1117545), and PRACE-2IP and 3IP (EU FP7 GA RI-283493 and RI-312763) on EPCC Archer.

We measured the electrical resistivity of iron and iron-silicon alloy to 100 GPa. The resistivity of iron was also calculated to core pressures. Combined with the first geophysical model accounting for saturation resistivity of core metal, the present results show that the thermal conductivity of the outermost core is greater than 90 W/m/K. These values are significantly higher than conventional estimates, implying rapid secular core cooling, an inner core younger than 1 Ga, and ubiquitous melting of the lowermost mantle during the early Earth. An enhanced conductivity with depth suppresses convection in the deep core, such that its center may have been stably stratified prior to the onset of inner core crystallization. A present heat flow in excess of 10 TW is likely required to explain the observed dynamo characteristics.

The Seebeck coefficient, thermoelectric power factor, electrical conductivity, and electronic thermal conductivity of the orthorhombic Pbnm phase of SrRuO3 are studied comprehensively by combining first-principles density functional calculations and Boltzmann transport theory. The influence of exchange-correlation functional on the Seebeck coefficient is carefully investigated. We show that the best agreement with experimental data is achieved when SrRuO3 is described as being at the limit of a half-metal. Furthermore, we analyze the role of individual symmetry-adapted atomic distortions on the Seebeck coefficient, highlighting a particularly strong sensitivity to R-4(+) oxygen rotational motions, which may shed light on how to manipulate the Seebeck coefficient. We confirm that the power factor of SrRuO3 can only be slightly improved by carrier doping. Our results provide a complete understanding of the thermoelectric properties of SrRuO3 and an interesting insight on the relationship between exchange-correlation functionals, atomic motions, and thermoelectric quantities.

Attosecond electron dynamics in small- and medium-sized molecules, induced by an ultrashort strong optical pulse, is studied computationally for a frozen nuclear geometry. The importance of exchange and correlation effects on the nonequilibrium electron dynamics induced by the interaction of the molecule with the strong optical pulse is analyzed by comparing the solution of the time-dependent Schrödinger equation based on the correlated field-free stationary electronic states computed with the equationof-motion coupled cluster singles and doubles and the complete active space multi-configurational self-consistent field methodologies on one hand, and various functionals in real-time time-dependent density functional theory (TDDFT) on the other. We aim to evaluate the performance of the latter approach, which is very widely used for nonlinear absorption processes and whose computational cost has a more favorable scaling with the system size. We focus on LiH as a toy model for a nontrivial molecule and show that our conclusions carry over to larger molecules, exemplified by ABCU (C10H19N). The molecules are probed with IR and UV pulses whose intensities are not strong enough to significantly ionize the system. By comparing the evolution of the time-dependent field-free electronic dipole moment, as well as its Fourier power spectrum, we show that TD-DFT performs qualitatively well in most cases. Contrary to previous studies, we find almost no changes in the TD-DFT excitation energies when excited states are populated. Transitions between states of different symmetries are induced using pulses polarized in different directions. We observe that the performance of TD-DFT does not depend on the symmetry of the states involved in the transition.

The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures. Funding Agencies|NSF [1434897]; American Chemical Society Petroleum Research Fund [54075-ND10]; ULg; Actions De Recherche Concertees (ACR) grants from Communaute Francaise de Belgique [10/15-03, 15/19-09]; COST network EUSpec [MP1306]; COST network XLIC [CM1204]; EU [RI-312763]; Knut & Alice Wallenberg Foundation (KAW) project "Isotopic Control for Ultimate Material Properties"; Swedish Foundation for Strategic Research (SSF) program [SRL10-002]