We use density functional theory plus dynamical mean-field theory (DFT+DMFT) to study multiple control parameters for tuning the metal-insulator transition (MIT) in CaVO$_3$ thin films. We focus on separating the effects resulting from substrate-induced epitaxial strain from those related to the reduced thickness of the film. We show that tensile epitaxial strain of around 3-4% is sufficient to induce a transition to a paramagnetic Mott-insulating phase. This corresponds to the level of strain that could be achieved on a SrTiO$_3$ substrate. Using free-standing slab models, we then demonstrate that reduced film thickness can also cause a MIT in CaVO$_3$, however, only for thicknesses of less than 4 perovskite units. Our calculations indicate that the MIT in such ultra-thin films results mainly from a surface-induced crystal-field splitting between the $t_{2g}$-orbitals, favoring the formation of an orbitally-polarized Mott insulator. This surface-induced crystal-field splitting is of the same type as the one resulting from tensile epitaxial strain, and thus the two effects can also cooperate. Furthermore, our calculations confirm an enhancement of correlation effects at the film surface, resulting in a reduced quasiparticle spectral weight in the outermost layer, whereas bulk-like properties are recovered within only a few layers away from the surface.

We report on the strengths and limitations of scanning tunnelling microscopy (STM) when used for characterising atomic-scale features of quasi two-dimensional materials, such as graphene and single layers of hexagonal boron nitride, which may present strong corrugations when grown epitaxially on a substrate with a lattice mismatch. As a paradigmatic test case, we choose single-layer and bilayer graphene on Ru(0001), because their STM images show both a long-range moiré modulation and complex atomic-scale distortions of the graphene lattice. Through high-resolution STM measurements, we first determine with high accuracy the moiré epitaxial relations of the single layer and the bilayer with respect to the metal substrate. In particular, we also provide direct evidence for the existence of AA-stacked bilayer graphene domains on Ru(0001). We then demonstrate that the local strain distribution, as inferred from the same STM images, can be affected by large errors, so that apparent giant strains arise in some regions of the moiré as an imaging artefact. With the aid of density functional theory simulations, we track down the origin of these fictitious distortions in the high directionality of the graphene π-orbital density combined with the large corrugation of the sample. The proposed theoretical model correctly accounts for the observed dependence of the apparent strain on the STM tip–sample separation and on the different degree of curvature of the second graphene layer with respect to the single layer.

We investigate the effect of epitaxial strain on the Mott metal-insulator transition (MIT) in perovskite systems with d^1 and d^2 electron configuration of the transition metal (TM) cation. We first discuss the general trends expected from the changes in the crystal-field splitting and in the hopping parameters that are induced by epitaxial strain. We argue that the strain-induced crystal-field splitting generally favors the Mott-insulating state, whereas the strain-induced changes in the hopping parameters favor the metallic state under compressive strain and the insulating state under tensile strain. Thus, the two effects can effectively cancel each other under compressive strain, while they usually cooperate under tensile strain, in this case favoring the insulating state. We then validate these general considerations by performing electronic structure calculations for several d^1 and d^2 perovskites, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We isolate the individual effects of strain-induced changes in either hopping or crystal-field by performing DMFT calculations where we fix one type of parameter to the corresponding unstrained DFT values. These calculations confirm our general considerations for SrVO_3 (d^1) and LaVO_3 (d^2), whereas the case of LaTiO_3 (d^1) is distinctly different, due to the strong effect of the octahedral tilt distortion in the underlying perovskite crystal structure. Our results demonstrate the possibility to tune the electronic properties of correlated TM oxides by using epitaxial strain, which allows to control the strength of electronic correlations and the vicinity to the Mott MIT.

The effect of epitaxial strain on the structural and electronic properties of LaVO3 is investigated through density functional theory (DFT) and dynamical mean-field theory (DMFT). Two different growth orientations of the crystal are considered, one preserving the bulk Pbnm space-group symmetry and another giving rise to a symmetry lowering to P2(1)/m. In the nonmagnetic DFT structures, the two growth orientations are equally favored for all tensile strains considered here, as well as for compressive strains weaker than -3%. For stronger compressive strains, the P2(1)/m orientation is favored and shows a complete suppression of octahedral tilts along the out-of-plane direction. Magnetically ordered structures do not show a complete tilt suppression, but the trend points to a similar reduction of the out-of-plane V-O-V bond angles under compressive strain. Our DMFT calculations show that, in accord with room-temperature experiments, the bulk paramagnetic Mott-insulating state of LaVO3 is robust against epitaxial strains attainable in thin films, since the suppression of orbital fluctuations counteracts the effect of bandwidth increase with compressive strain. Under stronger compressive strains, the straightening of the V-O-V bonds in the P2(1)/m geometry interferes with the suppression of orbital fluctuations and hence perturbs the Mott phase more strongly, albeit not enough to achieve a metallic phase.

The infrared absorption spectra of jennite, tobermorite 14Å, anomalous tobermorite 11Å, and normal tobermorite 11Å are simulated within a density-functional-theory scheme. The atomic coordinates and the cell parameters are optimized resulting in structures which agree with previous studies. The vibrational frequencies and modes are obtained for each mineral. The vibrational density of states is analyzed through extensive projections on silicon tetrahedra, oxygen atoms, OH groups, and water molecules. The coupling with the electric field is achieved through the use of density functional perturbation theory, which yields Born effective charges and dielectric constants. The simulated absorption spectra reproduce well the experimental spectra, thereby allowing for a detailed interpretation of the spectral features in terms of the underlying vibrational modes. In the far-infrared part of the absorption spectra, the interplay between Ca and Si related vibrations leads to differences which are sensitive to the calcium/silicon ratio of the mineral.

We investigate the intercalation of hydrogen at the graphene/SiC(0001) interface through atomistic models characterized by very low strains both in the epitaxial graphene and in the SiC substrate. Adsorption of H at the interface is always stable but shows energy variations larger than 1eV between different locations of the interface. An interface model presenting a strong interaction of graphene with the substrate, corresponding to the experimental situation, shows that adsorption at the interface is on average 0.75eV less stable than at the surface of the buffer layer. At variance, a model having a much weaker graphene/SiC interaction results in hydrogenation energies that are comparable in the two cases. The structural modifications occurring upon H intercalation show a partial conversion of the buffer layer into quasi-free standing graphene, accompanied by a marked downward relaxation of the hydrogenated Si atom and a local steric repulsion between the latter and the overlying graphene.

The adsorption of atomic H above the carbon buffer layer in the graphene/SiC(0001) interface system is investigated within density functional theory through a set of realistic interface models that do not impose large artificial strains on the graphene. We find that hydrogen binding energies above the buffer layer are two to four times higher than on free-standing graphene and display important spatial variations across the unit cell. Adsorption on Si-bonded, fourfold-coordinated C atoms is strongly unfavorable (unstable, or metastable with very low barriers), while all threefold-coordinated C sites lead to stable configurations often showing H binding energies larger than in H-2. We identify the origin of these large binding energies in the local strengthening of the graphene/SiC interface bonding around the adsorption site due to the H-induced deformation of the graphene buffer layer. The most stable adsorption sites are those having two or three C nearest neighbors (almost) atop the underlying surface Si atoms, because the presence of the adsorbed H atom favors the formation of a local sp(3) arrangement of the graphene. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

An efficient method for computing the Landauer-Buettiker conductance of an open quantum system within DFT+U is presented. The Hubbard potential is included in electronic structure and transport calculations as a simple renormalization of the non-local pseudopotential coefficients by restricting the integration for the on-site occupations within the cutoff spheres of the pseudopotential. We apply the methodology to the case of an Au monatomic chain in presence of a CO molecule adsorbed on it. We show that the Hubbard U correction removes the spurious magnetization in the pristine Au chain at the equilibrium spacing, as well as the unphysical contribution of d electrons to the conductance, resulting in a single (spin-degenerate) transmission channel and a more realistic conductance of 1 G_0 . We find that the conductance reduction due to CO adsorption is much larger for the atop site than for the bridge site, so that the general picture of electron transport in stretched Au chains given by the local density approximation remains valid at the equilibrium Au-Au spacing within DFT+U.

CO adsorption on an Au monatomic chain is studied within density functional theory in nanocontact geometries as a function of the contact stretching. We compare the bridge and atop adsorption sites of CO, finding that the bridge site is energetically favored at all strains studied here. Atop adsorption gives rise to an almost complete suppression of the ballistic conductance of the nanocontact, while adsorption at the bridge site results in a conductance value close to 0.6G(0), in agreement with previous experimental data. We show that only the bridge site can qualitatively account for the evolution of the conductance as a function of the contact stretching observed in the experimental conductance traces. The numerical discrepancy between the theoretical and experimental conductance slopes is rationalized through a simple model for the elastic response of the metallic leads. We also verify that our conductance values are not affected by the specific choice of the nanocontact geometry by comparing two different atomistic models for the tips.

We address the energetic stability of the graphene/SiC(0001) interface and the associated binding mechanism by studying a series of low-strain commensurate interface structures within a density functional scheme. Among the structures with negligible strain, the 6 root 3x6 root 3R30 degrees SiC periodicity shows the lowest interface energy, providing a rationale for its frequent experimental observation. The interface stability is driven by the enhanced local reactivity of the substrate-bonded graphene atoms undergoing sp(2)-to-sp(3) rehybridization (pyramidalization). By this mechanism, relaxed structures of higher stability exhibit more pronounced graphene corrugations at the atomic scale.

We study the energetics, the electronic structure, and the ballistic transport of an infinite Au monatomic chain with an adsorbed CO molecule. We find that the bridge adsorption site is energetically favored with respect to the atop site, both at the equilibrium Au-Au spacing of the chain and at larger spacings. Instead, a substitutional configuration requires a very elongated Au-Au bond, well above the rupture distance of the pristine Au chain. The electronic structure properties can be described by the Blyholder model, which involves the formation of bonding/antibonding pairs of 5 sigma and 2 pi* states through the hybridization between molecular levels of CO and metallic states of the chain. In the atop geometry, we find an almost vanishing conductance due to the 5 sigma antibonding states giving rise to a Fano-like destructive interference close to the Fermi energy. In the bridge geometry, instead, the same states are shifted to higher energies and the conductance reduction with respect to pristine Au chain is much smaller. We also examine the effects of strain on the ballistic transport, finding opposite behaviors for the atop and bridge conductances. Only the bridge geometry shows a strain dependence compatible with the experimental conductance traces.

Using density functional calculations, we address the energetics of the interface between the SiC(0001) substrate and the first covalently bonded epitaxial graphene layer. We consider a 63x63R30^o geometry showing the experimental periodicity, a simplified 3x3R30^o geometry presenting a strained graphene layer, and an almost commensurate 4x4 geometry where SiC and graphene have the same orientation. The total energies of the structurally relaxed interface systems indicate that the 63x63R30^o geometry is the most stable, in agreement with its experimental occurrence. The binding energy is found to correlate with the vertical spread of the C atoms in the graphene layer, with a larger extension corresponding to a higher binding energy. For the 63x63R30^o geometry, the height variation of the graphene layer displays the experimentally observed modulation with an apparent 6x6 periodicity. The charge transfer also correlates with the height of the graphene atoms, being more significant in graphene regions which are strongly attached to the SiC substrate.

A set of low-strain commensurate interface structures for epitaxial graphene on SiC(0001) are obtained by combining rotated graphene supercells with m×m or m3×m3R30° SiC cells. For two among the interfaces with lowest strain, corresponding to 4×4 and 63×63R30° SiC periodicities, we analyze the binding energy of the graphene/SiC(0001) interface in terms of bond energies resulting from the (partial) saturation of individual Si dangling bonds by C atoms of graphene. These bond energies are determined as a function of the relative lateral displacement between the surface Si atoms and the graphene honeycomb. We find that this model energy does not explain the variation of the binding energies obtained from direct density functional calculations for the full interfacial systems. Moreover, for a given interface, the lateral rearrangements of the Si and C atomic positions found upon relaxation do not lead to any significant increase of the model binding energy. These results indicate that the deformations of the graphene layer cannot be neglected in the modeling of the binding energy of the graphene/SiC(0001) system.

Using density functional calculations, we address the energetics of the interface between the SiC(0001) substrate and the first covalently bonded epitaxial graphene layer. We consider a 63×63R30° geometry showing the experimental periodicity, a simplified 3×3R30° geometry presenting a strained graphene layer, and an almost commensurate 4×4 geometry where SiC and graphene have the same orientation. The total energies of the structurally relaxed interface systems indicate that the 63×63R30° geometry is the most stable, in agreement with its experimental occurrence. The binding energy is found to correlate with the vertical spread of the C atoms in the graphene layer, with a larger extension corresponding to a higher binding energy. For the 63×63R30° geometry, the height variation of the graphene layer displays the experimentally observed modulation with an apparent 6×6 periodicity. The charge transfer also correlates with the height of the graphene atoms, being more significant in graphene regions which are strongly attached to the SiC substrate.

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Recent experiments showed that the conductance of Pt nanocontacts and nanowires is measurably reduced by adsorption of CO. We present DFT calculations of the electronic structure and ballistic conductance of a Pt monoatomic chain and a CO molecule adsorbed in an on‐top position. We find that the main electronic molecule‐chain interaction occurs via the 5σ and 2π* orbitals of the molecule, involved in a donation/back‐donation process similar to that of CO on transition‐metal surfaces. The ideal ballistic conductance of the monoatomic chain undergoes a moderate reduction by about 1.0 G0 (from 4 G0 to 3.1 G0) upon adsorption of CO. By repeating all calculations with and without spin‐orbit coupling, no substantial spin‐orbit induced change emerges either in the chain‐molecule interaction mechanism or in the conductance.

We carry out a first-principles density-functional study of the interaction between a monatomic Pt wire and a CO molecule, comparing the energies of different adsorption configurations (bridge, on top, substitutional, and tilted bridge) and discussing the effects of spin-orbit (SO) coupling on the electronic structure and on the ballistic conductance of two of these systems (bridge and substitutional). We find that when the wire is unstrained, the bridge configuration is energetically favored, while the substitutional geometry becomes possible only after the breaking of the Pt-Pt bond next to CO. The interaction can be described by a donation/backdonation process similar to that occurring when CO adsorbs on transition-metal surfaces, a picture which remains valid also in the presence of SO coupling. The ballistic conductance of the (tipless) nanowire is not much reduced by the adsorption of the molecule on the bridge and on-top sites. However, it shows a significant drop in the substitutional case. The differences in the electronic structure due to the SO coupling influence the transmission only at energies far away from the Fermi level so that fully and scalar-relativistic conductances do not differ significantly.