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The dispersion of the electronic states of epitaxial graphene (Gr) depends significantly on the strength of the bonding with the underlying substrate. We report on empty electron states in cobalt-intercalated Gr grown on Ir(111), studied by angle-resolved inverse photoemission spectroscopy and x-ray absorption spectroscopy, complemented with density functional theory calculations. The weakly bonded Gr on Ir preserves the peculiar spectroscopic features of the Gr band structure, and the empty spectral densities are almost unperturbed. Upon intercalation of a Co layer, the electronic response of the interface changes, with an intermixing of the Gr π* bands and Co d states, which breaks the symmetry of π/σ states, and a downshift of the upper part of the Gr Dirac cone. Similarly, the image potential of Ir(111) is unaltered by the Gr layer, while a downward shift is induced upon Co intercalation, as unveiled by the image state energy dispersion mapped in a large region of the surface Brillouin zone. [ABSTRACT FROM AUTHOR]

Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons--electron (e) and hole (h) pairs interacting by Coulomb force--overcomes the band gap, giving rise to a new phase: the "excitonic insulator" (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk MoS2 above 30 GPa. [ABSTRACT FROM AUTHOR]

We report on the electronic properties of an artificial system obtained by the intercalation of equiatomic FeCo layers under graphene grown on Ir(111). Upon intercalation, the FeCo film grows epitaxially on Ir(111), resulting in a lattice-mismatched system. By performing density functional theory calculations, we show that the intercalated FeCo layer leads to a pronounced corrugation of the graphene film. At the same time, the FeCo intercalated layers induce a clear transition from a nearly undisturbed to a strongly hybridized graphene π-band, as measured by angle-resolved photoemission spectroscopy. A comparison of experimental results with the computed band structure and the projected density of states unveils a spin-selective hybridization between the π band of graphene and FeCo-3d states. Our results demonstrate that the reduced dimensionality, as well as the hybridization within the FeCo layers, induces a narrowing and a clear splitting of Fe 3d-up and Fe 3d-down-spin bands of the confined FeCo layers with respect to bulk Fe and Co. [ABSTRACT FROM AUTHOR]

In this work we show the possibility to extract Kohn-Sham orbitals, orbital energies, and exchange correlation potentials from accurate Quantum Monte Carlo (QMC) densities for atoms (He, Be, Ne) and molecules (H2, Be2, H2O, and C2H4). The Variational Monte Carlo (VMC) densities based on accurate Jastrow Antisymmetrised Geminal Power wave functions are calculated through different estimators. Using these reference densities, we extract the Kohn-Sham quantities with the method developed by Zhao, Morrison, and Parr (ZMP) [Phys. Rev. A 50, 2138 (1994)]. We compare these extracted quantities with those obtained form CISD densities and with other data reported in the literature, finding a good agreement between VMC and other high-level quantum chemistry methods. Our results demonstrate the applicability of the ZMP procedure to QMC molecular densities, that can be used for the testing and development of improved functionals and for the implementation of embedding schemes based on QMC and Density Functional Theory. [ABSTRACT FROM AUTHOR]

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (–I, –Br, –Cl, –F) and other chemical functionalizations (–H, –OH, –CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar –OH and –CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases. [ABSTRACT FROM AUTHOR]

The article explains the ab initio optical absorption spectra of various size-expanded xDNA base assemblies. The optical response is found to get highly affected by the modifications of the frontier electron orbitals.

The study of the optical absorption spectra of DNA bases and base pairs, carried out by means of time dependent density functional theory is described. The spectra for the isolated bases are compared and used to assess the accuracy of the method and the quality of the exchange correlation functional and this approach turns out to be a reliable tool to describe the response of the nucleobases.

Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons–electron–hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators. [ABSTRACT FROM AUTHOR]

G-quadruplex is a quadruple helical form of nucleic acids that can appear in guanine-rich parts of the genome. The basic unit is the G-tetrad, a planar assembly of four guanines connected by eight hydrogen bonds. Its rich topology and its possible relevance as a drug target for a number of diseases have stimulated several structural studies. The superior stiffness and electronic π-π overlap between consecutive G-tetrads suggest exploitation for nanotechnologies. Here we inspect the intimate link between the structure and the electronic properties, with focus on charge transfer parameters. We show that the electronic couplings between stacked G-tetrads strongly depend on the three-dimensional atomic structure. Furthermore, we reveal a remarkable correlation with the topology: a topology characterized by the absence of syn-anti G-G sequences can better support electronic charge transfer. On the other hand, there is no obvious correlation of the electronic coupling with usual descriptors of the helix shape. We establish a procedure to maximize the correlation with a global helix shape descriptor. [ABSTRACT FROM AUTHOR]

The accurate calculation of electronic excited states of large and electronically correlated biological chromophores in their complex protein environment still represents a challenge for quantum chemistry. Two ab initio techniques are recently emerging as candidates to correctly tackle this issue: Quantum Monte Carlo (QMC) calculations for the ground state geometry optimization, and Many Body Green’s Function Theory (MBGFT) for exited state energies. In the present work we use the Variational Monte Carlo (VMC) to carry out structural optimizations and we present an extension of MBGFT to complex environments, using a Quantum Mechanics/Molecular Mechanics framework. This technique is applied to evaluate the optical properties of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin, the protein responsible for dim light vision in the vertebrates. Vertical energies for the bright excitation of RPSB calculated solving the Bethe-Salpeter equation in gas phase and in the protein environment correspond to 2.19eV and 2.58eV, respectively. These data are in fair agreement with the available experimental findings. The comparison between these excitation energies and that obtained for a gas phase calculation on a retinal geometry obtained in the protein environment (2.03eV) reveals the essential role of the protein field in the spectral tuning of the molecule. The proposed VMC/MBGFT methodologies can therefore be applied to obtain a reliable ab initio evaluation of optical properties of biological chromophores. [ABSTRACT FROM AUTHOR]

We investigate from first principles the electronic and optical properties of edge-modulated armchair graphene nanoribbons, including both quasiparticle corrections and excitonic effects. Exploiting the oscillating behavior of the ribbon energy gap, we show that minimal width-modulations are sufficient to obtain confinement of both electrons and holes, thus forming optically active quantum dots with unique properties, such as the coexistence of dotlike and extended excitations and the fine tunability of optical spectra, with great potential for optoelectronic applications. [ABSTRACT FROM AUTHOR]

We present an efficient scheme to calculate the chiroptical response of molecular systems within time dependent density functional theory using either a real-time propagation or a frequency-dependent Sternheimer method. The scheme avoids the commonly used sum over empty orbitals and has a very favorable scaling with system size. Moreover, the method is general and can be easily implemented. In the present work, we implemented it using a real-space pseudo-potential representation of the wave-functions and Hamiltonian. The specific use of non-local pseudo-potentials implies that a gauge correction term in the angular momentum operator must be included to ensure that the total scheme is fully gauge invariant. Applications to small organic chiral molecules are shown and discussed, addressing some deficiencies of present exchange–correlation functionals to describe the absolute position of the excitations. However, the shape or sign of the dichroism spectra comes out in excellent agreement with available experiments. [ABSTRACT FROM AUTHOR]

We present calculations on energy- and time-resolved two photon photoemission spectra of images states in Cu(1 0 0) and Cu(1 1 1) surfaces. The surface is modeled by a 1D effective potential and the states are propagated within a real-space, real-time method. To obtain the energy-resolved spectra we employ a geometrical approach based on a subdivision of space into two regions. We treat electronic inelastic effects by taking into account the scattering rates calculated within a GW scheme. To get further insight into the decaying mechanism we have also studied the effect of the variation of the classical Hartree potential during the excitation. This effect turns out to be small. [Copyright &y& Elsevier]

Gas-phase action spectroscopy shows unambiguously that the low-energy absorptions by tetramethylthiotetrathiafulvalene and tetrathianaphthalene cations in solution phase are due to monomers and not π-dimers. [ABSTRACT FROM AUTHOR]

In view of their applicability in optoelectronics, we review here the relevant structural, electronic, and optical features of the inorganic Pb-free halide perovskite class. In particular, after discussing the reasons that have motivated their introduction in opposition to their more widely investigated organic-inorganic counterparts, we highlight milestones already achieved in their synthesis and characterization and show how the use of ab initio ground and excited state methods is relevant in predicting their properties and in disclosing yet unsolved issues which characterize both ternary and quaternary stoichiometry double-perovskites. [ABSTRACT FROM AUTHOR]

Abstract: The suitability of the time-dependent density-functional theory (TDDFT) approach for the theoretical study of the optical properties of biomolecules is demonstrated by several examples. We critically discuss the limitations of available TDDFT implementations to address some of the present open questions in the description of the excited-state dynamics of biological complexes. The key objective of the present work is to address the performance of TDDFT in the linear response regime of the bio-molecular systems to the visible or near UV radiation – measured by, e.g. optical absorption or optical dichroism spectra. Although these spectra are essentially determined by the electronic degrees of freedom of small, optically active regions within the usually large biological systems, they can also be strongly influenced by environment effects (solvent, hosting protein, temperature, etc.). Moreover, many key biological processes consist of photo-induced dynamics (photoisomerisation, etc.), and their description requires a coupled treatment of electronic and nuclear degrees of freedom. We illustrate these aspects with a selection of paradigmatic biomolecular systems: chromophores in fluorescent proteins, porphyrins, DNA basis, the azobenzene dye, etc. To cite this article: A. Castro et al., C. R. Physique 10 (2009). [Copyright &y& Elsevier]

Carbon nanotubes continue to be model systems for studies of confinement and interactions. This is particularly true in the case of so-called "ultraclean" carbon nanotube devices offering the study of quantum dots with extremely low disorder. The quality of such systems, however, has increasingly revealed glaring discrepancies between experiment and theory. Here, we address the outstanding anomaly of exceptionally large orbital magnetic moments in carbon nanotube quantum dots. We perform low temperature magnetotransport measurements of the orbital magnetic moment and find it is up to 7 times larger than expected from the conventional semiclassical model. Moreover, the magnitude of the magnetic moment monotonically drops with the addition of each electron to the quantum dot directly contradicting the widely accepted shell filling picture of single-particle levels. We carry out quasiparticle calculations, both from first principles and within the effective-mass approximation, and find the giant magnetic moments can only be captured by considering a self-energy correction to the electronic band structure due to electron-electron interactions. [ABSTRACT FROM AUTHOR]

Density functional theory provides the most widespread framework for the realistic description of the electronic structure of solids, but the description of strongly correlated systems has remained so far elusive. We consider a particular limit of electrons and ions in which a one-band description becomes exact all the way from the weakly correlated metallic regime to the strongly correlated Mott-Hubbard regime. We provide a necessary condition a density functional should fulfill to describe Mott-Hubbard behavior in this one-band limit and show that it is not satisfied by standard and widely used local, semilocal, and hybrid functionals. We illustrate the condition in the case of few-atom systems and provide an analytic approximation to the exact exchange-correlation potential based on a variational wave function which shows explicitly the correct behavior, combining in a neat way lattice and continuum methods. [ABSTRACT FROM AUTHOR]