Theoretical study of the dynamics of atomic hydrogen adsorbed on graphene multilayers

We present a theoretical study of the dynamics of H atoms adsorbed on graphene bilayers with Bernal stacking. First, through extensive density functional theory calculations, including van der Waals Interactions, we obtain the activation barriers involved in the desorption and migration processes of a single H atom. These barriers, along with attempt rates and the energetics of H pairs, are used as input parameters in kinetic Monte Carlo simulations to study the time evolution of an initial random distribution of adsorbed H atoms. The simulations reveal that, at room temperature, H atoms occupy only one sublattice before they completely desorb or form clusters. This sublattice selectivity in the distribution of H atoms may last for sufficiently long periods of time upon lowering the temperature down to 0 ◦C. The final fate of the H atoms, namely, desorption or cluster formation, depends on the actual relative values of the activation barriers which can be tuned by doping. In some cases, a sublattice selectivity can be obtained for periods of time experimentally relevant even at room temperature. This result shows the possibility for observation and applications of the ferromagnetic state associated with such distributionThis work was supported by MINECO under Grant Nos. FIS2013-47328 and FIS2012-37549, by CAM under Grant Nos. S2013/MIT-3007, P2013/MIT-2850, and by Generalitat Valenciana under Grant PROMETEO/2012/01

Geometry and quantum delocalization of interstitial oxygen in silicon

The problem of the geometry of interstitial oxygen in silicon is settled by proper consideration of the quantum delocalization of the oxygen atom around the bond-center position. The calculated infrared absorption spectrum accounts for the 517 and 1136 cm$^{-1}$ bands in their position, character, and isotope shifts. The asymmetric lineshape of the 517 cm$^{-1}$ peak is also well reproduced. A new, non-infrared-active, symmetric-stretching mode is found at 596 cm$^{-1}$. First-principles calculations are presented supporting the nontrivial quantum delocalization of the oxygen atom.Comment: uuencoded, compressed postscript file for the whole. 4 pages (figures included), accepted in PR

Electronic and structural characterization of divacancies in irradiated graphene

We provide a thorough study of a carbon divacancy, a fundamental but almost unexplored point defect in graphene. Low temperature scanning tunneling microscopy (STM) imaging of irradiated graphene on different substrates enabled us to identify a common two-fold symmetry point defect. Our first principles calculations reveal that the structure of this type of defect accommodates two adjacent missing atoms in a rearranged atomic network formed by two pentagons and one octagon, with no dangling bonds. Scanning tunneling spectroscopy (STS) measurements on divacancies generated in nearly ideal graphene show an electronic spectrum dominated by an empty-states resonance, which is ascribed to a spin-degenerated nearly flat band of $\pi$-electron nature. While the calculated electronic structure rules out the formation of a magnetic moment around the divacancy, the generation of an electronic resonance near the Fermi level, reveals divacancies as key point defects for tuning electron transport properties in graphene systems.Comment: 5 page

Ab initio energetics and kinetics study of H2 and CH4 in the SI clathrate hydrate

We present ab initio results at the density functional theory level for the energetics and kinetics of H2 and CH4 in the SI clathrate hydrate. Our results complement a recent article by some of the authors [G.Román-Pérez et.al., Phys.Rev.Lett. 105, 145901 (2010)] in that we show additional results of the energy landscape of H2 and CH 4 in the various cages of the host material, as well as further results for energy barriers for all possible diffusion paths of H2 and CH4 through the water framework. We also report structural data of the low-pressure phase SI and the higher-pressure phases SII and S

Low-energy quantum dynamics of atoms at defects. Interstitial oxygen in silicon

The problem of the low-energy highly-anharmonic quantum dynamics of isolated impurities in solids is addressed by using path-integral Monte Carlo simulations. Interstitial oxygen in silicon is studied as a prototypical example showing such a behavior. The assignment of a "geometry" to the defect is discussed. Depending on the potential (or on the impurity mass), there is a "classical" regime, where the maximum probability-density for the oxygen nucleus is at the potential minimum. There is another regime, associated to highly anharmonic potentials, where this is not the case. Both regimes are separated by a sharp transition. Also, the decoupling of the many-nuclei problem into a one-body Hamiltonian to describe the low-energy dynamics is studied. The adiabatic potential obtained from the relaxation of all the other degrees of freedom at each value of the coordinate associated to the low-energy motion, gives the best approximation to the full many-nuclei problem.Comment: RevTeX, 6 pages plus 4 figures (all the figures were not accesible before

Unraveling the intrinsic and robust nature of van hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis

Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of vanHove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1º to 10º) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM imagesThis work was supported by Spain’s MICINN under Grants No. MAT2010-14902, No. CSD2010-00024, and No. CSD2007-00050, and by Comunidad de Madrid under Grant No. S2009/MAT-1467. M. M. U., I. B., P. M, J.-Y.V., L. M., and J. M. G.-R. also acknowledge the PHC Picasso program for financial support (Project No. 22885NH). I. B. was supported by a Ramón y Cajal project of the Spanish MEC. L. M., P. M., and J.-Y.V. acknowledge support from Fondation Nanosciences (Dispograph project

Estados superficiales de silicio

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física, Fecha de lectura 08-197

Theoretical study of magnetic moments induced by defects at the SiC(110) surface

4 páginas, 6 figuras, 1 tabla.-- PACS number(s): 71.20.Mq, 75.70.Rf, 81.05.U−The effect of different surface defects on the atomic and electronic structures of cubic β-SiC(110) surface are studied by means of a first-principles calculation based on density-functional theory using the siesta code. In the calculations, different spin populations at each atom are allowed. We find that while adsorption of atomic O, N, or H on surface C atoms do not induce magnetic moments on SiC(110), Si vacancies, substitutional C at the Si sites, and H or F adsorbed on Si surface sites induce localized magnetic moments as large as 0.7 μB at the C atoms close to the defect. The local magnetic moment arrangement varies from ferromagnetic in the case of H adsorption to antiferromagnetic in the Si vacancy and substitutional C cases. The case of H adsorption on Si surface atoms is discussed in detail. It is concluded that magnetism is mainly owing to the local character of the C valence orbitals.Financial support of the Spanish Ministry of Science and Innovation through Grants No. FIS2009-12712 and No. CSD2007-00050 is acknowledged.Peer reviewe

A charge density wave model for reconstructed monolayers of Co on Cu(100)

We have grown in ultra high vacuum Co on top of a clean unreconstructed Cu(100) surface. The system shows upon absorption of one third of a Co monolayer a c(2 × 2) structure as low energy electron diffraction measurements reveal. The electronic structure of the 1 × 1 Co-Cu(100) is calculated. We then interpret the c(2 × 2) reconstruction as being due to a ferromagnetic induced charge density waveThis work was supported in part by the U.S.-Spain Friendship and Cooperative Treaty - Complementary Agreement No.Peer reviewe