Search

We analyze theoretically four-terminal electronic devices composed of two crossed graphene nanoribbons (GNRs) and show that they can function as beam splitters or mirrors. These features are identified for electrons in the low-energy region where a single valence or conduction band is present. Our modeling is based on p z orbital tight binding with Slater-Koster-type matrix elements fitted to accurately reproduce the low-energy bands from density functional theory calculations. We analyze systematically all devices that can be constructed with either zigzag or armchair GNRs in AA and AB stackings. From Green's function theory the elastic electron transport properties are quantified as a function of the ribbon width. We find that devices composed of relatively narrow zigzag GNRs and AA-stacked armchair GNRs are the most interesting candidates to realize electron beam splitters with a close to 50:50 ratio in the two outgoing terminals. Structures with wider ribbons instead provide electron mirrors, where the electron wave is mostly transferred into the outgoing terminal of the other ribbon, or electron filters where the scattering depends sensitively on the wavelength of the propagating electron. We also test the robustness of these transport properties against variations in the intersection angle, stacking pattern, lattice deformation (uniaxial strain), inter-GNR separation, and electrostatic potential differences between the layers. These generic features show that GNRs are interesting basic components to construct electronic quantum optical setups., This work was supported by the project Spanish Ministerio de Economía y Competitividad (MINECO) through the Grants no. FIS2017-83780-P (Graphene Nanostructures “GRANAS”) and no.MAT2016-78293-C6-4R, the Basque Departamento de Educación through the PhD fellowship no. PRE_2019_2_0218 (S.S.), the University of the Basque Country through the Grant no. IT1246-19, and the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant no. 863098).

Graphene nanoribbons (GNRs), low-dimensional platforms for carbon-based electronics, show the promising perspective to also incorporate spin polarization in their conjugated electron system. However, magnetism in GNRs is generally associated with localized states around zigzag edges, difficult to fabricate and with high reactivity. Here we demonstrate that magnetism can also be induced away from physical GNR zigzag edges through atomically precise engineering topological defects in its interior. A pair of substitutional boron atoms inserted in the carbon backbone breaks the conjugation of their topological bands and builds two spin-polarized boundary states around them. The spin state was detected in electrical transport measurements through boron-substituted GNRs suspended between the tip and the sample of a scanning tunneling microscope. First-principle simulations find that boron pairs induce a spin 1, which is modified by tuning the spacing between pairs. Our results demonstrate a route to embed spin chains in GNRs, turning them into basic elements of spintronic devices., We gratefully acknowledge financial support from Spanish Agencia Estatal de Investigación (AEI) (MAT2016-78293, PID2019-107338RB, FIS2017-83780-P, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618), from the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant. No. 863098), the Basque Departamento de Educación through the PhD fellowship No. PRE_2019_2_0218 (S.S.), the Xunta de Galicia (Centro de Investigación de Galicia accreditation 2019–2022, ED431G 2019/03), the University of the Basque Country (Grant IT1246-19), and the European Regional Development Fund (ERDF). I. P. also thanks Xunta de Galicia and European Union (European Social Fund, ESF) for the award of a predoctoral fellowship

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., The Bethe-Salpeter equation (BSE) is currently the state of the art in the description of neutral electronic excitations in both solids and large finite systems. It is capable of accurately treating charge-transfer excitations that present difficulties for simpler approaches. We present a local basis set formulation of the BSE for molecules where the optical spectrum is computed with the iterative Haydock recursion scheme, leading to a low computational complexity and memory footprint. Using a variant of the algorithm we can go beyond the Tamm-Dancoff approximation. We rederive the recursion relations for general matrix elements of a resolvent, show how they translate into continued fractions, and study the convergence of the method with the number of recursion coefficients and the role of different terminators. Due to the locality of the basis functions the computational cost of each iteration scales asymptotically as O(N3) with the number of atoms, while the number of iterations typically is much lower than the size of the underlying electron-hole basis. In practice we see that, even for systems with thousands of orbitals, the runtime will be dominated by the O(N2) operation of applying the Coulomb kernel in the atomic orbital representation., We acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1083 project, the ANR ORGAVOLT project, and the Spanish MINECO MAT2013-46593-C6-2-P project. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, Una manera de hacer Europa. F.F. acknowledges support from the EXTRA programme of the “Universita degli Studi di Milano-Bicocca” and from the Erasmus Placement programme for student mobility.

D. Stradi et al., Monolayer graphene grown on Ru(0001) surfaces forms a superstructure with periodic modulations in its geometry and electronic structure. The large dimension and inhomogeneous features of this superstructure make its description and subsequent analysis a challenge for theoretical modeling based on density functional theory. In this work, we compare two different approaches to describe the same physical properties of this surface, focusing on the geometry and the electronic states confined at the surface. In the more complex approach, the actual moiré structure is taken into account by means of large unit cells, whereas in the simplest one, the graphene moiré is completely neglected by representing the system as a stretched graphene layer that adapts pseudomorphically to Ru(0001). As shown in previous work, the more complex model provides an accurate description of the existing experimental observations. More interestingly, we show that the simplified stretched models, which are computationally inexpensive, reproduce qualitatively the main features of the surface electronic structure. They also provide a simple and comprehensive picture of the observed electronic structure, thus making them particularly useful for the analysis of these and maybe other complex interfaces. © 2013 American Physical Society., Work supported by the MICINN Projects No. FIS2010-15127, No. FIS2010-18847, No. CTQ2010-17006, No. FIS-2010-19609-C09-00, No. ACI2008-0777, No. 2010C-07-25200, and No. CSD2007-00010, the CAM program NANOBIOMAGNET S2009/MAT1726 and the Gobierno Vasco-UPV/EHU Project No. IT-366-07. S.B. acknowledges financial support from MEC under FPU Grant No. AP-2007-00157. D.S. acknowledges financial support from the FPI-UAM grant program.

A minimal model based on density-functional theory is proposed and solved to explain the unusual chemisorption properties of carbon-monooxide (CO) molecules on Cu(110)-(2 × 1)-O quasi-one-dimensional (1D) surface reported in Feng. The striking features of CO adsorption include (1) the strong lifting of the host Cu atom by 1 Å, and (2) the highly anisotropic CO-CO interaction leading to self-assembly into a nanograting structure. Our model implies that the 1D nature of the surface band is the key to these two features. We illustrate how formation of a chemical bond through specific orbital interactions between an adsorbate and 1D dispersive states of the substrate can impact the surface geometrical and electronic structure. © 2012 American Physical Society., We thank DOE-BES Division of Chemical Sciences, Geosciences, and Biosciences for support through Grant No. DE-FG02-09ER16056, W. M. Keck foundation, Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-00) and G.V.-UPV/EHU (Grant No. IT-366-07) for financial support.

The interpretation of scanning tunneling spectroscopy (STS) and scanning tunneling microscopy measurements of epitaxial graphene on lattice-mismatched substrates is a challenging problem, because of the spatial modulation in the electronic structure imposed by the formation of a moiré pattern. Here we describe the electronic structure of graphene adsorbed on Ru(0001) by means of density functional theory calculations that include van der Waals interactions and are performed on a large 11×11 unit cell to account for the observed moiré patterns. Our results show the existence of localized electronic states in the high and low areas of the moiré at energies close to and well above the Fermi level, respectively. Localization is due to the spatial modulation of the graphene-Ru(0001) interaction and is at the origin of the various peaks observed in STS spectra. © 2012 American Physical Society., Work supported by the MICINN Projects No. FIS2010-15127, No. FIS2010-18847, No. CTQ2010- 17006, No. FIS-2010-19609-C09-00, No. ACI2008-0777, No. 2010C-07-25200, and No. CSD2007-00010, the CAM program NANOBIOMAGNET S2009/MAT1726, and the Gobierno Vasco-UPV/EHU Project No. IT-366-07. S.B. acknowledges financial support from MEC under FPU Grant No. AP-2007-00157.

4 páginas, 3 figuras.-- PACS numbers: 73.22.Pr, 68.37.Ef, 68.55.-a, 71.15.Mb.-- et al., Elaborate density functional theory (DFT) calculations that include the effect of van der Waals (vdW) interactions have been carried out for graphene epitaxially grown on Ru(0001). The calculations predict a reduction of structural corrugation in the observed moiré pattern of about 25% (∼0.4 Å) with respect to DFT calculations without vdW corrections. The simulated STM topographies are close to the experimental ones in a wide range of bias voltage around the Fermi level., Work supported by the MICINN projects FIS2010-15127, FIS2010-18847, CTQ2010-17006, FIS2010-19609-C02-00, ACI2008-0777, 2010C-07- 25200, and CSD2007-00010, the CAM program NANOBIOMAGNET S2009/MAT1726, and the Gobierno Vasco—UPV/EHU project IT-366-07.

6 páginas, 4 figuras.-- PACS number(s): 73.90.+f, 75.70.-i, 61.48.De, We report ab initio calculations of the structural, electronic, and magnetic properties of a graphene monolayer substitutionally doped with Co (Cosub) atoms. These calculations are done within density-functional theory using the generalized gradient approximation. We focus in Co because among traditional ferromagnetic elements (Fe, Co, and Ni), only Cosub atoms induce spin polarization in graphene. Our results show the complex magnetism of Co substitutional impurities in graphene, which is mapped into simple models such as the π-vacancy and Heisenberg model. The links established in our work can be used to bring into contact the engineering of nanostructures with the results of π models in defective graphene. In principle, the structures considered here can be fabricated using electron irradiation to create defects and depositing Co at the same time., We acknowledge support from Basque Dep. de Educación and the UPV/EHU (Grant No. IT-366-07) and the Spanish MCI (Grant No. FIS2007-66711-C02-02).

9 páginas, 6 figuras, 2 tablas.-- PACS number(S): 33.15.Fm, 68.43.-h, 33.15.Kr, Density functional theory calculations are used to investigate the role of substrate-induced cooperative effects on the adsorption of water on a partially oxidized transition-metal surface, O(2×2)/Ru(0001). Focusing particularly on the dimer configuration, we analyze the different contributions to its binding energy. A significant reinforcement of the intermolecular hydrogen bond (H bond), also supported by the observed frequency shifts of the vibration modes, is attributed to the polarization of the donor molecule when bonded to the Ru atoms in the substrate. This result is further confirmed by our calculations for a water dimer interacting with a small Ru cluster, which clearly show that the observed effect does not depend critically on fine structural details and/or the presence of coadsorbates. Interestingly, the cooperative reinforcement of the H bond is suppressed when the acceptor molecule, instead of the donor, is bonded to the surface. This simple observation can be used to rationalize the relative stability of different condensed structures of water on metallic substrates., We acknowledge support from Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio Innovación y Ciencia (Grant No. FIS2007-66711- C02-00), and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Guipúzcoa. M.V.F.-S. acknowledges support from DOE under Grant No. DE-FG02-09ER16052.

Using density-functional theory we found that, depending on coverage, coadsorbed oxygen can act both as a promoter and as an inhibitor of the dissociation of water on Ru(0001), the transition between these two behaviors occurring at ∼0.2 ML. The key factor that determines this transition is the adsorption energy of the reaction products, OH in particular. The chemistry of this coadsorbed system is dictated by the effective coordination of the Ru atoms that participate in the bonding of the different species. In particular, we observed that a low coverage of oxygen increases the adsorption energy of the OH fraction on the Ru surface. This surprising extra stabilization of the OH with the coadsorption of oxygen can be understood in the context of the metallic bonding and could well correspond to a general trend for the coadsorption of electronegative species on metallic surfaces., We acknowledge support from the Basque Departamento de Educación, UPV/EHU Grant No. IT-366-07, the Spanish Ministerio de Educación y Ciencia Grant No. FIS2007-66711-C02-02, and ETORTEK research contracts funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa. The work at the Berkeley Lab was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The work of A.M. was financed by the Marie Curie Outgoing International Fellowship, Project No. 514412.

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of ∼0.2a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is ∼2.4 at velocities slightly above the threshold (v∼0.4a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local., This work has been funded by the EC’s Marie Curie program, UK’s BNFL and NERC, the Spanish MEC, the UPV/EHU and the Basque Government through the Etortek program.

Time-dependent density-functional theory is used to calculate quantum-size effects in the energy loss of antiprotons interacting with a confined two-dimensional electron gas. The antiprotons follow a trajectory normal to jellium circular clusters of variable size, crossing every cluster at its geometrical center. Analysis of the characteristic time scales that define the process is made. For high-enough velocities, the interaction time between the projectile and the target electrons is shorter than the time needed for the density excitation to travel along the cluster. The finite-size object then behaves as an infinite system, and no quantum-size effects appear in the energy loss. For small velocities, the discretization of levels in the cluster plays a role and the energy loss does depend on the system size. A comparison to results obtained using linear theory of screening is made, and the relative contributions of electron-hole pair and plasmon excitations to the total energy loss are analyzed. This comparison also allows us to show the importance of a nonlinear treatment of the screening in the interaction process. © 2006 The American Physical Society., This work was supported, in part, by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2004-06490-C03-00), and the EU Network of Excellence NANOQUANTA (Grant No. NMP4-CT-2004-500198). A.G.B. acknowledges financial support by the DIPC.

A Comment on the Letter by H. Hakkinen, R. N. Barnett, and U. Landman, Phys. Rev. Lett. 82, 3264 (1999).