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2-D materials are considered to have great potential to integrate future generations of nanoelectronic devices aiming to outperform the limitations that will eventually reach the conventional silicon technology. In this letter, a field-effect transistor composed by half-hydrogenated-silicene (HHS) connected to pristine silicene regions is theoretically investigated. Using ab-initio ballistic transport simulations based on DFT, it is shown that this device has an extremely high ${I}_{ \mathrm{ON}}/{I}_{ \mathrm{OFF}}$ ratio, for very small bias voltages and for channel lengths smaller than 10 nm. In particular, an expression for ${I}_{ \mathrm{ON}}/{I}_{ \mathrm{OFF}}$ = exp {[3.35A−1] $w$ } is obtained, where ${w}$ is the channel length. Finally, we demonstrated that the peculiar electronic structure of the HHS allows a fine-tuning of the charge transport mechanism resulting in ultra-low values for the subthreshold slope close to 24.5 meV /dec, which overcomes the inferior limit of 60 meV /dec claimed for conventional devices.

The water-solvated excess electron (EE) is a key chemical agent whose hallmark signature, its asymmetric optical absorption spectrum, continues to be a topic of debate. While nearly all investigation has focused on the liquid-water solvent, the fact that the crystalline-water solvated EE shows a very similar visible absorption pattern has remained largely unexplored. Here, we present spin-polarized density-functional theory calculations subject to periodic boundary conditions of the interplay between an EE and a number of intrinsic lattice defects in ice Ih. Our results show that the optical absorption signatures in the presence of three unsaturated hydrogen bonds (HB) are very similar to those observed experimentally. Its low-energy side can be attributed to transitions between the EE ground state and a single localized excited level, in a picture that is different from that for the liquid solvent, where this portion has been associated with hydrogen-like s → p excitations. The blue tail, on the other hand, relates to transitions between the EE ground state and delocalized excited states, which is in line with the bound-to-continuum transition interpretations for the EE in liquid water. Finally, we find that, depending on the number of dangling HBs participating in the EE trap, its charge density may spontaneously break the spin degeneracy through exchange interactions with the surrounding electrons, displaying the many-electron quantum nature of the EE problem in ice Ih.

We put forward a tight-binding model for rhombohedral topological insulator materials with the space group ${D}_{3d}^{5}(R\overline{3}m)$. The model describes the bulk band structure of these materials over the whole Brillouin zone. Within this framework, we also describe the topological nature of surface states, characterized by a Dirac conelike dispersion and the emergence of surface projected bulk states near to the Dirac point in energy. We find that the breaking of the ${R}_{3}$ symmetry as one moves away from the $\mathrm{\ensuremath{\Gamma}}$ point has an important role in the hybridization of the ${p}_{x}, {p}_{y}$, and ${p}_{z}$ atomic orbitals. In our tight-binding model, the latter leads to a band mixing matrix element ruled by a single parameter. We show that our model gives a good description of the strategies/mechanisms proposed in the literature to eliminate and/or energy shift the bulk states away from the Dirac point, such as stacking faults and the introduction of an external applied electric field.

The structural and electronic properties of few layers of blue phosphorus and their van der Waals heterostructures with graphene were investigated by means of first-principles electronic structure calculations. We study the four energetically most stable stacking configurations for multilayers of blue phosphorus. For all of them, the indirect band-gap semiconductor character, are preserved. We show that the properties of monolayer graphene and single-layer (bilayer) blue phosphorus are preserved in the van der Waals heterostructures. Further, our results reveal that under a perpendicular applied electric field, the position of the band structure of blue phosphorus with respect to that of graphene is tunable, enabling the effective control of the Schottky barrier height. Indeed, for the bilayer blue phosphorene on top of graphene, it is possible to even move the system into an Ohmic contact and induce a doping level of the blue phosphorene. All of these features are fundamental for the design of new nanodevices based on van der Waals heterostructures.

The matrix metalloproteinase family (MMP) constitutes a family of zinc (Zn) proteases that catalyze the breaking of peptide bonds in proteins. These enzymes are very promising drug targets, since they are involved in remodeling and degradation of the extracellular matrix, which is a key process required for cancer metastasis, and thus, their reaction mechanism has been an area of intensive research. Early proposal based on acid base catalyzed hydrolysis, suggested that a conserved zinc bound water molecule acted as the nucleophile attacking the peptide bond carbon, after being activated by essential glutamate. The possibility of a direct nucleophilic attack by the enzyme, performed by the glutamate was also suggested. These are the key yet unsolved issues about MMP reaction mechanism. In the present work, we used hybrid quantum/classical calculations to analyze the structure and energetics of different possible hydrolysis reaction paths. The results support a water mediated mechanism, where both the nucleophile water molecule and the carbonyl oxygen of the scissile peptide bond are coordinated to zinc in the reactive configuration, while the essential glutamate acts as the base accepting the proton from the nucleophilic water. Formation of the carbon–oxygen bond and breaking of carbon–nitrogen bond were found to be concerted events, with a computed barrier of 14.8 kcal/mol. Substrate polarization was found to be important for the observed reaction mechanism, and a substantial change in the metal coordination environment was observed, particularly, regarding the zinc–histidine coordination.

A new acylated glycoside, 4-O-( 4’,6’-di-O-galloyl- b - D -glucopyranosyl)-trans-p-coumaric acid, named microcoryn ( 1 ) , together with sixteen known phenolic compounds, 5-O-(6'-O-galloyl- b - D -glucopyranosyl)-gentisic acid (2), ellagic acid (3), gallic acid (4), kaempferol (5), quercetin (6), 3-O-galloyl- b - D -glucose (7), 2,3,6-tri-O-galloyl- b - D -glucose (8), 1,2,4,6-tetra-O-galloyl- b - D -glucose (9), 1,2,3,4,6-penta-O-galloyl- b - D -glucose (10), 4,6-hexahydroxydiphenoyl- b - D -glucose (11), gemin D (12), tellimagrandin I (13), tellimagrandin II (14), isocoriariin F (15), oenothein C (16), and oenothein B (17) were isolated from the leaves of Eucalyptus microcorys . The structure of the new compound was elucidated by spectroscopic data, especially by 2D NMR techniques. This is the first phytochemical investigation of this plant’s leaf extract.

It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport properties calculations, that this assumption can not be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy levels positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrodes separation., Comment: 14 pages, 13 figures, to appear in Nanoscale

A magnetic tunnel junction consisting of a boron nitride nanoribbon contacted by two semi-infinite electrodes composed of (3,0) ferromagnetic zig-zag graphene nanoribbons was investigated. Performing spin-polarized ab initio transport calculations based on a scheme that combines non-equilibrium Green's function with density functional theory (NEGF+DFT) we predict that such system could act as spin-filter (the efficiency reaches 50%) and present a magnetoristance of 105%, depending on the length of the boron nitride region.

The respiration of metal oxides by the bacterium Geobacter sulfurreducens requires the assembly of a small peptide (the GS pilin) into conductive filaments termed pili. We gained insights into the contribution of the GS pilin to the pilus conductivity by developing a homology model and performing molecular dynamics simulations of the pilin peptide in vacuo and in solution. The results were consistent with a predominantly helical peptide containing the conserved α-helix region required for pilin assembly but carrying a short carboxy-terminal random-coiled segment rather than the large globular head of other bacterial pilins. The electronic structure of the pilin was also explored from first principles and revealed a biphasic charge distribution along the pilin and a low electronic HOMO-LUMO gap, even in a wet environment. The low electronic band gap was the result of strong electrostatic fields generated by the alignment of the peptide bond dipoles in the pilin's α-helix and by charges from ions in solution and amino acids in the protein. The electronic structure also revealed some level of orbital delocalization in regions of the pilin containing aromatic amino acids and in spatial regions of high resonance where the HOMO and LUMO states are, which could provide an optimal environment for the hopping of electrons under thermal fluctuations. Hence, the structural and electronic features of the pilin revealed in these studies support the notion of a pilin peptide environment optimized for electron conduction.

We investigate the transport properties (IxV curves and zero bias transmittance) of pristine graphene nanoribbons (GNRs) as well as doped with boron and nitrogen using an approach that combines nonequilibrium Green's functions and density functional theory (DFT) [NEGF-DFT]. Even for a pristine nanoribbon we verify a spin-filter effect under finite bias voltage when the leads have an antiparallel magnetization. The presence of the impurities at the edges of monohydrogenated zigzag GNRs changes dramatically the charge transport properties inducing a spin-polarized conductance. The IxV curves for these systems show that depending on the bias voltage the spin polarization can be inverted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The fact that the resistance of propagating electrons in solids depends on their spin orientation has led to a new field called spintronics. With the parallel advances in nanoscience, it is now possible to talk about nanospintronics. Many works have focused on the study of charge transport along nanosystems, such as carbon nanotubes, graphene nanoribbons, or metallic nanowires, and spin dependent transport properties at this scale may lead to new behaviors due to the manipulation of a small number of spins. Metal nanowires have been studied as electric contacts where atomic and molecular insertions can be constructed. Here we describe what might be considered the ultimate spin device, namely, a Au thin nanowire with one Co atom bridging its two sides. We show that this system has strong spin dependent transport properties and that its local symmetry can dramatically change them, leading to a significant spin polarized conductance.

We show, using ab initio calculations based on density functional theory, that for hydrogen-passivated Si nanowires (SiNWs), the relative contribution of surface atoms to the band-edge states varies according to the way these surface atoms are bonded to the core ones. The largest influence occurs when these bonds are oriented along the wire's growth direction, which occurs either on the symmetric (001) 1 x 1 or the monohydrated (111) 1 x 1 surfaces. These results are obtained for wires grown along the [110] direction, with hexagonal cross-sections and facets corresponding to (111) and (001) planes, as observed experimentally. Their diameters range from about 10 to 35 A. On the basis of our results, we propose that particular facets should be more appropriate to be functionalized in order to build SiNW-based sensors. As an example, we have investigated the effect of NH2 adsorbed on some of these surfaces on the electronic structure of these wires.

We theoretically investigate, as a function of the stretching, the behaviour of the thermoelectric properties – the Seebeck coefficient (S), the electronic heat conductance (κel) and the figure of merit (ZT) – of a molecule-based junction composed of a benzene-1,4-dithiolate molecule (BDT) coupled to Au(111) surfaces at room temperature. We show that the thermoelectric properties of a single molecule junction can be tuned by mechanic stretching. The Seebeck coefficient is positive, indicating that it is dominated by the HOMO. Furthermore, it increases as the HOMO level, which is associated to the sulphur atom, tends towards energies close to the Fermi energy. By modelling the transmission coefficient of the system as a single Lorentzian peak, we propose a scheme to obtain the maximum ZT of any molecular junction.

We describe a procedure to calculate charge transport properties across a nanosystem. This scheme is based on a Green's Function formalism to treat a non-equilibrium problem, coupled to the Density Functional Theory to describe the electronic structure. As an illustration, we perform calculations for the charge transport across a (5,5) carbon nanotube with a vacancy.

A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga1-xMnxAs diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J$^{Mn-Mn}_n$, from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J$^{Mn-Mn}_n$ with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.

Metal nanowires and in particular gold nanowires have received a great deal of attention in the past few years. Experiments on gold nanowires have prompted theory and simulation to help answer questions posed by these studies. Here we present results of computer simulations for the formation, evolution and breaking of very thin Au nanowires. We also discuss the influence of contaminants, such as atoms and small molecules, and their effect on the structural and mechanical properties of these nanowires.

We propose the use of the Si atom in the experimentally observed C59Si molecule as a possible way to controllably anchor fullerene molecules on a Si surface, due to the formation of a strong bond to one of the Si surface atoms. All our results are based on ab initio total energy density functional theory, and we obtain that the binding energy is on the order of 2.1 eV, approximately 1.4 eV more stable than a C60 bonded in a similar situation. A possible route to obtain such adsorption via a (C59Si)2 dimer is examined, and we find the whole process to be exothermic by approximately 0.2 eV.

A unified band structure model is proposed to explain the magnetic ordering in Mn-doped semiconductors. This model is based on the p–d and d–d level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II–VI and III–V semiconductors such as CdTe, GaAs, ZnO, and GaN. The model can also be used to explain the interesting behavior of GaMnN, which changes from ferromagnetic ordering to antiferromagnetic ordering as the Mn concentration increases. This model, therefore, is useful to provide a simple guideline for future band structure engineering of magnetic semiconductors.

We propose the use of the Si atom in the C 5 9 Si molecule as a possible way to controllably anchor fullerene molecules on an Si surface, due to the formation of a strong bond to one of the Si surface atoms. We considered a monohydride Si(100) surface with the H removed from one of the Si surface atoms, and the C 5 9 Si was adsorbed at this site. Through ab initio calculations based on Density Functional Theory, we obtained a final Si s u r f a c e -Si C 5 9 S i bond distance of 2.37 A, and a binding energy of -3.0 eV. Considering the reaction H s u r f a c e + C 5 9 SiH → (C 5 9 Si) a d s o r b e d + (H 2 ) g a s , we find this process to be exothermic by 0.41 eV. Upon adsorption of an isolated molecule, we find four levels in the gap, three empty and one singly occupied, all of them being basically localized at the C 5 9 Si molecule.

We have performed ab initio calculations within the density-functional theory for Ga1−xMnxAs diluted semiconductors. Total energy results unambiguously show that a quasi-localized hole, with predominant p-like character, surrounds the fully polarized Mn d5-electrons. The calculations indicate that the holes form a relatively dispersionless impurity band, thus rendering effective-mass descriptions of hole states open to challenge. We obtain estimates both for the s = 1/2 hole and S = 5/2 Mn exchange coupling, and for the distance dependence of the effective Mn–Mn exchange interaction. The results demonstrate that the effective Mn–Mn coupling is always ferromagnetic, and thus non-RKKY, and is intermediated by the antiferromagnetic coupling of each Mn spin to the holes.

Using ab initio calculations, based on the density functional theory, we have investigated the electronic, magnetic and structural properties of a Mn neutral impurity on the bare and hydrogenated Si(1 0 0) surface, as well as on bulk Si. We have observed that the most stable site for adsorption on the bare surface is near the second layer, below a substrate dimer, whereas for the hydrogenated surface it is in the through, between two dimer rows. Hydrogenation makes the surface less reactive and increases the formation energy of the impurity in the surface. In the bare surface, we have found a substitutional site that has the same formation energy as an interstitial one, and this can be an important route for the development of dilute magnetic semiconductors based on Si.

Using ab initio density functional theory total energy calculations, we study the influence of H, B, C, N, O, and S in the rupture of a gold nanowire. In particular, using an as realistic as possible model for a suspended gold nanowire under stress, we observe that the Au wire always breaks at an Au–Au bond, with a maximum bond length between 3.0 and 3.1 A. Therefore, the experimentally observed large Au–Au bonds before the rupture of the nanowire (≈3.6 A) are probably due to the presence of light impurities (X) forming Au–X–Au bonds. We obtain that the maximum Au–Au distance, for X equals C or N, is of the order of 3.9 A, whereas for B and O it is of the order of 4.1 A. On the other hand, for H this maximum distance before the rupture of the wire is approximately 3.6 A, being the best candidate to explain the experimental results. For both C and H impurities, we present a detailed analysis of the neck atoms electronic structures, and compare them with similar results for the pure nanowire.

We have investigated the formation evolution and breaking of a gold nanowire using molecular dynamics simulation. Results reproduce many features observed experimentally. Suspended gold nanowires have recently been produced in an ultra-high vacuum and were imaged by electron microscopy. Here we simulate gold nanowires under stress. In particular, we discuss the mechanisms of formation, evolution and breaking of these atomically thin Au nanowires.

We study the structural, electronic and magnetic properties of Fe and Mn monomers and dimers adsorbed on a semiconductor (8,0) single-wall carbon nanotube. Using spin-polarized total-energy ab initio calculations based on the density-functional theory, we find for both Fe and Mn monomers the hexagonal center as the most stable adsorption site. For Fe dimers in the most stable configuration, the atoms are on opposite C–C bond-centered positions (BC–BC), with an Fe–Fe distance of 2.23 A. For Mn dimers, on the other hand, the most stable configurations are found to be with the Mn atoms adsorbed on top of C atoms, for a high-spin state (Mn–Mn distance of 2.6 A), and a slightly lower-energy BC–BC configuration for the low-spin state (Mn–Mn distance of 2.53 A). These results show that the dimer configurations are not simply related to the isolated monomer structures. The resulting magnetic moments for the adsorbed dimers are found to be similar to their corresponding values in the free diatomic molecules.

We perform a detailed theoretical study, within the density-functional theory, of the electronic structure and magnetic properties of Ga1−xMnxAs diluted semiconductors. Ab initio total energy results provide evidence that the appearance of a ferromagnetic state in these materials is due to an exchange coupling between the localized ↑ S= 5 2 Mn spins mediated by quasi-localized ↓ holes, with strong p-like character, surrounding the fully polarized Mn d5-electrons. From total energy differences, we find the effective Mn–Mn coupling always ferromagnetic, with the Mn–Mn interaction intermediated by an antiferromagnetic coupling of each Mn spin to the holes.

In this work the structural, electronic and magnetic properties of one-dimensional Fe chains interacting with a semiconductor single-wall carbon nanotube are investigated using first-principles spin-polarized calculations. A systematic study of several configurations of a Fe wire interacting with a nanotube both from outside as well as inside is presented, and the resulting equilibrium distances, binding energies and magnetizations are discussed. The most stable configurations are compared with previous results obtained for a single Fe atom interacting with the tube.

The energetics and structural properties of atoms and molecules on a substitutional Si atom in single wall carbon nanotubes (SWCN) are investigated using first principle calculations based on density-functional theory. A detailed analysis is performed for the geometry and the electronic structures of a Si-doped semiconducting (10,0) carbon nanotube interacting with F, Cl, H, CH3, and SiH3. A common feature for the systems with these atoms or molecules is the presence of one half-filled level close to the top of the valence band. The specific position of this level in the gap depends on the chemisorbed species and the binding energy between this species and the Si atom.

In this Letter, we study the structural and electronic properties of single-layer and bilayer phosphorene with graphene. We show that both the properties of graphene and phosphorene are preserved in the composed heterostructure. We also show that via the application of a perpendicular electric field, it is possible to tune the position of the band structure of phosphorene with respect to that of graphene. This leads to control of the Schottky barrier height and doping of phosphorene, which are important features in the design of new devices based on van der Waals heterostructures.

We have performed an ab initio investigation of the electronic properties of the graphene sheet adsorbed by Ru adatoms (Ru/graphene). For a particular set of triangular arrays of Ru adatoms, we find the formation of four (spin-polarized) Dirac cones attributed to a suitable overlap between two hexagonal lattices: one composed by the C sites of the graphene sheet, and the other formed by the surface potential induced by the Ru adatoms. Upon the presence of spin-orbit coupling (SOC) nontrivial band gaps take place at the Dirac cones promoting several topological phases. Depending on the Ru concentration, the system can be topologically characterized among the phases i) Quantum Spin Hall (QSH), ii) Quantum Anomalous Hall (QAH), iii) metal iv) or trivial insulator. For each concentration, the topological phase is characterized by the ab-initio calculation of the Chern number., Comment: 8 pages, 6 figures

We theoretically studied the phase transition between the cubic-diamond and β-Sn structures for the Si 0.5 Ge 0.5 alloy. We performed first principles calculations based on the density-functional theory, using the local-density approximation, and the variable cell shape molecular dynamics approach. The alloy was modeled using an ordered zincblende type lattice, and we obtained a transition pressure of 8.35 GPa. We also calculated the enthalpy curves at zero as well as at the transition pressure. At zero pressure, the enthalpy difference between the two phases is 0.016 Ry/atom, and there is a barrier of approximately 0.026 Ry/atom. At the transition pressure, the enthalpy barrier is still 0.018 Ry/atom, a sign of the strong first order character of this phase transformation.

The adsorption of the Ge monomer on the Si(1 0 0) surface is studied through first principles calculations based on the density-functional theory. We have observed that, for a given position of the monomer on the surface, there are many local minima which differ in the substrate local configuration of the buckling of the silicon dimers. We show that this local configuration may also play an important role for the diffusion of these ad-atoms.

We have performed systematical ab initio studies of the electronic and structural properties of the disordered phase of the Si1−xGex alloy. For the defect free alloy, we find that the Si–Si bond length has a tendency to maintain its natural value (x=0) all the way up to x>0.5, similarly to what is found experimentally. We also analyze the vacancy, and note, as expected for a structurally as well as chemically disordered solid, that there is no local symmetry around the vacancy site. Finally, we perform some studies of the alloy under pressure all the way up to 30 GPa, and find that the structure is highly distorted in some cases, indicating that an amorphous phase may be found in the pressure region of the diamond–β-tin phase transition.

Motivated by the controversy between quantum chemists and solid-state physicists, and by recent experimental results, spin-polarized density-functional (DFT) calculations are used to probe electron correlation in the Si(100) reconstructed surface. The ground state displays antiferromagnetic spin polarization for low dimer inclinations indicating, not magnetic order, but the importance of Mott-like correlations among dangling bonds. The lowest energy corresponds to a higher dimer inclination with no spin. DFT energies, however, should be taken with caution here. Our results together with quantum-chemical findings suggest dimers with highly correlated electrons that tend to buckle due to interactions with other dimers., Comment: 5 pages, 1 eps figure, 1 table; RevTeX v3.1. To appear in Surface Science (proceedings of the European Conference On Surface Science, ECOSS-19, Madrid, Sept. 5-8, 2000)

We study, using first principles calculations, the electronic and structural properties of an isolated vacancy and self-interstitial defects in germanium. For the vacancy we analyze the lattice relaxations for the charge states (++), (+), and (0), and show that the atomic displacements, albeit small, are not completely negligible for the second-nearest-neighbor atoms to the vacancy site. Moreover, we show that the relaxations decay slowly along the zigzags that go through the vacancy site. For the self-interstitial we present an analysis of the charge density, which supports the description of this defect as a four-atoms ring strongly attached to the lattice, which we have named a kite-defect.

A complete microscopic picture of the germanium vacancy is presented, and our results are compared with recent measurements. We analyze, through first principles calculations, the structural relaxations, Jahn-Teller distortions, and orbitals for the charge states $(++,+,0,\ensuremath{-},\ensuremath{-}\ensuremath{-}).$ The formation energies for the different charge states are presented, as well as the positions of the $(++/+),$ $(+/0),$ $(0/\ensuremath{-}),$ and $(\ensuremath{-}/\ensuremath{-}\ensuremath{-})$ levels, and we obtain that the vacancy in Ge is not an Anderson negative-$U$ system, as opposed to the silicon vacancy. We propose as an explanation a much smaller electron-lattice coupling for the E mode in germanium than in silicon.

The electronic and structural properties of an isolated vacancy and self-interstitial defects in germanium are studied through ‘parameter free’ calculations. We analyze the lattice relaxations and Jahn–Teller distortions for several charge states. Our results for ionization levels of the Ge-vacancy are in fair agreement with experimental results. In contrast to the silicon vacancy we obtain that the germanium vacancy does not present an Anderson negative-U. For the self-interstitial defect we obtain, similarly to silicon, that the lowest-energy configuration is the split dumbbell configuration.

We study, using ab initio calculations, small structures that are present at the early stages of growth of Ge on the Si(1 0 0) surface. Ad-atoms, ad-dimers, and ad-trimers are investigated. We consider different configurations of the adsorbed structures, and present results for their relaxed geometries and relative energies.

This work describes a neural network model which simulates a discrete part of the dopaminergic striatal circuitry involved in reinforcement learning. We consider the proposal that learning by reinforcement is acquired by a heterosynaptic mechanism affecting plasticity of corticostriatal synapses, under the modulatory control of DA neurons. The simulation results seem to be successful since they are consistent with the literature that shows a transfer of DA neuron firing from the primary reinforcement stimulus to the conditioned stimulus after learning. We also present some simulations, using the same model, of two learning paradigms, blocking and overshadowing, which may be mediated by this dopamine circuitry.

Made available in DSpace on 2022-04-28T18:53:17Z (GMT). No. of bitstreams: 0 Previous issue date: 1999-02-01 We report elastic and rotationally inelastic cross sections for e- - XH3 collisions (X: N, P, As, Sb) at the static-exchange level of approximation. The energy range was from 7.5 up to 30 eV. Our fixed-nuclei scattering amplitudes were obtained through the Schwinger multichannel method with pseudopotentials (SMCPP) [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)]. The rotational cross sections were obtained with the help of the adiabatic-nuclei-rotation approximation. There is good agreement with available experimental elastic cross sections. In order to improve rotational cross sections at small scattering angles for the dipole-allowed 00→10 rotational excitation, we have combined the SMCPP and the first Born approximation of the full interaction potential and also of the dipole moment potential. To our knowledge this is the first time that rotational excitation cross sections for these molecules are reported. © 1999 American Institute of Physics. Inst. de Física Gleb Wataghin Universidade Estadual de Campinas UNICAMP, 13083-970, Campinas, São Paulo Departamento de Física Univ. Federal do Paraná UFPR, Caixa Postal 19081, 81531-990 Curitiba, Paraná Instituto de Física Univ. Estadual de São Paulo USP, Caixa Postal 66318, 05389-970, Sao Paulo, Sao Paulo

Filter diagonalization (Wall, M. R.; Neuhauser, D. J. Chem. Phys. 1995, 102, 8011) is a new and efficient method for extracting frequencies and damping constants from a short-time segment of any time-dependent signal, whether of quantum nature or not. In this letter, we will demonstrate that filter diagonalization can be used to follow the anharmonic vibrational dynamics of Si6. Using the technique, we can locate (resolve) the normal modes (including the widths) from even a very short-time correlation signal generated by ab initio molecular dynamics calculations. Our results show that filter diagonalization can reduce significantly the necessary sampling time with an ab initio molecular simulation and aid in understanding normal-mode dynamics for systems that are affected by anharmonicity.

We present the first use of ab initio molecular dynamics (AIMD) with correlated wavefunctions to study a gas phase reaction. Our benchmark system is the well-studied F + H2 reaction. The neutral, unstable species FH2 is created by removing an electron from an equilibrated anion (FH2)−, simulating the photoelectron spectroscopy experiments of Neumark and co-workers. These experiments attempt to probe the transition state (TS) region by creating a negative ion that may have a structure similar to the neutral reaction TS. We then follow the dynamics of the neutral system on its Born-Oppenheimer surface, both forward and backward in time, to see how close the anion equilibrium geometry truly is to the neutral TS for the F + H2 → HF + H reaction. We then perform AIMD from the actual neutral TS for comparison. All electronic energy and gradient calculations are at the complete active space self-consistent field level. We find for this system that our level of approximation leads only to a qualitatively correct description. It allows us, however, to show that at finite temperature the anion geometry is markedly different from the neutral TS structure in that instantaneous electron detachment forms an FH2 complex lying entirely in the reactants valley. We propose that analysis of the asymptotic fragments in the photodetachment experiment will almost invariably detect F + H2 and very rarely FH + H, and we also discuss which conditions would lead to a different conclusion.

The buckled structure of silicene leads to the possibility of new kinds of line defects that separate regions with reversed buckled phases. In the present work we show that these new grain boundaries have very low formation energies, one order of magnitude smaller than grain boundaries in graphene. These defects are stable along different orientations, and they can all be differentiated by STM images. All these defects present local dimerization between the Si atoms, with the formation of $\pi$-bonds. As a result, these defects are preferential adsorption sites when compared to the pristine region. Thus, the combination of low formation energy and higher reactivity of these defects may be cleverly used to design new nano-structures embedded in silicene., Comment: 8 pages, 8 figures

Graphane, hydrogenated graphene, can be patterned into electronic devices by selectively removing hydrogen atoms. The most simple of such devices is the so-called nanoroad, analogous to the graphene nanoribbon, where confinement-and the opening of a gap-is obtained without the need for breaking the carbon bonds. In this work we address the electronic transport properties of such systems considering different hydrogen impurities within the conduction channel. We show, using a combination of density functional theory and non-equilibrium Green's functions, that hydrogen leads to significant changes in the transport properties and in some cases to current polarization.

We show that the Si${}_{x}$Ge${}_{1\ensuremath{-}x}$ random alloys in the honeycomb monolayer structure are nontrivial topological insulators with the same topological classification ${Z}_{2}=1$ as pristine silicene and germanene. The application of an external electric field decreases the band gap up to its closure at a critical value of the field, $|{E}_{c}|$. For absolute values of the field larger than $|{E}_{c}|$ the band gap increases again, and the systems are now trivial insulators. Thus, similarly to what happens for zinc-blend Si${}_{x}$Ge${}_{1\ensuremath{-}x}$ alloys, these materials will bring new possibilities in the design of devices, such as band gap and mobility engineering, with the added feature that they will exhibit a quantum spin Hall effect due to the gap generated by the spin-orbit coupling.

A suggestion for substitutional Si doping in single-wall carbon nanotubes (SWNTs) through radial tube deformation is presented. Although the doping of buckyballs with substitutional Si atoms has been experimentally achieved, so far the synthesis of silicon-doped SWNT has not been reported. Using theoretical methods based on first-principles, the formation energies for Si doping on deformed (10,0) SWNTs are calculated, and their electronic and structural properties are investigated. It is established that, upon tube compression, the formation energy for Si doping in the large curvature region of the SWNT is reduced to values similar to the ones obtained for Si in buckyballs, indicating that this procedure may help to surmount the experimental difficulties associated with Si doping of SWNTs.