R. W. Nunes

Berry-phase treatment of the homogeneous electric field perturbation in insulators

A perturbation theory of the static response of insulating crystals to homogeneous electric fields that combines the modem theory of polarization (MTP) with the variation-perturbation framework is developed at unrestricted order of perturbation. First, we address conceptual issues related to the definition of such a perturbative approach. In particular, in our definition of an electric-field-dependent energy functional for periodic systems, the position operator appearing in the perturbation term is replaced by a Berry-phase expression, along the lines of the MTP. Moreover, due to the unbound nature of the perturbation, a regularization of the Ferry-phase expression for the polarization is needed in order to define a numerically stable variational procedure. Regularization is achieved by means of discretization, which can be performed either before or after the perturbation expansion. We compare the two possibilities and apply them to a model tight-binding Hamiltonian. Lowest-order as well as generic formulas are presented for the derivatives of the total energy, the normalization condition, the eigenequation, and the Lagrange parameters.

Structures of Si and Ge Nanowires in the Subnanometer Range

We report an ab initio investigation of several structures of pristine Si and Ge nanowires with diameters between 0.5 and 2.0 nm. We consider nanowires based on the diamond structure, high-density bulk structures, and fullerenelike structures. Our calculations indicate a transition from sp3 geometries to structures with higher coordination, for diameters below 1.4 nm. We find that diamond-structure nanowires are unstable for diameters smaller than 1 nm, undergoing considerable structural transformations towards amorphouslike wires. For diameters between 0.8 and 1 nm, filled-fullerene wires are the most stable. For even smaller diameters (approximately 0.5 nm), we find that a simple hexagonal structure is particularly stable for both Si and Ge.

Stability of antiphase line defects in nanometer-sized boron nitride cones

Departamento de F´isica, Universidade Estadual de Feira de Santana,Km 3 BR-116, 44031-460, Feira de Santana, BA, Brazil.(Dated: February 2, 2008)We investigate the stability of boron nitride conical sheets of nanometer size, using first-principlescalculations. Our results indicate that cones with an antiphase boundary (a line defect that containseither B-B or N-N bonds) can be more stable than those without one. We also find that dopingthe antiphase boundaries with carbon can enhance their stability, leading also to the appearanceof localized states in the bandgap. Among the structures we considered, the one with the smallestformation energy is a cone with a carbon-modified antiphase boundary that presents a spin splittingof ∼0.5 eV at the Fermi level.

Correlated magnetic states in extended one-dimensional defects in graphene.

Ab initio calculations indicate that while the electronic states introduced by tilt grain boundaries in graphene are only partially confined to the defect core, a translational grain boundary introduces states near the Fermi level that are very strongly confined to the core of the defect, and display a ferromagnetic instability. The translational boundary lies along a graphene zigzag direction and its magnetic state is akin to that which has been theoretically predicted to occur on zigzag edges of graphene ribbons. Unlike ribbon edges, the translational grain boundary is fully immersed within the bulk of graphene, hence its magnetic state is protected from the contamination and reconstruction effects that have hampered experimental detection of the magnetic ribbon states. Moreover, our calculations suggest that charge transfer between grain boundaries and the bulk in graphene is short ranged, with charge redistribution confined to ~5 Å from the geometric center of the 1D defects.

Real-space approach to calculation of electric polarization and dielectric constants.

We describe a real-space approach to the calculation of the properties of an insulating crystal in an applied electric field, based on the iterative determination of the Wannier functions (WFs) of the occupied bands. It has been recently shown that a knowledge of the occupied WFs allows the calculation of the spontaneous (zero-field) electronic polarization. Building on these ideas, we describe a method for calculating the electronic polarization and dielectric constants of a material in non-zero field. The method is demonstrated for a one-dimensional tight-binding Hamiltonian.

Electron states in boron nitride nanocones

We apply first-principles calculations to study the electronic structure of boron nitride nanocones with disclinations of different angles θ=nπ/3. Nanocones with odd values of n present antiphase boundaries that cause a reduction of the work function of the nanocones, relative to the bulk BN value, by as much as 2 eV. In contrast, nanocones with even values of n do not have such defects and present work functions that are very similar to the BN bulk value. These results should have strong consequences for the field emission properties of boron nitride nanocones and nanotubes.

A study of inner process double-resonance Raman scattering in bilayer graphene

The dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. In a previous resonance Raman investigation of graphene, the electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering the outer DR process. In this work we analyze the data considering the inner DR process, and obtain SWM parameters that are in better agreement with those obtained from other experimental techniques. This result possibly shows that there is still a fundamental open question concerning the double resonance process in graphene systems.

Design of molecular wires based on one-dimensional coordination polymers

The authors report the results of ab initio calculations for the structural and electronic properties of one-dimensional coordination polymers with the general formula [M(6-MP)2]n (where 6-MP=6-mercaptopurinate, and M=MnII, FeII, CoII, NiII, and CuII). A common stable structure, consistent with the experimental data for [Cd(6-MP)2]n, is found for all metal cations studied, with the exception of MnII. Polymers containing FeII, NiII, and CoII are found to be ferromagnetic semiconductors, while [Cu(6-MP)2]n shows a Peierls-unstable paramagnetic metallic phase that undergoes a transition to a ferromagnetic semiconductor one under small stretching.

Complex evolution of the electronic structure from polycrystalline to monocrystalline graphene: Generation of a new Dirac point

First principles calculations, employed to address the properties of polycrystalline graphene, indicate that the electronic structure of tilt grain boundaries in this system displays a rather complex evolution toward graphene bulk, as the tilt angle decreases, with the generation of a Dirac point, at the Fermi level, that lies not on the usual graphene Brillouin zone

Surface dangling-bond States and band lineups in hydrogen-terminated Si, Ge, and Ge/si nanowires.

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