Pseudo Goldstone Bosons Phenomenology in Minimal Walking Technicolor

We construct the non-linear realized Lagrangian for the Goldstone Bosons associated to the breaking pattern of SU(4) to SO(4). This pattern is expected to occur in any Technicolor extension of the standard model featuring two Dirac fermions transforming according to real representations of the underlying gauge group. We concentrate on the Minimal Walking Technicolor quantum number assignments with respect to the standard model symmetries. We demonstrate that for, any choice of the quantum numbers, consistent with gauge and Witten anomalies the spectrum of the pseudo Goldstone Bosons contains electrically doubly charged states which can be discovered at the Large Hadron Collider.Comment: 25 pages, 5 figure

Inducing energy gaps in graphene monolayer and bilayer

In this paper we propose a mechanism for the induction of energy gaps in the spectrum of graphene and its bilayer, when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20-30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature

Bromination of Graphene and Graphite

We present a density functional theory study of low density bromination of graphene and graphite, finding significantly different behaviour in these two materials. On graphene we find a new Br2 form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant Br+-Br- has an empty pz-orbital located in the graphene electronic pi-cloud. Bromination opens a small (86meV) band gap and strongly dopes the graphene. In contrast, in graphite we find Br2 is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable Br2 concentration in graphite, finding low density bromination to be endothermic. Graphene may be a useful substrate for stabilising normally unstable transient molecular states

Calculated properties of nitrogen-vacancy complexes in beryllium- and magnesium-doped GaN

The properties of defect complexes consisting of a nitrogen vacancy with a substitutional beryllium or magnesium atom on neighboring lattice sites in hexagonal GaN are calculated using the AIMPRO local-density-functional theory method. Both types of defects VN−BeGa and VN−MgGa are bound with respect to their isolated constituents. They do not appear to have any electronic levels in the bandgap, and are expected to be neutral defects. Important structural differences are found. In its minimum energy configuration, the Be atom in the VN−BeGa complex lies nearly in the same plane as the three equivalent N atoms nearest to it. Thus, it has shorter Be−N bonds than the Ga−N distance in the bulk crystal, while the Mg atom in the VN−MgGa complex occupies a position closer the lattice site of the Ga atom it replaces. Hence, the VN−BeGa complex has a larger open volume than the VN−MgGa complex. This is consistent with positron annihilation experiments [Saarinen et al., J. Cryst. Growth 246, 281 (2002); Hautakangas et al., Phys. Rev. Lett. 90, 137402 (2003)]. The frequency of the highest local vibrational mode of the VN−BeGa center is calculated to be within 3–4 % of an infrared absorption line detected in Be-doped GaN [Clerjaud (private communication)].Peer reviewe

Passivation of copper in silicon by hydrogen

The structures and energies of model defects consisting of copper and hydrogen in silicon are calculated using the AIMPRO local-spin-density functional method. For isolated copper atoms, the lowest energy location is at the interstitial site with Td symmetry. Substitutional copper atoms are found to adopt a configuration with D2d symmetry. We conclude that the symmetry is lowered from Td due to the Jahn-Teller effect. Interstitial hydrogen atoms are found to bind strongly to substitutional copper atoms with an energy that is more than the difference in formation energy over the interstitial site for Cu. The resulting complex has C2v symmetry in the −2 charge state where the H atom is situated about 1.54 Å away from the Cu atom in a [100] direction. In other charge states the symmetry of the defect is lowered to Cs or C1. A second hydrogen atom can bind to this complex with nearly the same energy as the first. Two structures are found for copper dihydride complexes that have nearly equal energies; one with C2 symmetry, and the other with Cs symmetry. The binding energy for a third hydrogen atom is slightly more than for the first. Calculated electronic levels for the model defects relative to one another are found to be in fair to good agreement with experimental data, except for the copper-dihydride complex. The copper trihydride complex has no deep levels in the bandgap, according to our calculations.Peer reviewe

Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition

Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown

Calculated properties of point defects in Be-doped GaN

The properties of several point defects in hexagonal gallium nitride that can compensate beryllium shallow acceptors (BeGa) are calculated using the AIMPRO method based on local density functional theory. BeGa itself is predicted to have local vibrational modes (LVM’s) very similar to magnesium acceptors. The highest frequency is about 663cm−1. Consistent with other recent studies, we find that interstitial beryllium double donors and single-donor beryllium split interstitial pairs at gallium sites are very likely causes of compensation. The calculations predict that the split interstitial pairs possess three main LVM’s at about 1041, 789, and 738cm−1. Of these, the highest is very close to the experimental observation in Be-doped GaN. Although an oxygen donor at the nearest-neighboring site to a beryllium acceptor (BeGa−ON) is also a prime suspect among defects that are possibly responsible for compensation, its highest frequency is calculated to be about 699cm−1 and hence is not related in any way to the observed center. Another mode for this defect is estimated to be about 523cm−1 and is localized on the ON atom. These two vibrations of BeGa−ON are thus equivalent to those for the isolated substitutional centers perturbed by the presence of their impurity partners.Peer reviewe

Strained graphene: tight-binding and density functional calculations

We determine the band structure of graphene under strain using density functional calculations. The ab-initio band strucure is then used to extract the best fit to the tight-binding hopping parameters used in a recent microscopic model of strained graphene. It is found that the hopping parameters may increase or decrease upon increasing strain, depending on the orientation of the applied stress. The fitted values are compared with an available parametrization for the dependence of the orbital overlap on the distance separating the two carbon atoms. It is also found that strain does not induce a gap in graphene, at least for deformations up to 10%

Theory of Threading Edge and Screw Dislocations in GaN

The atomic structures, electrical properties, and line energies for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (1010) surfaces