26 research outputs found
Impurity State and Variable Range Hopping Conduction in Graphene
The variable range hopping theory, as formulated for exponentially localized
impurity states, does not necessarily apply in the case of graphene with
covalently attached impurities. We analyze the localization of impurity states
in graphene using the nearest-neighbor, tight-binding model of an
adatom-graphene system with Green's function perturbation methods. The
amplitude of the impurity state wave function is determined to decay as a power
law with exponents depending on sublattice, direction, and the impurity
species. We revisit the variable range hopping theory in view of this result
and find that the conductivity depends as a power law of the temperature with
an exponent related to the localization of the wave function. We show that this
temperature dependence is in agreement with available experimental results
Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau Levels
The electronic structure of bilayer graphene under pressure develops very
interesting features with an enhancement of the trigonal warping and a
splitting of the parabolic touching bands at the K point of the reciprocal
space into four Dirac cones, one at K and three along the T symmetry lines. As
pressure is increased, these cones separate in reciprocal space and in energy,
breaking the electron-hole symmetry. Due to their energy separation, their
opposite Berry curvature can be observed in valley Hall effect experiments and
in the structure of the Landau levels. Based on the electronic structure
obtained by Density Functional Theory, we develop a low energy Hamiltonian that
describes the effects of pressure on measurable quantities such as the Hall
conductivity and the Landau levels of the system.Comment: 11 pages, 9 figure
Analysis of periodic Schrodinger operators: regularity and approximation of eigenfunctions.
Let V be a real valued potential that is smooth everywhere on R 3 , except at a periodic, discrete set S of points, where it has singularities of the Coulomb-type Z/r . We assume that the potential V is periodic with period lattice L . We study the spectrum of the Schrödinger operator H=−Δ+V acting on the space of Bloch waves with arbitrary, but fixed, wavevector k . Let T≔R 3 /L . Let u be an eigenfunction of H with eigenvalueλ and let ϵ>0 be arbitrarily small. We show that the classical regularity of the eigenfunction u is u∊H 5/2−ϵ (T) in the usual Sobolev spaces, and u∊K m 3/2−ϵ (T\S) in the weighted Sobolev spaces. The regularity index m can be as large as desired, which is crucial for numerical methods. For any choice of the Bloch wavevector k , we also show that H has compact resolvent and hence a complete eigenfunction expansion. The case of the hydrogen atom suggests that our regularity results are optimal. We present two applications to the numerical approximation of eigenvalues: using wave functions and using piecewise polynomials
Low energy phases of bilayer Bi predicted by structure search in two dimensions
We employ an ab-initio structure search algorithm to explore the
configurational space of Bi in quasi two dimensions. A confinement potential
restricts the movement of atoms within a pre-defined thickness during structure
search calculations within the minima hopping method to find the stable and
metastable forms of bilayer Bi. In addition to recovering the two known
low-energy structures (puckered monoclinic and buckled hexagonal), our
calculations predict three new structures of bilayer Bi. We call these
structures the , , and phases of bilayer Bi, which are,
respectively, 63, 72, and 83 meV/atom higher in energy than that of the
monoclinic ground state, and thus potentially synthesizable using appropriate
substrates. We also compare the structural, electronic, and vibrational
properties of the different phases. The puckered monoclinic, buckled hexagonal,
and phases exhibit a semiconducting energy gap, whereas and
phases are metallic. We notice an unusual Mexican-hat type band
dispersion leading to a van Hove singularity in the buckled hexagonal bilayer
Bi. Notably, we find symmetry-protected topological Dirac points in the
electronic spectrum of the phase. The new structures suggest that
bilayer Bi provides a novel playground to study distortion-mediated
metal-insulator phase transitions
Photoluminescence from nanocrystalline graphite monofluoride
We synthesize and study the structural and optical properties of
nanocrystalline graphene monofluoride and graphite monofluoride, which are
carbon-based wide bandgap materials. Using laser excitations 2.41 - 5.08 eV, we
identify six emission modes of graphite monofluoride, spanning the visible
spectrum from red to violet. The energy and linewidth of the modes point to
defect-induced midgap states as the source of the photoemission. We discuss
possible candidates. Our findings open the window to electro-optical
applications of graphene fluoride.Comment: 11 pages including supporting information, 2 figure
Metal-substituted Ti8C12 metallocarbohedrynes: toward less reactive clusters as building blocks of cluster-assembled materials
To form cluster-assembled materials, the clusters should have low reactivity and be characterized by a closed-shell electronic configuration with a large gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO). Using spin-polarized density functional theory calculations, we investigate the M-substituted Ti8C12 metallocarbohedrynes to search for less reactive clusters as building blocks for cluster-assembled materials (M = Be, Mg, Ca, Sr, Ba and Sc, Y). The selected atoms in the correct stoichiometry would produce a metallocarbohedryne that is isoelectronic with the Ti8C122+, which has a closed-shell electronic configuration and an enhanced HOMO-LUMO gap of 1.735 eV. According to our results, the HOMO-LUMO gaps of the M-substituted Ti8C12 metallocarbohedrynes are in the range of 0.715-0.979 eV for the case of Be, Mg, Ca, Sr and Ba and in the range of 0.865-1.294 eV for the case of Sc and Y. Among all the M-substituted metallocarbohedrynes we consider here, one of the isomers of Ti6Sc2C12 is not only energetically more favorable but also exhibits a larger HOMO-LUMO gap of 1.294 eV. This result indicates that the Ti6Sc2C12(4) metallocarbohedryne should be less reactive than the Ti8C12 metallocarbohedryne which has a narrow HOMO-LUMO gap of 0.146 eV. Moreover, we show that the intercluster interaction between two individual Ti6Sc2C12(4) metallocarbohedrynes is relatively weak compared to the Ti8C12 dimer