47,017 research outputs found
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -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 -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
Magnetism of Covalently Functionalized Carbon Nanotubes
We investigate the electronic structure of carbon nanotubes functionalized by
adsorbates anchored with single C-C covalent bonds. We find that, despite the
particular adsorbate, a spin moment with a universal value of 1.0 per
molecule is induced at low coverage. Therefore, we propose a mechanism of
bonding-induced magnetism at the carbon surface. The adsorption of a single
molecule creates a dispersionless defect state at the Fermi energy, which is
mainly localized in the carbon wall and presents a small contribution from the
adsorbate. This universal spin moment is fairly independent of the coverage as
long as all the molecules occupy the same graphenic sublattice. The magnetic
coupling between adsorbates is also studied and reveals a key dependence on the
graphenic sublattice adsorption site.Comment: final version, improved discussion about calculations and defect
concentratio
First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties
We present a theoretical study using density functional calculations of the
structural, electronic and magnetic properties of 3d transition metal, noble
metal and Zn atoms interacting with carbon monovacancies in graphene. We pay
special attention to the electronic and magnetic properties of these
substitutional impurities and found that they can be fully understood using a
simple model based on the hybridization between the states of the metal atom,
particularly the d shell, and the defect levels associated with an
unreconstructed D3h carbon vacancy. We identify three different regimes
associated with the occupation of different carbon-metal hybridized electronic
levels:
(i) bonding states are completely filled for Sc and Ti, and these impurities
are non-magnetic;
(ii) the non-bonding d shell is partially occupied for V, Cr and Mn and,
correspondingly, these impurties present large and localized spin moments;
(iii) antibonding states with increasing carbon character are progressively
filled for Co, Ni, the noble metals and Zn. The spin moments of these
impurities oscillate between 0 and 1 Bohr magnetons and are increasingly
delocalized.
The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v
symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct
position at the border between regimes (ii) and (iii) and shows a more complex
behavior: while is non-magnetic at the level of GGA calculations, its spin
moment can be switched on using GGA+U calculations with moderate values of the
U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th,
200
A project of a new detector for direct Dark Matter search: MACHe3
MACHe3 (MAtrix of Cells of superfluid He3) is a project of a new detector for
direct Dark Matter (DM) search. A cell of superfluid He3 has been developed and
the idea of using a large number of such cells in a high granularity detector
is proposed.This contribution presents, after a brief description of the
superfluid He3 cell, the simulation of the response of different matrix
configurations allowing to define an optimum design as a function of the number
of cells and the volume of each cell. The exclusion plot and the predicted
interaction cross-section for the neutralino as a photino are presented.Comment: 8 pages, 7 figures, Proceedings of Dark Matter 2000 (Marina Del Rey,
Los Angeles, USA, 02/23/2000-02/25/2000
Interlayer superfluidity in bilayer systems of fermionic polar molecules
We consider fermionic polar molecules in a bilayer geometry where they are
oriented perpendicularly to the layers, which permits both low inelastic losses
and superfluid pairing. The dipole-dipole interaction between molecules of
different layers leads to the emergence of interlayer superfluids. The
superfluid regimes range from BCS-like fermionic superfluidity with a high
to Bose-Einstein (quasi-)condensation of interlayer dimers, thus
exhibiting a peculiar BCS-BEC crossover. We show that one can cover the entire
crossover regime under current experimental conditions.Comment: 4 pages, 4 figure
Landau levels of cold atoms in non-Abelian gauge fields
The Landau levels of cold atomic gases in non-Abelian gauge fields are
analyzed. In particular we identify effects on the energy spectrum and density
distribution which are purely due to the non-Abelian character of the fields.
We investigate in detail non-Abelian generalizations of both the Landau and the
symmetric gauge. Finally, we discuss how these non-Abelian Landau and symmetric
gauges may be generated by means of realistically feasible lasers in a tripod
scheme.Comment: 13 pages, 9 figure
Equivalence between the Lovelock-Cartan action and a constrained gauge theory
We show that the four-dimensional Lovelock-Cartan action can be derived from
a massless gauge theory for the group with an additional BRST trivial
part. The model is originally composed by a topological sector and a BRST exact
piece and has no explicit dependence on the metric, the vierbein or a mass
parameter. The vierbein is introduced together with a mass parameter through
some BRST trivial constraints. The effect of the constraints is to identify the
vierbein with some of the additional fields, transforming the original action
into the Lovelock-Cartan one. In this scenario, the mass parameter is
identified with Newton's constant while the gauge field is identified with the
spin-connection. The symmetries of the model are also explored. Moreover, the
extension of the model to a quantum version is qualitatively discussed.Comment: 17 pages. No figures. Final version accepted for publication at the
EPJ
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