396 research outputs found
Ni(111)|Graphene|h-BN Junctions as Ideal Spin Injectors
Deposition of graphene on top of hexagonal boron nitride (h-BN) was very
recently demonstrated while graphene is now routinely grown on Ni. Because the
in-plane lattice constants of graphite, h-BN, graphite-like BC2N and of the
close-packed surfaces of Co, Ni and Cu match almost perfectly, it should be
possible to prepare ideal interfaces between these materials which are
respectively, a semimetal, insulator, semiconductor, ferromagnetic and
nonmagnetic metals. Using parameter-free energy minimization and electronic
transport calculations, we show how h-BN can be combined with the perfect spin
filtering property of Ni|graphite and Co|graphite interfaces to make perfect
tunnel junctions or ideal spin injectors (SI) with any desired resistance-area
product.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Substrate-induced bandgap in graphene on hexagonal boron nitride
We determine the electronic structure of a graphene sheet on top of a
lattice-matched hexagonal boron nitride (h-BN) substrate using ab initio
density functional calculations. The most stable configuration has one carbon
atom on top of a boron atom, the other centered above a BN ring. The resulting
inequivalence of the two carbon sites leads to the opening of a gap of 53 meV
at the Dirac points of graphene and to finite masses for the Dirac fermions.
Alternative orientations of the graphene sheet on the BN substrate generate
similar band gaps and masses. The band gap induced by the BN surface can
greatly improve room temperature pinch-off characteristics of graphene-based
field effect transistors.Comment: 5 pages, 4 figures, Phys. Rev. B, in pres
First-principles study of the interaction and charge transfer between graphene and metals
Measuring the transport of electrons through a graphene sheet necessarily
involves contacting it with metal electrodes. We study the adsorption of
graphene on metal substrates using first-principles calculations at the level
of density functional theory. The bonding of graphene to Al, Ag, Cu, Au and
Pt(111) surfaces is so weak that its unique "ultrarelativistic" electronic
structure is preserved. The interaction does, however, lead to a charge
transfer that shifts the Fermi level by up to 0.5 eV with respect to the
conical points. The crossover from p-type to n-type doping occurs for a metal
with a work function ~5.4 eV, a value much larger than the work function of
free-standing graphene, 4.5 eV. We develop a simple analytical model that
describes the Fermi level shift in graphene in terms of the metal substrate
work function. Graphene interacts with and binds more strongly to Co, Ni, Pd
and Ti. This chemisorption involves hybridization between graphene -states
and metal d-states that opens a band gap in graphene. The graphene work
function is as a result reduced considerably. In a current-in-plane device
geometry this should lead to n-type doping of graphene.Comment: 12 pages, 9 figure
Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene
The in-plane lattice constants of close-packed planes of fcc and hcp Ni and
Co match that of graphite almost perfectly so that they share a common two
dimensional reciprocal space. Their electronic structures are such that they
overlap in this reciprocal space for one spin direction only allowing us to
predict perfect spin filtering for interfaces between graphite and (111) fcc or
(0001) hcp Ni or Co. First-principles calculations of the scattering matrix
show that the spin filtering is quite insensitive to amounts of interface
roughness and disorder which drastically influence the spin-filtering
properties of conventional magnetic tunnel junctions or interfaces between
transition metals and semiconductors. When a single graphene sheet is adsorbed
on these open -shell transition metal surfaces, its characteristic
electronic structure, with topological singularities at the K points in the two
dimensional Brillouin zone, is destroyed by the chemical bonding. Because
graphene bonds only weakly to Cu which has no states at the Fermi energy at the
K point for either spin, the electronic structure of graphene can be restored
by dusting Ni or Co with one or a few monolayers of Cu while still preserving
the ideal spin injection property.Comment: 12 pages, 11 figure
Stability of conductance oscillations in monatomic sodium wires
We study the stability of conductance oscillations in monatomic sodium wires
with respect to structural variations. The geometry, the electronic structure
and the electronic potential of sodium wires suspended between two sodium
electrodes are obtained from self-consistent density functional theory
calculations. The conductance is calculated within the framework of the
Landauer-B\"utttiker formalism, using the mode-matching technique as formulated
recently in a real-space finite-difference scheme [Phys. Rev. B \textbf{70},
195402 (2004)]. We find a regular even-odd conductance oscillation as a
function of the wire length, where wires comprising an odd number of atoms have
a conductance close to the quantum unit , and even-numbered
wires have a lower conductance. The conductance of odd-numbered wires is stable
with respect to geometry changes in the wire or in the contacts between the
wire and the electrodes; the conductance of even-numbered wires is more
sensitive. Geometry changes affect the spacing and widths of the wire
resonances. In the case of odd-numbered wires the transmission is on-resonance,
and hardly affected by the resonance shapes, whereas for even-numbered wires
the transmission is off-resonance and sensitive to the resonance shapes.
Predicting the amplitude of the conductance oscillation requires a
first-principles calculation based upon a realistic structure of the wire and
the leads. A simple tight-binding model is introduced to clarify these results.Comment: 16 pages, 20 figure
Real space finite difference method for conductance calculations
We present a general method for calculating coherent electronic transport in
quantum wires and tunnel junctions. It is based upon a real space high order
finite difference representation of the single particle Hamiltonian and wave
functions. Landauer's formula is used to express the conductance as a
scattering problem. Dividing space into a scattering region and left and right
ideal electrode regions, this problem is solved by wave function matching (WFM)
in the boundary zones connecting these regions. The method is tested on a model
tunnel junction and applied to sodium atomic wires. In particular, we show that
using a high order finite difference approximation of the kinetic energy
operator leads to a high accuracy at moderate computational costs.Comment: 13 pages, 10 figure
The Subject of the Prosecutor's Activity in Court Proceedings on Cases Involving Changing a Convict’s Type of Correctional Institution
The relevance of the work consists in the clearest definition of the subject of Prosecutor's supervision when the court considers materials on changing a convict’s type of correctional institution. The Prosecutor's office's supervision of the execution of sentences in places of deprivation of liberty should be aimed at unconditionally observing the rights and legitimate interests of persons serving criminal sentences, however, there are still serious problems in correctional institutions that negatively affect the implementation of the tasks and goals of criminal punishment. The reason for this is still cases of “superficial” and non-professional approach to the study of the personality of convicts who are subjects of legal relations that occur when their behavior changes (in a positive or negative direction) in places of deprivation of liberty. The Prosecutor's supervision and its activities in this regard are considered as one of the guarantees of the rights of convicts to improve their legal status, which fully implements the constitutional principle of respect for the dignity of the individual, humanism, justice and the rule of law. The materials of Prosecutor's checks, judicial practice, analysis of statistical materials, materials of dissertations, monographs and journals included in the international global scientific databases studied in the course of the research allowed us to determine the most effective ways to solve the problems considered. These include the need for direct, timely and step-by-step participation of prosecutors in this process, as well as high requirements for their professional and personal qualities. The selection and analysis of these factors allowed the authors to determine in detail the subject of the Prosecutor's activity in the mentioned study – it is an active and qualified activity of the Prosecutor (Prosecutor's supervision) in the field of criminal enforcement legislation, aimed at identifying, preventing (preventing) and eliminating violations of the rights and legitimate interests of convicted persons serving a sentence of imprisonment. The main research methods used were: analysis of theoretical and regulatory sources; comparison; generalization and analysis of documents. Their application allowed the author to analyze the subject in question in the relationship and interdependence of its constituent elements, their integrity, comprehensiveness and objectivity
Perturbation theory for the one-dimensional optical polaron
The one-dimensional optical polaron is treated on the basis of the
perturbation theory in the weak coupling limit. A special matrix diagrammatic
technique is developed. It is shown how to evaluate all terms of the
perturbation theory for the ground-state energy of a polaron to any order by
means of this technique. The ground-state energy is calculated up to the eighth
order of the perturbation theory. The effective mass of an electron is obtained
up to the sixth order of the perturbation theory. The radius of convergence of
the obtained series is estimated. The obtained results are compared with the
results from the Feynman polaron theory.Comment: 9 pages, 2 figures, RevTeX, to be published in Phys. Rev. B (2001)
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