232 research outputs found
The effect of vacancy-induced magnetism on electronic transport in armchair carbon nanotubes
The influence of local magnetic moment formation around three kinds of
vacancies on the electron conduction through metallic single-wall carbon
nanotubes is studied by use of the Landauer formalism within the coherent
regime. The method is based on the single-band tight-binding Hamiltonian, a
surface Green's function calculation, and the mean-field Hubbard model. The
numerical results show that the electronic transport is spin-polarized due to
the localized magnetic moments and it is strongly dependent on the geometry of
the vacancies. For all kinds of vacancies, by including the effects of local
magnetic moments, the electron scattering increases with respect to the
nonmagnetic vacancies case and hence, the current-voltage characteristic of the
system changes. In addition, a high value for the electron-spin polarization
can be obtained by applying a suitable gate voltage.Comment: 6 pages, 6 figure
Dielectric Function of Diluted Magnetic Semiconductors in the Infrared Regime
We present a study of the dielectric function of metallic (III,Mn)V diluted
magnetic semiconductors in the infrared regime. Our theoretical approach is
based on the kinetic exchange model for carrier induced (III,Mn)V
ferromagnetism. The dielectric function is calculated within the random phase
approximation and, within this metallic regime, we treat disorder effects
perturbatively and thermal effects within the mean field approximation. We also
discuss the implications of this calculations on carrier concentration
measurements from the optical f-sum rule and the analysis of plasmon-phonon
coupled modes in Raman spectra.Comment: 6 pages, 6 figures include
Pinning and switching of magnetic moments in bilayer graphene
We examine the magnetic properties of the localized states induced by lattice
vacancies in bilayer graphene with an unrestricted Hartree-Fock calculation. We
show that with realistic values of the parameters and for experimentally
accessible gate voltages we can have a magnetic switching between an
unpolarized and a fully polarized system.Comment: 9 pages, 4 figure
Renormalization group approach to anisotropic superconductivity
The superconducting instability of the Fermi liquid state is investigated by
considering anisotropic electron-boson couplings. Both electron-electron
interactions and anisotropic electron-boson couplings are treated with a
renormalization-group method that takes into account retardation effects.
Considering a non-interacting circular Fermi surface, we find analytical
solutions for the flow equations and derive a set of generalized Eliashberg
equations. Electron-boson couplings with different momentum dependences are
studied, and we find superconducting instabilities of the metallic state with
competition between order parameters of different symmetries. Numerical
solutions for some couplings are given to illustrate the frequency dependence
of the vertices at different coupling regimes.Comment: 9 pages, 7 figures. Final version as published in Phys. Rev.
Hole Pairs in the Two-Dimensional Hubbard Model
The interactions between holes in the Hubbard model, in the low density,
intermediate to strong coupling limit, are investigated. Dressed spin polarons
in neighboring sites have an increased kinetic energy and an enhanced hopping
rate. Both effects are of the order of the hopping integral and lead to an
effective attraction at intermediate couplings. Our results are derived by
systematically improving mean field calculations. The method can also be used
to derive known properties of isolated spin polarons.Comment: 4 page
Effect of edge decoration on the energy spectrum of semi-infinite lattices
Analytical studies of the effect of edge decoration on the energy spectrum of
semi-infinite one-dimensional (1D) lattice chain with Peierls phase transition
and zigzag edged graphene (ZEG) are presented by means of transfer matrix
method, in the frame of which the sufficient and necessary conditions for the
existence of the edge states are determined. For 1D lattice chain, the
zero-energy edge state exists when Peierls phase transition happens regardless
whether the decoration exists or not, while the non-zero-energy edge states can
be induced and manipulated through adjusting the edge decoration. On the other
hand, the semi-infinite ZEG model with nearest-neighbor interaction can be
mapped into the 1D lattice chain case. The non-zero-energy edge states can be
induced by the decoration as well, and we can obtain the condition of the
decoration on the edge for the existence of the novel edge states.Comment: 6 pages,4 figure
Robust signatures in the current-voltage characteristics of DNA molecules oriented between two graphene nanoribbon electrodes
In this work we numerically calculate the electric current through three
kinds of DNA sequences (telomeric, \lambda-DNA, and p53-DNA) described by
different heuristic models. A bias voltage is applied between two zig-zag edged
graphene contacts attached to the DNA segments, while a gate terminal modulates
the conductance of the molecule. The calculation of current is performed by
integrating the transmission function (calculated using the lattice Green's
function) over the range of energies allowed by the chemical potentials. We
show that a telomeric DNA sequence, when treated as a quantum wire in the fully
coherent low-temperature regime, works as an excellent semiconductor. Clear
steps are apparent in the current-voltage curves of telomeric sequences and are
present independent of lengths and sequence initialisation at the contacts. The
current-voltage curves suggest the existence of stepped structures independent
of length and sequencing initialisation at the contacts. We also find that the
molecule-electrode coupling can drastically influence the magnitude of the
current. The difference between telomeric DNA and other DNA, such as
\lambda-DNA and DNA for the tumour suppressor p53, is particularly visible in
the length dependence of the current
Confinement of electrons in layered metals
We analyze the out of plane hopping in models of layered systems where the
in--plane properties deviate from Landau's theory of a Fermi liquid. We show
that the hopping term acquires a non trivial energy dependence, due to the
coupling to in plane excitations, and can be either relevant or irrelevant at
low energies or temperatures. The latter is always the case if the Fermi level
lies close to a saddle point in the dispersion relation.Comment: 4 pages, 1 eps figur
Self-energy corrections to anisotropic Fermi surfaces
The electron-electron interactions affect the low-energy excitations of an
electronic system and induce deformations of the Fermi surface. These effects
are especially important in anisotropic materials with strong correlations,
such as copper oxides superconductors or ruthenates. Here we analyze the
deformations produced by electronic correlations in the Fermi surface of
anisotropic two-dimensional systems, treating the regular and singular regions
of the Fermi surface on the same footing. Simple analytical expressions are
obtained for the corrections, based on local features of the Fermi surface. It
is shown that, even for weak local interactions, the behavior of the
self-energy is non trivial, showing a momentum dependence and a self-consistent
interplay with the Fermi surface topology. Results are compared to experimental
observations and to other theoretical results.Comment: 13 pages, 10 figure
- …