16,858 research outputs found
Magnetic forces and localized resonances in electron transfer through quantum rings
We study the current flow through semiconductor quantum rings. In high
magnetic field the current is usually injected to the arm of the ring preferred
by classical magnetic forces. However, for narrow magnetic field intervals that
appear periodically on the magnetic field scale the current is injected to the
other arm of the ring. We indicate that the appearance of the anomalous --
non-classical -- current circulation results from Fano interference involving
localized resonant states. The identification of the Fano interference is based
on the comparison of the solution of the scattering problem with the results of
the stabilization method. The latter employs the bound-state type calculations
and allows to extract both the energy of metastable states localized within the
ring and the width of resonances by analysis of the energy spectrum of a finite
size system in function of its length. The Fano resonances involving states of
anomalous current circulation become extremely narrow on both magnetic field
and energy scales. This is consistent with the orientation of the Lorentz force
that tends to keep the electron within the ring and thus increases the lifetime
of the electron localization within the ring. Absence of periodic Fano
resonances in electron transfer probability through a quantum ring containing
an elastic scatterer is also explained.Comment: This paper explains the origins of anomalous (non-classical) current
circulation reported in http://arxiv.org/abs/1004.219
A theoretical model for single molecule incoherent scanning tunneling spectroscopy
Single molecule scanning tunneling spectroscopy (STS), with dephasing due to
elastic and inelastic scattering, is of some current interest. Motivated by
this, we report an extended Huckel theory (EHT) based mean-field
Non-equilibrium Green's function (NEGF) transport model with electron-phonon
scattering treated within the self-consistent Born approximation (SCBA).
Furthermore, a procedure based on EHT basis set modification is described. We
use this model to study the effect of the temperature dependent dephasing, due
to low lying modes in far-infrared range for which hw<<kT, on the resonant
conduction through highest occupied molecular orbital (HOMO) level of a phenyl
dithiol molecule sandwiched between two fcc-Au(111) contacts. Furthermore, we
propose to include dephasing in room temperature molecular resonant conduction
calculations.Comment: 12 pages, 5 figure
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
Interplay of bulk and edge states in transport of two-dimensional topological insulators
We study transport in two-terminal metal/quantum spin-Hall insulator
(QSHI)/metal junctions. We show that the conductance signals originating from
the bulk and the edge contributions are not additive. While for a long junction
the transport is determined by the edge states contribution, for a short
junction, the conductance signal is built from both bulk and edge states in the
ratio which depends on the width of the sample. Further, in the topological
insulator regime the conductance for short junctions shows a non-monotonic
behavior as a function of the sample length. Surprisingly this non-monotonic
behavior of conductance can be traced to the formation of an effectively
propagating solution which is robust against scalar disorder. Our predictions
should be experimentally verifiable in HgTe QWs and BiSe thin films.Comment: 9 pages, 8 figure
Z2 Monopoles, Vortices, and the Deconfinement Transition in Mixed Action SU(2) Gauge Theory
Adding separate chemical potentials lambda and gamma for Z2 monopoles and
vortices respectively in the Villain form of the mixed fundamental-adjoint
action for the SU(2) lattice gauge theory, we investigate their role in the
interplay between the deconfinement and bulk phase transitions using Monte
Carlo techniques. Setting lambda to be nonzero, we find that the line of
deconfinement transitions is shifted in the coupling plane but it behaves
curiously also like the bulk transition line for large enough adjoint coupling,
as for lambda=0. In a narrow range of couplings, however, we find separate
deconfinement and bulk phase transitions on the same lattice for nonzero and
large lambda, suggesting the two to be indeed coincident in the region where a
first order deconfinement phase transition is seen. In the limit of large
lambda and gamma, we obtain only lines of second order deconfinement phase
transitions, as expected from universality.Comment: 18 pages, 10 figures include
Strong flavour changing effective operator contributions to single top quark production
We study the effects of dimension six effective operators on the production
of single top quarks at the LHC. The operator set considered includes terms
with effective gluon interactions and four-fermion terms. Analytic expressions
for the several partonic cross sections of single top production will be
presented, as well as the results of their integration on the parton density
functions.Comment: 20 pages, 7 fig
On the Observability of "Invisible" / "Nearly Invisible" Charginos
It is shown that if the charginos decay into very soft leptons or hadrons +
due to degeneracy/ near- degeneracy with the LSP or the sneutrino,
the observability of the recently proposed signal via the single photon (+ soft
particles) + channel crucially depends on the magnitude of the \SNU
mass due to destructive interferences in the matrix element squared. If the
\SNU's and, consequently, left-sleptons are relatively light, the size of the
signal, previously computed in the limit \MSNU \to \infty only, is
drastically reduced. We present the formula for the signal cross section in a
model independent way and discuss the observability of the signal at LEP 192
and NLC energies.Comment: 27 pages, Late
Electrical Conductance of Molecular Wires
Molecular wires (MW) are the fundamental building blocks for molecular
electronic devices. They consist of a molecular unit connected to two continuum
reservoirs of electrons (usually metallic leads). We rely on Landauer theory as
the basis for studying the conductance properties of MW systems. This relates
the lead to lead current to the transmission probability for an electron to
scatter through the molecule. Two different methods have been developed for the
study of this scattering. One is based on a solution of the Lippmann-Schwinger
equation and the other solves for the {\bf t} matrix using Schroedinger's
equation. We use our methodology to study two problems of current interest. The
first MW system consists of 1,4 benzene-dithiolate (BDT) bonded to two gold
nanocontacts. Our calculations show that the conductance is sensitive to the
chemical bonding between the molecule and the leads. The second system we study
highlights the interesting phenomenon of antiresonances in MW. We derive an
analytic formula predicting at what energies antiresonances should occur in the
transmission spectra of MW. A numerical calculation for a MW consisting of
filter molecules attached to an active molecule shows the existence of an
antiresonance at the energy predicted by our formula.Comment: 14 pages, 5 figure
Effects of Disorder and Momentum Relaxation on the Intertube Transport of Incommensurate Carbon Nanotube Ropes and Multiwall Nanotubes
We study theoretically the electrical transport between aligned carbon
nanotubes in nanotube ropes, and between shells in multiwall carbon nanotubes.
We focus on transport between two metallic nanotubes (or shells) of different
chiralities with mismatched Fermi momenta and incommensurate periodicities. We
perform numerical calculations of the transport properties of such systems
within a tight-binding formalism. For clean (disorder-free) nanotubes the
intertube transport is strongly suppressed as a result of momentum
conservation. For clean nanotubes, the intertube transport is typically
dominated by the loss of momentum conservation at the contacts. We discuss in
detail the effects of disorder, which also breaks momentum conservation, and
calculate the effects of localised scatterers of various types. We show that
physically relevant disorder potentials lead to very dramatic enhancements of
the intertube conductance. We show that recent experimental measurements of the
intershell transport in multiwall nanotubes are consistent with our theoretical
results for a model of short-ranged correlated disorder.Comment: References adde
Extending additivity from symmetric to asymmetric channels
We prove a lemma which allows one to extend results about the additivity of
the minimal output entropy from highly symmetric channels to a much larger
class. A similar result holds for the maximal output -norm. Examples are
given showing its use in a variety of situations. In particular, we prove the
additivity and the multiplicativity for the shifted depolarising channel.Comment: 8 pages. This is the latest version of the first half of the original
paper. The other half will appear in another pape
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