17,683 research outputs found
Conversion of neutron stars to strange stars as the central engine of gamma-ray bursts
We study the conversion of a neutron star to a strange star as a possible
energy source for gamma-ray bursts. We use different recent models for the
equation of state of neutron star matter and strange quark matter. We show that
the total amount of energy liberated in the conversion is in the range of (1-4)
10^{53} ergs (one order of magnitude larger than previous estimates) and is in
agreement with the energy required to power gamma-ray burst sources at
cosmological distances.Comment: ApJ, 530, 2000 February 20, Lxxx (in press
Charmonium systems after the deconfinement transition
The behavior of charmonia after the deconfinement transition is investigated
on quenched lattices. Analysis of temporal correlators on fine lattices at
temperatures upto 3 T_c show that the J/psi and eta_c survive the deconfinement
transition with little significant changes, and survive as bound states at
least upto 2.25 T_c. The spatially excited chi_c states suffer serious system
modifications, maybe dissolution, already a little above T_c.Comment: Contribution to Quark Matter 2004. 4 page
Non-equilibrium current and electron pumping in nanostructures
We discuss a numerical method to study electron transport in mesoscopic
devices out of equilibrium. The method is based on the solution of operator
equations of motion, using efficient Chebyshev time propagation techniques. Its
peculiar feature is the propagation of operators backwards in time. In this way
the resource consumption scales linearly with the number of states used to
represent the system. This allows us to calculate the current for
non-interacting electrons in large one-, two- and three-dimensional lead-device
configurations with time-dependent voltages or potentials. We discuss the
technical aspects of the method and present results for an electron pump device
and a disordered system, where we find transient behaviour that exists for a
very long time and may be accessible to experiments.Comment: 4 pages, 3 figures. Contribution to the International Conference on
Magnetism (ICM) 2009 in Karlsruh
Strongly modulated transmissions in gapped armchair graphene nanoribbons with sidearm or on-site gate voltage
We propose two schemes of field-effect transistor based on gapped armchair
graphene nanoribbons connected to metal leads, by introducing sidearms or
on-site gate voltages. We make use of the band gap to reach excellent
switch-off character. By introducing one sidearm or on-site gate to the
graphene nanoribbon, conduction peaks appear inside the gap regime. By further
applying two sidearms or on-site gates, these peaks are broadened to conduction
plateaus with a wide energy window, thanks to the resonance from the dual
structure. The position of the conduction windows inside the gap can be fully
controlled by the length of the sidearms or the on-site gate voltages, which
allows "on" and "off" operations for a specific energy window inside the gap
regime. The high robustness of both the switch-off character and the conduction
windows is demonstrated and shows the feasibility of the proposed dual
structures for real applications.Comment: 6 pages, 6 figure
Efficient implementation of the nonequilibrium Green function method for electronic transport calculations
An efficient implementation of the nonequilibrium Green function (NEGF)
method combined with the density functional theory (DFT) using localized
pseudo-atomic orbitals (PAOs) is presented for electronic transport
calculations of a system connected with two leads under a finite bias voltage.
In the implementation, accurate and efficient methods are developed especially
for evaluation of the density matrix and treatment of boundaries between the
scattering region and the leads. Equilibrium and nonequilibrium contributions
in the density matrix are evaluated with very high precision by a contour
integration with a continued fraction representation of the Fermi-Dirac
function and by a simple quadratureon the real axis with a small imaginary
part, respectively. The Hartree potential is computed efficiently by a
combination of the two dimensional fast Fourier transform (FFT) and a finite
difference method, and the charge density near the boundaries is constructed
with a careful treatment to avoid the spurious scattering at the boundaries.
The efficiency of the implementation is demonstrated by rapid convergence
properties of the density matrix. In addition, as an illustration, our method
is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and
a LaMnOSrMnO superlattice, demonstrating its applicability to a wide
variety of systems.Comment: 20 pages, 11 figure
Finite Element Modelling of Bends and Creases during Folding Ultra Thin Stainless Steel Foils
Finite Element Modelling of an ultra thin foil of SUS 304 stainless steel is carried out. These foils are 20 mm and below in thickness. The development of stresses and strains during folding of these foils is studied. The objective of this study is to induce qualities of paper in the foils of stainless steel such that a public sculpture of origami can be built with the foil. Finite Element modelling of the fold, reverse fold, junctions of multiple folds as well as the finger-dents are carried out to quantify the extent of straining the steel foil would undergo while an object of origami is folded with it. It is important to know the extent of straining the foil would undergo during folding operation. With this knowledge, the through-thickness microstructure and microtexture can be studied which influence the fracture toughness and low cycle fatigue properties of the steel foil. The foil with the requisite qualities of paper can then be manufactured
Sample-specific and Ensemble-averaged Magnetoconductance of Individual Single-Wall Carbon Nanotubes
We discuss magnetotransport measurements on individual single-wall carbon
nanotubes with low contact resistance, performed as a function of temperature
and gate voltage. We find that the application of a magnetic field
perpendicular to the tube axis results in a large magnetoconductance of the
order of e^2/h at low temperature. We demonstrate that this magnetoconductance
consists of a sample-specific and of an ensemble-averaged contribution, both of
which decrease with increasing temperature. The observed behavior resembles
very closely the behavior of more conventional multi-channel mesoscopic wires,
exhibiting universal conductance fluctuations and weak localization. A
theoretical analysis of our experiments will enable to reach a deeper
understanding of phase-coherent one-dimensional electronic motion in SWNTs.Comment: Replaced with published version. Minor changes in tex
Embedding approach for dynamical mean field theory of strongly correlated heterostructures
We present an embedding approach based on localized basis functions which
permits an efficient application of the dynamical mean field theory (DMFT) to
inhomogeneous correlated materials, such as semi-infinite surfaces and
heterostructures. In this scheme, the semi-infinite substrate leads connected
to both sides of the central region of interest are represented via complex,
energy-dependent embedding potentials that incorporate one-electron as well as
many-body effects within the substrates. As a result, the number of layers
which must be treated explicitly in the layer-coupled DMFT equation is greatly
reduced. To illustrate the usefulness of this approach, we present numerical
results for strongly correlated surfaces, interfaces, and heterostructures of
the single-band Hubbard model.Comment: 8 pages, 4 figures; typos correcte
Partial Isometries of a Sub-Riemannian Manifold
In this paper, we obtain the following generalisation of isometric
-immersion theorem of Nash and Kuiper. Let be a smooth manifold of
dimension and a rank subbundle of the tangent bundle with a
Riemannian metric . Then the pair defines a sub-Riemannian
structure on . We call a -map into a Riemannian
manifold a {\em partial isometry} if the derivative map restricted
to is isometric; in other words, . The main result states that
if then a smooth -immersion satisfying
can be homotoped to a partial isometry which is
-close to . In particular we prove that every sub-Riemannian manifold
admits a partial isometry in provided .Comment: 13 pages. This is a revised version of an earlier submission (minor
revision
Current induced light emission and light induced current in molecular tunneling junctions
The interaction of metal-molecule-metal junctions with light is considered
within a simple generic model. We show, for the first time, that light induced
current in unbiased junctions can take place when the bridging molecule is
characterized by a strong charge-transfer transition. The same model shows
current induced light emission under potential bias that exceeds the molecular
excitation energy. Results based on realistic estimates of molecular-lead
coupling and molecule-radiation field interaction suggest that both effects
should be observable.Comment: 5 pages, 3 figures, RevTeX
- …