18,939 research outputs found
Oscillating chiral currents in nanotubes: a route to nanoscale magnetic test tubes
With a view to optimising the design of carbon-nanotube (CNT) windmills and
to maximising the internal magnetic field generated by chiral currents, we
present analytical results for the group velocity components of an electron
flux through chiral carbon nanotubes. Chiral currents are shown to exhibit a
rich behaviour and can even change sign and oscillate as the energy of the
electrons is increased. We find that the transverse velocity and associated
angular momentum of electrons is a maximum for non-metallic CNTs with a chiral
angle of 18. Such CNTs are therefore the optimal choice for CNT windmills
and also generate the largest internal magnetic field for a given longitudinal
current. For a longitudinal current of order amps, this field can be
of order Teslas, which is sufficient to produce interesting spintronic
effects and a significant contribution to the self inductance.Comment: 4 pages, 1 figur
Superconductivity-Induced Anderson Localisation
We have studied the effect of a random superconducting order parameter on the
localization of quasi-particles, by numerical finite size scaling of the
Bogoliubov-de Gennes tight-binding Hamiltonian. Anderson localization is
obtained in d=2 and a mobility edge where the states localize is observed in
d=3. The critical behavior and localization exponent are universal within error
bars both for real and complex random order parameter. Experimentally these
results imply a suppression of the electronic contribution to thermal transport
from states above the bulk energy gap.Comment: 4 pages, revtex file, 3 postscript figure
General Green's function formalism for transport calculations with spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic multilayers
A novel, general Green's function technique for elastic spin-dependent
transport calculations is presented, which (i) scales linearly with system size
and (ii) allows straightforward application to general tight-binding
Hamiltonians (spd in the present work). The method is applied to studies of
conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP
(current perpendicular to planes) geometry in the limit of large coherence
length. The magnetic materials considered are Co and Ni, with various
non-magnetic materials from the 3d, 4d, and 5d transition metal series.
Realistic tight-binding models for them have been constructed with the use of
density functional calculations. We have identified three qualitatively
different cases which depend on whether or not the bands (densities of states)
of a non-magnetic metal (i) form an almost perfect match with one of spin
sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost
pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d
character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise
to a large GMR ratio turn out to be (i) a strong spin polarization of the
magnetic metal, (ii) a large energy offset between the conduction band of the
non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii)
strong interband scattering in one of spin sub-bands of a magnetic metal. The
present results show that GMR oscillates with variation of the thickness of
either non-magnetic or magnetic layers, as observed experimentally.Comment: 22 pages, 9 figure
Forming disk galaxies in wet major mergers. I. Three fiducial examples
Using three fiducial Nbody+SPH simulations, we follow the merging of two disk
galaxies with a hot gaseous halo component each, and examine whether the merger
remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by
violent relaxation during the merging and most of their stars form a classical
bulge, while the remaining form a thick disk and its bar. A new stellar disk
forms subsequently and gradually in the remnant from the gas accreted mainly
from the halo. It is vertically thin and well extended in its equatorial plane.
A bar starts forming before the disk is fully in place, contrary to what is
assumed in idealised simulations of isolated bar-forming galaxies. It has
morphological features such as ansae and boxy/peanut bulges. Stars of different
ages populate different parts of the box/peanut. A disky pseudobulge forms
also, so that by the end of the simulation, all three types of bulges coexist.
The oldest stars are found in the classical bulge, followed by those of the
thick disk, then by those in the thin disk. The youngest stars are in the
spiral arms and the disky pseudobulge. The disk surface density profiles are of
type II (exponential with downbending), and the circular velocity curves are
flat and show that the disks are submaximum in these examples: two clearly so
and one near-borderline between maximum and submaximum. On average, only
roughly between 10 and 20% of the stellar mass is in the classical bulge of the
final models, i.e. much less than in previous simulations.Comment: 17 pages, 8 figures, accepted for publication in ApJ. V2: replaced
Figure 4 with correct versio
Negative 4-Probe Conductances of Mesoscopic Superconducting Wires
We analyze the longitudinal 4-probe conductance of mesoscopic normal and
superconducting wires and predict that in the superconducting case, large
negative values can arise for both the weakly disordered and localized regimes.
This contrasts sharply with the behaviour of the longitudinal 4-probe
conductance of normal wires, which in the localized limit is always
exponentially small and positive.Comment: Latex, 3 figures available on request to [email protected]
(Simon Robinson
High Dimensional Classification with combined Adaptive Sparse PLS and Logistic Regression
Motivation: The high dimensionality of genomic data calls for the development
of specific classification methodologies, especially to prevent over-optimistic
predictions. This challenge can be tackled by compression and variable
selection, which combined constitute a powerful framework for classification,
as well as data visualization and interpretation. However, current proposed
combinations lead to instable and non convergent methods due to inappropriate
computational frameworks. We hereby propose a stable and convergent approach
for classification in high dimensional based on sparse Partial Least Squares
(sparse PLS). Results: We start by proposing a new solution for the sparse PLS
problem that is based on proximal operators for the case of univariate
responses. Then we develop an adaptive version of the sparse PLS for
classification, which combines iterative optimization of logistic regression
and sparse PLS to ensure convergence and stability. Our results are confirmed
on synthetic and experimental data. In particular we show how crucial
convergence and stability can be when cross-validation is involved for
calibration purposes. Using gene expression data we explore the prediction of
breast cancer relapse. We also propose a multicategorial version of our method
on the prediction of cell-types based on single-cell expression data.
Availability: Our approach is implemented in the plsgenomics R-package.Comment: 9 pages, 3 figures, 4 tables + Supplementary Materials 8 pages, 3
figures, 10 table
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