6,435 research outputs found
Ridge Estimation of Inverse Covariance Matrices from High-Dimensional Data
We study ridge estimation of the precision matrix in the high-dimensional
setting where the number of variables is large relative to the sample size. We
first review two archetypal ridge estimators and note that their utilized
penalties do not coincide with common ridge penalties. Subsequently, starting
from a common ridge penalty, analytic expressions are derived for two
alternative ridge estimators of the precision matrix. The alternative
estimators are compared to the archetypes with regard to eigenvalue shrinkage
and risk. The alternatives are also compared to the graphical lasso within the
context of graphical modeling. The comparisons may give reason to prefer the
proposed alternative estimators
Technological learning for innovating towards sustainable cultivation practices: the Vietnamese smallholder rose sector
Deregulation and globalisation has altered the views of public involvement in development and led to strategies focusing on private sector participation. An implicit assumption seems to be that these linkages will enhance the technological capacity of smallholder producers by way of more cost-efficient technologies trickling down through the value chain or by quality requirements inducing best practices. The argument put forward in this paper is that sustainable non traditional agricultural chain development requires more purposeful actions and institutional transitions, both in the public and private spheres, targeting improved upstream innovative capacities. Empirical findings from a Dutch-Vietnamese partnership on sustainable floriculture development are used. Research revealed that the pest and disease control solutions applied by smallholder rose growers were incremental adaptations of experiences obtained in former food crop cultivation practices. Floriculture however may require more drastic changes in cultivation practices to make the sector more environmentally benign. In the case of smallholder Vietnamese flower producers, this implies adaptation of knowledge and skills currently not present. An important hindrance in promoting this knowledge and skills appears to be the weak vertical linkages between flower growers and public and private research and development organizations
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Confined magnetic guiding orbit states
We show how snake-orbit states which run along a magnetic edge can be
confined electrically. We consider a two-dimensional electron gas (2DEG)
confined into a quantum wire, subjected to a strong perpendicular and steplike
magnetic field . Close to this magnetic step new, spatially confined
bound states arise as a result of the lateral confinement and the magnetic
field step. The number of states, with energy below the first Landau level,
increases as becomes stronger or as the wire width becomes larger. These
bound states can be understood as an interference between two
counter-propagating one-dimensional snake-orbit states.Comment: 4 pages, 4 figure
An efficient finite-difference scheme for computation of electron states in free-standing and core-shell quantum wires
The electron states in axially symmetric quantum wires are computed by means
of the effective-mass Schroedinger equation, which is written in cylindrical
coordinates phi, rho, and z. We show that a direct discretization of the
Schroedinger equation by central finite differences leads to a non-symmetric
Hamiltonian matrix. Because diagonalization of such matrices is more complex it
is advantageous to transform it in a symmetric form. This can be done by the
Liouville-like transformation proposed by Rizea et al. (Comp. Phys. Comm. 179
(2008) 466-478), which replaces the wave function psi(rho) with the function
F(rho)=psi(rho)sqrt(rho) and transforms the Hamiltonian accordingly. Even
though a symmetric Hamiltonian matrix is produced by this procedure, the
computed wave functions are found to be inaccurate near the origin, and the
accuracy of the energy levels is not very high. In order to improve on this, we
devised a finite-difference scheme which discretizes the Schroedinger equation
in the first step, and then applies the Liouville-like transformation to the
difference equation. Such a procedure gives a symmetric Hamiltonian matrix,
resulting in an accuracy comparable to the one obtained with the finite element
method. The superior efficiency of the new finite-difference (FDM) scheme is
demonstrated for a few rho-dependent one-dimensional potentials which are
usually employed to model the electron states in free-standing and core-shell
quantum wires. The new scheme is compared with the other FDM schemes for
solving the effective-mass Schroedinger equation, and is found to deliver
energy levels with much smaller numerical error for all the analyzed
potentials. Moreover, the PT symmetry is invoked to explain similarities and
differences between the considered FDM schemes
Spin- and valley-dependent transport through arrays of ferromagnetic silicene junctions
We study ballistic transport of Dirac fermions in silicene through arrays of
barriers, of width , in the presence of an exchange field and a tunable
potential of height or depth . The spin- and valley-resolved
conductances as functions of or , exhibit resonances away from the Dirac
point (DP) and close to it a pronounced dip that becomes a gap when a critical
electric field is applied. This gap widens by increasing the number of
barriers and can be used to realize electric field-controlled switching of the
current. The spin and valley polarizations of the current near the
DP increase with or and can reach 100\% for certain of their values.
These field ranges widen significantly by increasing the number of barriers.
Also, and oscillate nearly periodically with the separation between
barriers or wells and can be inverted by reversing .Comment: 9 pages, 43 figures, to appear in PRB, figure resolutions reduced for
siz
Polaron effects in electron channels on a helium film
Using the Feynman path-integral formalism we study the polaron effects in
quantum wires above a liquid helium film. The electron interacts with
two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one
dimension (1D) by an harmonic potential. The obtained results are valid for
arbitrary temperature (), electron-phonon coupling strength (), and
lateral confinement (). Analytical and numerical results are
obtained for limiting cases of , , and . We found the
surprising result that reducing the electron motion from 2D to quasi-1D makes
the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
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