5,331 research outputs found
A commuting q-analogue of the addition formula for disk polynomials
Starting from the addition formula for -disk polynomials, which is an
identity in non-commuting variables, we establish a basic analogue in commuting
variables of the addition and product formula for disk polynomials. These
contain as limiting cases the addition and product formula for little
-Legendre polynomials. As tends to the addition and product formula
for disk polynomials are recovered
The Analysis of Two Esl/efl Websites: Englishclub and Activities for Esl Students
This study reviewed two well known ESL/EFL websites namely EnglishClub and Activities for ESL Students using the website evaluation framework proposed by Hasan and Abuelrub (2011). The writers found that Activities for ESL Students met 79.92% of the website evaluation criteria with 211 of the total score; while EnglishClub met 79.54% of the website evaluation criteria with 210 of the total score. Thus there was no significant difference between these two websites. Both EnglishClub and Activities for ESL Students are good for ESL/EFL learner
Palladium gates for reproducible quantum dots in silicon
We replace the established aluminium gates for the formation of quantum dots
in silicon with gates made from palladium. We study the morphology of both
aluminium and palladium gates with transmission electron microscopy. The native
aluminium oxide is found to be formed all around the aluminium gates, which
could lead to the formation of unintentional dots. Therefore, we report on a
novel fabrication route that replaces aluminium and its native oxide by
palladium with atomic-layer-deposition-grown aluminium oxide. Using this
approach, we show the formation of low-disorder gate-defined quantum dots,
which are reproducibly fabricated. Furthermore, palladium enables us to further
shrink the gate design, allowing us to perform electron transport measurements
in the few-electron regime in devices comprising only two gate layers, a major
technological advancement. It remains to be seen, whether the introduction of
palladium gates can improve the excellent results on electron and nuclear spin
qubits defined with an aluminium gate stack
Electronic, dynamical and superconducting properties of CaBeSi
We report first-principles calculations on the normal and superconducting
state of CaBe(x)Si(2-x) (x=1), in the framework of density functional theory
for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB2
and this makes possible a direct comparison of the electronic and vibrational
properties and the electron-phonon interaction of the two materials. Despite
the many similarities with MgB2 (e.g. sigma bands at the Fermi level and a
larger Fermi surface nesting), according to our calculations CaBeSi has a very
low critical temperature (Tc ~ 0.4 K, consistent with the experiment). CaBeSi
exhibits a complex gap structure, with three gaps at Fermi level: besides the
two sigma and pi gaps, present also in MgB2, the appearance of a third gap is
related to the anisotropy of the Coulomb repulsion, acting in different way on
the bonding and antibonding electronic pi states.Comment: 6 pages, 5 figure
A Laplace operator and harmonics on the quantum complex vector space
The aim of this paper is to study the q-Laplace operator and q-harmonic
polynomials on the quantum complex vector space generated by z_i,w_i,
i=1,2,...,n, on which the quantum group GL_q(n) (or U_q(n)) acts. The
q-harmonic polynomials are defined as solutions of the equation Delta_qp=0,
where p is a polynomial in z_i,w_i, i=1,2,...,n, and the q-Laplace operator
Delta_q is determined in terms of q-derivatives. The q-Laplace operator Delta_q
commutes with the action of GL_q(n). The projector H_{m,m'}: A_{m,m'} -->
H_{m,m'} is constructed, where A_{m,m'} and H_{m,m'} are the spaces of
homogeneous (of degree m in z_i and of degree m' in w_i) polynomials and
homogeneous q-harmonic polynomials, respectively. By using these projectors, a
q-analogue of the classical zonal spherical and associated spherical harmonics
are constructed. They constitute an orthogonal basis of H_{m,m'}. A q-analogue
of separation of variables is given. The quantum algebra U_q(gl_n), acting on
H_{m,m'}, determines an irreducible representation of U_q(gl_n). This action is
explicitly constructed. The results of the paper lead to the dual pair
(U_q(sl_2), U_q(gl_n)) of quantum algebras.Comment: 26 pages, LaTe
Strongly nonexponential time-resolved fluorescence of quantum-dot ensembles in three-dimensional photonic crystals
We observe experimentally that ensembles of quantum dots in three-dimensional (3D) photonic crystals reveal strongly nonexponential time-resolved emission. These complex emission decay curves are analyzed with a continuous distribution of decay rates. The log-normal distribution describes the decays well for all studied lattice parameters. The distribution width is identified with variations of the radiative emission rates of quantum dots with various positions and dipole orientations in the unit cell. We find a striking sixfold change of the width of the distribution by varying the lattice parameter. This interpretation qualitatively agrees with the calculations of the 3D projected local density of states. We therefore conclude that fluorescence decay of ensembles of quantum dots is highly nonexponential to an extent that is controlled by photonic crystals
Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars
The physical structure of the envelopes around a sample of fourteen massive
(1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scales
using maps and spectra in submillimeter continuum and lines of C17O, CS and
H2CO. The total column densities and the temperature profiles are obtained by
fitting self-consistent dust models to submillimeter photometry. Both the
molecular line and dust emission data indicate density gradients ~r^{-alpha},
with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally found
for low-mass objects. This flattening may indicate that in massive young
stellar objects, nonthermal pressure is more important for the support against
gravitational collapse, while thermal pressure dominates for low-mass sources.
We find alpha=2 for two hot core-type sources, but regard this as an upper
limit since in these objects, the CS abundance may be enhanced in the warm gas
close to the star.Comment: To be published in The Astrophysical Journal. 54 pages including 14
figures Revised version with references adde
Anisotropic Pauli spin blockade in hole quantum dots
We present measurements on gate-defined double quantum dots in Ge-Si
core-shell nanowires, which we tune to a regime with visible shell filling in
both dots. We observe a Pauli spin blockade and can assign the measured leakage
current at low magnetic fields to spin-flip cotunneling, for which we measure a
strong anisotropy related to an anisotropic g-factor. At higher magnetic fields
we see signatures for leakage current caused by spin-orbit coupling between
(1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic
spin-flip mechanisms, we can choose the magnetic field direction with the
longest spin lifetime for improved spin-orbit qubits
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