2,100 research outputs found
Photocatalytic oxidation of NOx under indoor conditions using a functional wall covering
There is an increasing concern about the indoor air environment, where we spend most of our time. The air filling closed spaces is one of the main factors impoverishing health and creating the so-called "sick building syndrome". Indoor air quality can be affected by microbial contaminants, gases or particulates as well as other energy stressors. The newest tool for achieving depollution is a technology called heterogeneous photocatalytic oxidation, which uses energy from light to activate chemical reactions to degrade indoor air pollutants. This research was carried out to document the performance of a photocatalytic wall-covering under indoor conditions. The experimental results indicate that the photocatalytic oxidation is an effective air purification technology for indoor air pollutants
Preferred growth direction of III-V nanowires on differently oriented Si substrates
One of the nanowire characteristics is its preferred elongation direction. Here, we investigated the impact of Si substrate crystal orientation on the growth direction of GaAs nanowires. We first studied the self-catalyzed GaAs nanowire growth on Si (111) and Si (001) substrates. SEM observations show GaAs nanowires on Si (001) are grown along four directions without preference on one or some of them. This non-preferential nanowire growth on Si (001) is morphologically in contrast to the extensively reported vertical preferred GaAs nanowire growth on Si (111) substrates. We propose a model based on the initial condition of an ideal Ga droplet formation on Si substrates and the surface free energy calculation which takes into account the dangling bond surface density for different facets. This model provides further understanding of the different preferences in the growth of GaAs nanowires along selected directions depending on the Si substrate orientation. To verify the prevalence of the model, nanowires were grown on Si (311) substrates. The results are in good agreement with the three-dimensional mapping of surface free energy by our model. This general model can also be applied to predictions of nanowire preferred growth directions by the vapor-liquid-solid growth mode on other group IV and III-V substrates
Fundamental studies of the adhesion of explosives to textile and non-textile surfaces
This paper describes the use of atomic force microscopy (AFM) to investigate the interactions between explosives crystals and different surfaces. Crystals of TNT, PETN and RDX were mounted onto tipless AFM cantilevers and repeatedly brought into contact with a range of surfaces (n = 15), including textile and non-textile surfaces. The adhesion force during each contact was measured, and the results are presented in this work. The results suggest that explosives crystals display a higher adhesion to smoother, non-textile surfaces, particularly glass. This finding may be of use for forensic explosives investigators when deciding the best types of debris to target for explosives recovery
Entanglement in quantum computers described by the XXZ model with defects
We investigate how to generate maximally entangled states in systems
characterized by the Hamiltonian of the XXZ model with defects. Some proposed
quantum computers are described by such model. We show how the defects can be
used to obtain EPR states and W states when one or two excitations are
considered.Comment: 4 pages, 1 figur
Confinement and scaling in deep inelastic scattering
We show that parton confinement in the final state generates large
corrections to Bjorken scaling, thus leaving less room for the logarithmic
corrections. In particular, the -scaling violations at large are
entirely described in terms of power corrections. For treatment of these
non-perturbative effects, we derive a new expansion in powers of for
the structure function that is free of infra-red singularities and which
reduces corrections to the leading term. The leading term represents scattering
from an off-mass-shell parton, which keeps the same virtual mass in the final
state. It is found that this quasi-free term is a function of a new variable
, which coincides with the Bjorken variable for . The
two variables are very different, however, at finite . In particular, the
variable depends on the invariant mass of the spectator particles.
Analysis of the data at large shows excellent scaling in the variable , and determines the value of the diquark mass to be close to zero. -scaling allows us to extract the structure function near the elastic
threshold. It is found to behave as . Predictions for the
structure functions based on -scaling are made.Comment: Discussion of target mass corrections is added. Accepted for
publication in Phys. Rev.
Impurity Energy Level Within The Haldane Gap
An impurity bond in a periodic 1D antiferromagnetic, spin 1 chain with
exchange is considered. Using the numerical density matrix renormalization
group method, we find an impurity energy level in the Haldane gap,
corresponding to a bound state near the impurity bond. When the level
changes gradually from the edge of the Haldane gap to the ground state energy
as the deviation changes from 0 to 1. It seems that there is
no threshold. Yet, there is a threshold when . The impurity level
appears only when the deviation is greater than ,
which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4
figure
Interethnic differences in neuroimaging markers and cognition in Asians, a population-based study
We examined interethnic differences in the prevalence of neuroimaging markers of cerebrovascular and neurodegenerative disease in 3 major Asian ethnicities (Chinese, Malays, and Indians), as well as their role in cognitive impairment. 3T MRI brain scans were acquired from 792 subjects (mean age: 70.0â±â6.5years, 52.1% women) in the multi-ethnic Epidemiology of Dementia In Singapore study. Markers of cerebrovascular disease and neurodegeneration were identified. Cognitive performance was evaluated using Mini Mental State Examination (MMSE), Montreal Cognitive Assessment (MoCA), and a neuropsychological assessment. Compared to Chinese, Malays had a higher burden of intracranial stenosis (OR: 2.28. 95%CI: 1.23-4.20) and cortical atrophy (ÎČ: -0.60. 95%CI: -0.78, -0.41), while Indians had a higher burden of subcortical atrophy (ÎČ: -0.23. 95%CI: -0.40, -0.06). Moreover, Malay and Indian ethnicities were likely to be cognitively impaired (OR for Malays: 3.79. 95%CI: 2.29-6.26; OR for Indians: 2.87. 95%CI: 1.74-4.74) and showed worse performance in global cognition (ÎČ for Malays: -0.51. 95%CI: -0.66, -0.37; and Indians: -0.32. 95%CI: -0.47, -0.17). A higher burden of cerebrovascular and neurodegenerative markers were found in Malays and Indians when compared to Chinese. Further research is required to fully elucidate the factors and pathways that contribute to these observed differences
Convergence of energy-dependent incommensurate antiferromagnetic neutron scattering peaks to commensurate resonance in underdoped bilayer cuprates
The recently discovered coexistence of incommensurate antiferromagnetic
neutron scattering peaks and commensurate resonance in underdoped
YBaCuO is calling for an explanation. Within the t-J model, the
doping and energy dependence of the spin dynamics of the underdoped bilayer
cuprates in the normal state is studied based on the fermion-spin theory by
considering the bilayer interactions. Incommensurate peaks are found at
and at low energies with
initially increasing with doping at low dopings and then saturating at
higher dopings. These incommensurate peaks are suppressed, and the parameter
is reduced with increasing energy. Eventually it converges to the
resonance peak. Thus the recently observed coexistence is
interpreted in terms of bilayer interactions.Comment: 15 pages, Revtex, five figures are included, accepted for publication
in Phys. Rev.
Two-particle localization and antiresonance in disordered spin and qubit chains
We show that, in a system with defects, two-particle states may experience
destructive quantum interference, or antiresonance. It prevents an excitation
localized on a defect from decaying even where the decay is allowed by energy
conservation. The system studied is a qubit chain or an equivalent spin chain
with an anisotropic () exchange coupling in a magnetic field. The chain
has a defect with an excess on-site energy. It corresponds to a qubit with the
level spacing different from other qubits. We show that, because of the
interaction between excitations, a single defect may lead to multiple localized
states. The energy spectra and localization lengths are found for
two-excitation states. The localization of excitations facilitates the
operation of a quantum computer. Analytical results for strongly anisotropic
coupling are confirmed by numerical studies.Comment: Updated version, 13 pages, 5 figures To appear in Phys. Rev. B (2003
Theory of bound polarons in oxide compounds
We present a multilateral theoretical study of bound polarons in oxide
compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary
electron-phonon coupling is used for calculation of the energies of thermal
dissociation, photoionization (optically induced release of an electron (hole)
from the ground self-consistent state), as well as optical absorption to the
non-relaxed excited states. Unlike the case of free strong-coupling polarons,
where the ratio \kappa of the photoionization energy to the thermal
dissociation energy was shown to be always equal to 3, here this ratio depends
on the Froehlich coupling constant \alpha and the screened Coulomb interaction
strength \beta. Reasonable variation of these two parameters has demonstrated
that the magnitude of \kappa remains usually in the narrow interval from 1 to
2.5. This is in agreement with atomistic calculations and experimental data for
hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation
energy for the ground self-consistent state and the energy of the optically
induced charge transfer process (hops of a hole between O^{2-} ions) have been
calculated using the quantum-chemical method INDO. Results obtained within the
two approaches for hole O polarons bound by the cation vacancies (V^-) in
MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to
experimental data and to each other. We discuss a surprising closeness of the
results obtained on the basis of independent models and their agreement with
experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses:
[email protected], [email protected]
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