14,482 research outputs found
Cation mono- and co-doped anatase TiO nanotubes: An {\em ab initio} investigation of electronic and optical properties
The structural, electronic, and optical properties of metal (Si, Ge, Sn, and
Pb) mono- and co-doped anatase TiO nanotubes are investigated, in order
to elucidate their potential for photocatalytic applications. It is found that
Si doped TiO nanotubes are more stable than those doped with Ge, Sn, or
Pb. All dopants lower the band gap, except the (Ge, Sn) co-doped structure, the
decrease depending on the concentration and the type of dopant.
Correspondingly, a redshift in the optical properties for all kinds of dopings
is obtained. Even though a Pb mono- and co-doped TiO nanotube has the
lowest band gap, these systems are not suitable for water splitting, due to the
location of the conduction band edges, in contrast to Si, Ge, and Sn mono-doped
TiO nanotubes. On the other hand, co-doping of TiO does not improve
its photocatalytic properties. Our findings are consistent with recent
experiments which show an enhancement of light absorption for Si and Sn doped
TiO nanotubes.Comment: revised and updated, 23 pages (preprint style), 7 figures, 5 table
Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb
Effects of thermal spin disorder and excess Mn on the electronic spectrum of
half-metallic NiMnSb are studied using first-principles calculations.
Temperature-dependent spin disorder, introduced within the vector disordered
local moment model, causes the valence band at the point to broaden
and shift upwards, crossing the Fermi level and thereby closing the
half-metallic gap above room temperature. The spectroscopic signatures of
excess Mn on the Ni, Sb, and empty sites (Mn, Mn,
and Mn) are analyzed. Mn is spectroscopically
invisible. The relatively weak coupling of Mn and Mn
spins to the host strongly deviates from the Heisenberg model, and the spin of
Mn is canted in the ground state. While the half-metallic gap is
preserved in the collinear ground state of Mn, thermal spin
disorder of the weakly coupled Mn spins destroys it at low
temperatures. This property of Mn may be the source of the
observed low-temperature transport anomalies.Comment: 5 pages, 7 figures, updated version with minor revisions and an
additional figure, accepted in Phys. Rev. B (Rapid Communication
Symplectic Reduction and Symmetry Algebra in Boundary Chern-Simons theory
We derive the Kac-Moody algebra and Virasoro algebra in Chern-Simons theory
with boundary by using the symplectic reduction method and the Noether
procedures.Comment: References are adde
Signals of Unparticles in Low Energy Parity Violation and NuTeV Experiment
We have studied the possible signals of unparticle in atomic parity
violation(APV) along an isotope chain and in the NuTeV experiment. The effects
of unparticle physics could be observed in APV, if the uncertainty in relative
neutron/proton radius shift is less than a few
times by measuring the parity violating electron scattering. The
constraints imposed by NuTeV experiment on unparticle physics are discussed in
detail. If the NuTeV results are confirmed by future experiments, we suggest
that unparticle could account for a part of NuTeV anomaly. There exist certain
regions for the unparticle parameters (, ,
and ), where the NuTeV discrepancy could be
completely explained by unparticle effects and the strange quark asymmetry,
even with or without the contributions from the isoscalarity violation etc. It
is remarkable that these parameter regions are consistent with the constraints
from Comment: 19 pages, 7 figure
Study of axial strain induced torsion of single wall carbon nanotubes by 2D continuum anharmonic anisotropic elastic model
Recent molecular dynamic simulations have found chiral single wall carbon
nanotubes (SWNTs) twist during stretching, which is similar to the motion of a
screw. Obviously this phenomenon, as a type of curvature-chirality effect, can
not be explained by usual isotropic elastic theory of SWNT. More interestingly,
with larger axial strains (before buckling), the axial strain induced torsion
(a-SIT) shows asymmetric behaviors for axial tensile and compressing strains,
which suggests anharmonic elasticity of SWNTs plays an important role in real
a-SIT responses. In order to study the a-SIT of chiral SWNTs with actual sizes,
and avoid possible deviations of computer simulation results due to the
finite-size effect, we propose a 2D analytical continuum model which can be
used to describe the the SWNTs of arbitrary chiralities, curvatures, and
lengths, with the concerning of anisotropic and anharmonic elasticity of SWNTs.
This elastic energy of present model comes from the continuum limit of lattice
energy based on Second Generation Reactive Empirical Bond Order potential
(REBO-II), a well-established empirical potential for solid carbons. Our model
has no adjustable parameters, except for those presented in REBO-II, and all
the coefficients in the model can be calculated analytically. Using our method,
we obtain a-SIT responses of chiral SWNTs with arbitrary radius, chiralities
and lengthes. Our results are in reasonable agreement with recent molecular
dynamic simulations. [Liang {\it et. al}, Phys. Rev. Lett, , 165501
(2006).] Our approach can also be used to calculate other curvature-chirality
dependent anharmonic mechanic responses of SWNTs.Comment: 14 pages, 2 figure
Superconducting transition of a two-dimensional Josephson junction array in weak magnetic fields
The superconducting transition of a two-dimensional (2D) Josephson junction
array exposed to weak magnetic fields has been studied experimentally.
Resistance measurements reveal a superconducting-resistive phase boundary in
serious disagreement with the theoretical and numerical expectations. Critical
scaling analyses of the characteristics indicate contrary to the
expectations that the superconducting-to-resistive transition in weak magnetic
fields is associated with a melting transition of magnetic-field-induced
vortices directly from a pinned-solid phase to a liquid phase. The expected
depinning transition of vortices from a pinned-solid phase to an intermediate
floating-solid phase was not observed. We discuss effects of the
disorder-induced random pinning potential on phase transitions of vortices in a
2D Josephson junction array.Comment: 9 pages, 7 figures (EPS+JPG format), RevTeX
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