34,098 research outputs found
The observed spiral structure of the Milky Way
The spiral structure of the Milky Way is not yet well determined. The keys to
understanding this structure are to increase the number of reliable spiral
tracers and to determine their distances as accurately as possible. HII
regions, giant molecular clouds (GMCs), and 6.7-GHz methanol masers are closely
related to high mass star formation, and hence they are excellent spiral
tracers. We update the catalogs of Galactic HII regions, GMCs, and 6.7-GHz
methanol masers, and then outline the spiral structure of the Milky Way. We
collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7-GHz
methanol masers. If the photometric or trigonometric distance was not yet
available, we determined the kinematic distance using a Galaxy rotation curve
with the current IAU standard, = 8.5 kpc and = 220 km
s, and the most recent updated values of = 8.3 kpc and
= 239 km s, after we modified the velocities of tracers with the adopted
solar motions. With the weight factors based on the excitation parameters of
HII regions or the masses of GMCs, we get the distributions of these spiral
tracers. The distribution of tracers shows at least four segments of arms in
the first Galactic quadrant, and three segments in the fourth quadrant. The
Perseus Arm and the Local Arm are also delineated by many bright HII regions.
The arm segments traced by massive star forming regions and GMCs are able to
match the HI arms in the outer Galaxy. We found that the models of three-arm
and four-arm logarithmic spirals are able to connect most spiral tracers. A
model of polynomial-logarithmic spirals is also proposed, which not only
delineates the tracer distribution, but also matches the observed tangential
directions.Comment: 22 Pages, 16 Figures, 7 Tables, updated to match the published
versio
One-Dimensional Transition Metal-Benzene Sandwich Polymers: Possible Ideal Conductors for Spin Transport
We investigate the electronic and magnetic properties of the proposed
one-dimensional transition metal (TM=Sc, Ti, V, Cr, and Mn)-benzene (Bz)
sandwich polymers by means of density functional calculations.
[V(Bz)] is found to be a quasi-half-metallic ferromagnet and
half-metallic ferromagnetism is predicted for [Mn(Bz)]. Moreover, we
show that stretching the [TM(Bz)] polymers could have dramatic
effects on their electronic and magnetic properties. The elongated
[V(Bz)] displays half-metallic behavior, and [Mn(Bz)]
stretched to a certain degree becomes an antiferromagnetic insulator. The
possibilities to stabilize the ferromagnetic order in [V(Bz)] and
[Mn(Bz)] polymers at finite temperature are discussed. We suggest
that the hexagonal bundles composed by these polymers might display intrachain
ferromagnetic order at finite temperature by introducing interchain exchange
coupling
Electronic, Mechanical, and Piezoelectric Properties of ZnO Nanowires
Hexagonal [0001] nonpassivated ZnO nanowires are studied with density
functional calculations. The band gap and Young's modulus in nanowires which
are larger than those in bulk ZnO increase along with the decrease of the
radius of nanowires. We find ZnO nanowires have larger effective piezoelectric
constant than bulk ZnO due to their free boundary. In addition, the effective
piezoelectric constant in small ZnO nanowires doesn't depend monotonously on
the radius due to two competitive effects: elongation of the nanowires and
increase of the ratio of surface atoms
Linear scaling calculation of band edge states and doped semiconductors
Linear scaling methods provide total energy, but no energy levels and
canonical wavefuctions. From the density matrix computed through the density
matrix purification methods, we propose an order-N (O(N)) method for
calculating both the energies and wavefuctions of band edge states, which are
important for optical properties and chemical reactions. In addition, we also
develop an O(N) algorithm to deal with doped semiconductors based on the O(N)
method for band edge states calculation. We illustrate the O(N) behavior of the
new method by applying it to boron nitride (BN) nanotubes and BN nanotubes with
an adsorbed hydrogen atom. The band gap of various BN nanotubes are
investigated systematicly and the acceptor levels of BN nanotubes with an
isolated adsorbed H atom are computed. Our methods are simple, robust, and
especially suited for the application in self-consistent field electronic
structure theory
First principles lattice dynamics of NaCoO
We report first principles linear response calculations on NaCoO. Phonon
frequencies and eigenvectors are obtained throughout the Brillouin zone for two
geometries with different Na site occupancies. While most of the phonon modes
are found to be unsensitive to the Na site occupancy, there are two modes
dominated by out-of-plane vibrations of Na giving very different frequencies
for different geometries. One of these two modes, the A mode, is
infrared-active, and can be used as a suitable sensor of Na
distribution/ordering. The longitudinal-transverse splitting of the zone-center
optical-mode frequencies, Born effective charges and the dielectric constants
are also reported, showing considerable anisotropy. The calculated frequencies
of Raman-active modes generally agree with the experimental values of
corresponding Na de-intercalated and/or hydrated compounds, while it requires
better experimental data to clarify the infrared-active mode frequencies.Comment: 12 pages, 2 figure
- β¦