1,973 research outputs found
Orbital-controlled magnetic transition between gapful and gapless phases in the Haldane system with t2g-orbital degeneracy
In order to clarify a key role of orbital degree of freedom in the spin S=1
Haldane system, we investigate ground-state properties of the t2g-orbital
degenerate Hubbard model on the linear chain by using numerical techniques.
Increasing the Hund's rule coupling in multi-orbital systems, in general, there
occurs a transition from an antiferromagnetic to a ferromagnetic phase. We find
that the antiferromagnetic phase is described as the Haldane system with spin
gap, while in the ferromagnetic phase, there exists the gapless excitation with
respect to orbital degree of freedom. Possible relevance of the present results
to actual systems is also discussed.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Spin, charge, and orbital correlations in the one-dimensional t2g-orbital Hubbard model
We present the zero-temperature phase diagram of the one-dimensional
t2g-orbital Hubbard model, obtained using the density-matrix renormalization
group and Lanczos techniques. Emphasis is given to the case for the electron
density n=5 corresponding to five electrons per site, of relevance for some
Co-based compounds. However, several other cases for electron densities between
n=3 and 6 are also studied. At n=5, our results indicate a first-order
transition between a paramagnetic (PM) insulator phase and a fully-polarized
ferromagnetic (FM) state by tuning the Hund's coupling. The results also
suggest a transition from the n=5 PM insulator phase to a metallic regime by
changing the electron density, either via hole or electron doping. The behavior
of the spin, charge, and orbital correlation functions in the FM and PM states
are also described in the text and discussed. The robustness of these two
states varying parameters suggests that they may be of relevance in more
realistic higher dimensional systems as well.Comment: 9 pages, 8 figure
Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex
We carry out three-dimensional MHD simulations of star formation in
turbulent, magnetized clouds, including ambipolar diffusion and feedback from
protostellar outflows. The calculations focus on relatively diffuse clouds
threaded by a strong magnetic field capable of resisting severe tangling by
turbulent motions and retarding global gravitational contraction in the
cross-field direction. They are motivated by observations of the Taurus
molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an
ordered large-scale magnetic field, as well as elongated condensations that are
generally perpendicular to the large-scale field. We find that stars form in
earnest in such clouds when enough material has settled gravitationally along
the field lines that the mass-to-flux ratios of the condensations approach the
critical value. Only a small fraction (of order 1% or less) of the nearly
magnetically-critical, condensed material is turned into stars per local
free-fall time, however. The slow star formation takes place in condensations
that are moderately supersonic; it is regulated primarily by magnetic fields,
rather than turbulence. The quiescent condensations are surrounded by diffuse
halos that are much more turbulent, as observed in the Taurus complex. Strong
support for magnetic regulation of star formation in this complex comes from
the extremely slow conversion of the already condensed, relatively quiescent
CO gas into stars, at a rate two orders of magnitude below the maximum,
free-fall value. We analyze the properties of dense cores, including their mass
spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap
Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains
We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model
coupled to a lattice distortion by a quantum Monte Carlo method. Investigating
the ground state energy of the static bond-alternating chain, we find that the
instability to a dimerized chain depends on the value of the spin-phonon
coupling, unlike the case of S=1/2. The spin state is the dimer state or the
uniform Haldane state depending on whether the lattice distorts or not,
respectively. At an intermediate value of the spin-phonon coupling, we find the
first-order transition between the two states. We also find the coexistence of
the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos,
replaced figure
The Dynamical State fo the Starless Dense Core FeSt 1-457: A Pulsating Globule?
High resolution molecular line observations of CS, HCO+, C18O and N2H+ were
obtained toward the starless globule FeSt 1-457 in order to investigate its
kinematics and chemistry. The HCO+ and CS spectra show clear self-reversed and
asymmetric profiles across the face of the globule. The sense of the observed
asymmetry is indicative of the global presence of expansion motions in the
outer layers of the globule. These motions appear to be subsonic and
significantly below the escape velocity of the globule. Comparison of our
observations with near-infrared extinction data indicate that the globule is
gravitationally bound. Taken together these considerations lead us to suggest
that the observed expansion has its origin in an oscillatory motion of the
outer layers of the globule which itself is likely in a quasi-stable state near
hydrostatic equilibrium. Analysis of the observed linewidths of CO and N2H+
confirm that thermal pressure is the dominant component of the cloud's internal
support. A simple calculation suggests that the dominant mode of pulsation
would be an l = 2 mode with a period of 0.3 Myr. Deformation of the globule due
to the large amplitude l = 2 oscillation may be responsible for the
double-peaked structure of the core detected in high resolution extinction
maps. Detailed comparison of the molecular-line observations and extinction
data provides evidence for significant depletion of C18O and perhaps HCO+ while
N2H+ may be undepleted to a cloud depth of about 40 magnitudes of visual
extinction.Comment: to appear in ApJ vol 665 20 August 2007
The nature of the dense core population in the pipe nebula: core and cloud kinematics from C18O observations
We present molecular-line observations of 94 dark cloud cores identified in
the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio
Observatory 12m telescope, we obtained spectra of these cores in the J=1-0
transition of C18O. We use the measured core parameters, i.e., antenna
temperature, linewidth, radial velocity, radius and mass, to explore the
internal kinematics of these cores as well as their radial motions through the
larger molecular cloud. We find that the vast majority of the dark extinction
cores are true cloud cores rather than the superposition of unrelated
filaments. While we identify no significant correlations between the core's
internal gas motions and the cores' other physical parameters, we identify
spatially correlated radial velocity variations that outline two main kinematic
components of the cloud. The largest is a 15pc long filament that is
surprisingly narrow both in spatial dimensions and in radial velocity.
Beginning in the Stem of the Pipe, this filament displays uniformly small C18O
linewidths (dv~0.4kms-1) as well as core to core motions only slightly in
excess of the gas sound speed. The second component outlines what appears to be
part of a large (2pc; 1000 solar mass) ring-like structure. Cores associated
with this component display both larger linewidths and core to core motions
than in the main cloud. The Pipe Molecular Ring may represent a primordial
structure related to the formation of this cloud.Comment: Accepted to ApJ. 14 pages, 11 figures. Complete table at end of
documen
Probing the Galactic cosmic ray flux with submillimeter and gamma ray data
The study of Galactic diffuse radiation combined with the knowledge
of the distribution of the molecular hydrogen in the Galaxy offers a unique
tool to probe the cosmic ray flux in the Galaxy. A methodology to study the
level of the cosmic ray "sea" and to unveil target-accelerator systems in the
Galaxy, which makes use of the data from the high resolution survey of the
Galactic molecular clouds performed with the NANTEN telescope and of the data
from gamma-ray instruments, has been developed. Some predictions concerning the
level of the cosmic ray "sea" and the -ray emission close to cosmic ray
sources for instruments such as Fermi and Cherenkov Telescope Array are
presented.Comment: Proceedings of the 4th Heidelberg International Symposium on High
Energy Gamma-Ray Astronom
Modeling the gamma-ray emission produced by runaway cosmic rays in the environment of RX J1713.7-3946
Diffusive shock acceleration in supernova remnants is the most widely invoked
paradigm to explain the Galactic cosmic ray spectrum. Cosmic rays escaping
supernova remnants diffuse in the interstellar medium and collide with the
ambient atomic and molecular gas. From such collisions gamma-rays are created,
which can possibly provide the first evidence of a parent population of runaway
cosmic rays. We present model predictions for the GeV to TeV gamma-ray emission
produced by the collisions of runaway cosmic rays with the gas in the
environment surrounding the shell-type supernova remnant RX J1713.7-3946. The
spectral and spatial distributions of the emission, which depend upon the
source age, the source injection history, the diffusion regime and the
distribution of the ambient gas, as mapped by the LAB and NANTEN surveys, are
studied in detail. In particular, we find for the region surrounding RX
J1713-3946, that depending on the energy one is observing at, one may observe
startlingly different spectra or may not detect any enhanced emission with
respect to the diffuse emission contributed by background cosmic rays. This
result has important implications for current and future gamma-ray experiments.Comment: version published on PAS
Molecular Clouds as Cosmic-Ray Barometers
The advent of high sensitivity, high resolution gamma-ray detectors, together
with a knowledge of the distribution of the atomic hydrogen and especially of
the molecular hydrogen in the Galaxy on sub-degree scales creates a unique
opportunity to explore the flux of cosmic rays in the Galaxy. We here present
the new data on the distribution of the molecular hydrogen from a large region
of the inner Galaxy obtained by the NANTEN Collaboration. We then introduce a
methodology which aims to provide a test bed for current and future gamma-ray
observatories to explore the cosmic ray flux at various positions in our
Galaxy. In particular, for a distribution of molecular clouds, as provided by
the NANTEN survey, and local cosmic ray density as measured at the Earth, we
estimate the expected GeV to TeV gamma-ray signal, which can then be compared
with observations and use to test the cosmic ray flux.Comment: PASJ (in press
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