3,198 research outputs found
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
Structure and apparent topography of TiO2 (110) surfaces
We present self-consistent ab-initio total-energy and electronic-structure
calculations on stoichiometric and non-stoichiometric TiO2 (110) surfaces.
Scanning tunneling microscopy (STM) topographs are simulated by calculating the
local electronic density of states over an energy window appropriate for the
experimental positive-bias conditions. We find that under these conditions the
STM tends to image the undercoordinated Ti atoms, in spite of the physical
protrusion of the O atoms, giving an apparent reversal of topographic contrast
on the stoichiometric 1x1 or missing-row 2x1 surface. We also show that both
the interpretation of STM images and the direct comparison of surface energies
favor an added-row structure over the missing-row structure for the
oxygen-deficient 2x1 surface.Comment: 6 pages, two-column style with 5 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#ng_tio
Multipole correlations of -orbital Hubbard model with spin-orbit coupling
We investigate the ground-state properties of a one-dimensional -orbital Hubbard model including an atomic spin-orbit coupling by using
numerical methods, such as Lanczos diagonalization and density-matrix
renormalization group. As the spin-orbit coupling increases, we find a
ground-state transition from a paramegnetic state to a ferromagnetic state. In
the ferromagnetic state, since the spin-orbit coupling mixes spin and orbital
states with complex number coefficients, an antiferro-orbital state with
complex orbitals appears. According to the appearance of the complex orbital
state, we observe an enhancement of octupole correlations.Comment: 3 pages, 3 figures, To appear in J. Phys. Soc. Jpn. Suppl.,
Proceedings of ICHE2010 (September 17-20, 2010, Hachioji, Japan
Catalogue of 12CO(J=1-0) and 13CO(J=1-0) Molecular Clouds in the Carina Flare Supershell
We present a catalogue of 12CO(J=1-0) and 13CO(J=1-0) molecular clouds in the
spatio-velocity range of the Carina Flare supershell, GSH 287+04-17. The data
cover a region of ~66 square degrees and were taken with the NANTEN 4m
telescope, at spatial and velocity resolutions of 2.6' and 0.1 km/s.
Decomposition of the emission results in the identification of 156 12CO clouds
and 60 13CO clouds, for which we provide observational and physical parameters.
Previous work suggests the majority of the detected mass forms part of a
comoving molecular cloud complex that is physically associated with the
expanding shell. The cloud internal velocity dispersions, degree of
virialization and size-linewidth relations are found to be consistent with
those of other Galactic samples. However, the vertical distribution is heavily
skewed towards high-altitudes. The robust association of high-z molecular
clouds with a known supershell provides some observational backing for the
theory that expanding shells contribute to the support of a high-altitude
molecular layer.Comment: To be published in PASJ Vol. 60, No. 6. (Issued on December 25th
2008). 35 pages (including 13 pages of tables), 7 figures. Please note that
formatting problems with the journal macro result in loss of rightmost data
columns in some long tables. These will be fixed in the final published
issue. In the meantime, please contact the authors for missing dat
Desalination of shale gas produced water: a rigorous design approach for zero-liquid discharge evaporation systems
Shale gas has recently emerged as a promising energy source to face the increasing global demand. This paper introduces a new rigorous optimization model for the simultaneous synthesis of single and multiple-effect evaporation (SEE/MEE) systems, considering mechanical vapor recompression (MVR) and energy recovery. The proposed model has been especially developed for the desalination of high-salinity produced water from shale gas hydraulic fracturing (“fracking”). Its main objective is to enhance the system energy efficiency through the reduction of brine discharges. Therefore, the outflow brine salinity should be near to salt saturation conditions to achieve zero liquid discharge (ZLD). The multiple-effect superstructure is comprised by several effects of horizontal-tube falling film evaporation. Due to the inclusion of the electric-driven mechanical compressor, no other external energy source is needed in the SEE/MEE system. A more accurate process design is attained through the calculation of the overall heat transfer coefficients in function of the individual coefficients for the falling boiling film and vapor condensation. Additionally, the SEE/MEE-MVR model allows the estimation of the major geometrical characteristics of the evaporation system. The non-linear programming (NLP)-based model is optimized using the CONOPT solver under GAMS by the minimization of the process total annualized cost. Thermal analysis is carried out to evaluate the effects of the feed salinity and geometrical parameters on system heat transfer performance. The results highlight the ability of the developed model to rigorously design SEE/MEE-MVR systems by improving their cost-effectively and reaching ZLD conditions
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
Huge Enhancement of Impurity Scattering due to Critical Valence Fluctuations in a Ce-Based Heavy Electron System
On the basis of the Ward-Pitaevskii identity, the residual resistivity
is shown to exhibit huge enhancement around the quantum critical
point of valence transition in Ce-based heavy electron systems. This explains a
sharp peak of observed in CeCuGe under the pressure at
16GPa where the superconducting trasition temperature also exhibit the
sharp peak.Comment: 5 pages, 1 figur
The mass function of dense molecular cores and the origin of the IMF
Context: Stars form in the cold dense cores of interstellar molecular clouds
and the detailed knowledge of the spectrum of masses of such cores is clearly a
key for the understanding of the origin of the IMF. To date, observations have
presented somewhat contradictory evidence relating to this issue. Aims: In this
paper we propose to derive the mass function of a complete sample of dense
molecular cores in a single cloud employing a robust method that uses uses
extinction of background starlight to measure core masses and enables the
reliable extension of such measurements to lower masses than previously
possible. Methods: We use a map of near-infrared extinction in the nearby Pipe
dark cloud to identify the population of dense cores in the cloud and measure
their masses. Results: We identify 159 dense cores and construct the mass
function for this population. We present the first robust evidence for a
departure from a single power-law form in the mass function of a population of
cores and find that this mass function is surprisingly similar in shape to the
stellar IMF but scaled to a higher mass by a factor of about 3. This suggests
that the distribution of stellar birth masses (IMF) is the direct product of
the dense core mass function and a uniform star formation efficiency of
30%+/-10%, and that the stellar IMF may already be fixed during or before the
earliest stages of core evolution. These results are consistent with previous
dust continuum studies which suggested that the IMF directly originates from
the core mass function. The typical density of ~10^4/cm^3 measured for the
dense cores in this cloud suggests that the mass scale that characterizes the
dense core mass function may be the result of a simple process of thermal
(Jeans) fragmentation.Comment: A&A accepte
The nature of the dense core population in the Pipe Nebula: A survey of NH3, CCS, and HC5N molecular line emission
Recent extinction studies of the Pipe Nebula (d=130 pc) reveal many cores
spanning a range in mass from 0.2 to 20.4 Msun. These dense cores were
identified via their high extinction and comprise a starless population in a
very early stage of development. Here we present a survey of NH3 (1,1), NH3
(2,2), CCS (2_1,1_0), and HC5N (9,8) emission toward 46 of these cores. An
atlas of the 2MASS extinction maps is also presented. In total, we detect 63%
of the cores in NH3 (1,1) 22% in NH3 (2,2), 28% in CCS, and 9% in HC5N
emission. We find the cores are associated with dense gas (~10^4 cm-3) with 9.5
< T_k < 17 K. Compared to C18O, we find the NH3 linewidths are systematically
narrower, implying that the NH3 is tracing the dense component of the gas and
that these cores are relatively quiescent. We find no correlation between core
linewidth and size. The derived properties of the Pipe cores are similar to
cores within other low-mass star-forming regions: the only differences are that
the Pipe cores have weaker NH3 emision and most show no current star formation
as evidenced by the lack of embedded infrared sources. Such weak NH3 emission
could arise due to low column densities and abundances or reduced excitation
due to relatively low core volume densities. Either alternative implies that
the cores are relatively young. Thus, the Pipe cores represent an excellent
sample of dense cores in which to study the initial conditions for star
formation and the earliest stages of core formation and evolution.Comment: 35 pages, 10 figures (excluding the appendix). For the complete
appendix contact [email protected]. Accepted for publication in ApJ
Absence of Edge Localized Moments in the Doped Spin-Peierls System CuGeSiO
We report the observation of nuclear quadrupole resonance (NQR) of Cu from
the sites near the doping center in the spin-Peierls system
CuGeSiO. The signal appears as the satellites in the Cu NQR
spectrum, and has a suppressed nuclear spin-lattice relaxation rate indicative
of a singlet correlation rather than an enhanced magnetic correlation near the
doping center. Signal loss of Cu nuclei with no neighboring Si is also
observed. We conclude from these observations that the doping-induced moments
are not in the vicinity of the doping center but rather away from it.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
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