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 t2gt_{\rm 2g}-orbital Hubbard model with spin-orbit coupling

We investigate the ground-state properties of a one-dimensional $t_{\rm 2g}$-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 $\Gamma_{4u}$ 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 $\rho_{0}$ 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 $\rho_{0}$ observed in CeCu$_2$Ge$_2$ under the pressure at $P\sim$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 CuGe1−x_{1-x}Six_{x}O3_3

We report the observation of nuclear quadrupole resonance (NQR) of Cu from the sites near the doping center in the spin-Peierls system CuGe$_{1-x}$Si$_{x}$O$_3$. 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