Electronic and magnetic structure of CsV2_2O5_5

We have studied the electronic structure of the spin-gapped system CsV$_2$O$_5$ by means of an ab initio calculation. Our analysis and a re-examination of the susceptibility data indicate that the behavior of this system is much closer to that of an alternating spin-1/2 antiferromagnetic chain with significant inter-dimer coupling and weaker inter-chain couplings than that of isolated dimers as was initially proposed. Comparison to the vanadate family members $\alpha '$-NaV$_2$O$_5$, $\gamma$-LiV$_2$O$_5$ and isostructural compounds like (VO)$_{2}$P$_{2}$O$_{7}$ (VOPO) is discussed.Comment: revised version. To appear in Phys. Rev.

Comparative study of FeCr2S4 and FeSc2S4: Spinels with orbitally active A site

Using first-principles density functional calculations, we perform a comparative study of two Fe based spinel compounds, FeCr2S4 and FeSc2S4. Though both systems contain an orbitally active A site with an Fe2+ ion, their properties are rather dissimilar. Our study unravels the microscopic origin of their behavior driven by the differences in hybridization of Fe d states with Cr/Sc d states and S p states in the two cases. This leads to important differences in the nature of the magnetic exchanges as well as the nearest versus next nearest neighbor exchange parameter ratios, resulting into significant frustration effects in FeSc2S4 which are absent in FeCr2S4.Comment: 5 pages, 4 figures Phys Rev B (rapid commun) to appear (2010

Chandra Observations of the X-Ray Jet of 3C273

We report results from Chandra observations of the X-ray jet of 3C~273 during the calibration phase in 2000 January. The zeroeth-order images and spectra from two 40-ks exposures with the HETG and LETG+ACIS-S show a complex X-ray structure. The brightest optical knots are detected and resolved in the 0.2-8 keV energy band. The X-ray morphology tracks well the optical. However, while the X-ray brightness decreases along the jet, the outer parts of the jet tend to be increasingly bright with increasing wavelength. The spectral energy distributions of four selected regions can best be explained by inverse Compton scattering of (beamed) cosmic microwave background photons. The model parameters are compatible with equipartition and a moderate Doppler factor, which is consistent with the one-sidedness of the jet. Alternative models either imply implausible physical conditions and energetics (the synchrotron self-Compton model) or are sufficiently ad hoc to be unconstrained by the present data (synchrotron radiation from a spatially or temporally distinct particle population).Comment: 3 figures; Figure 1 in color. Accepted for publication by ApJ Letter

Non-Quantum Behaviors of Configuration-Space Density Formulations of quantum mechanics

The trajectories of the pilot-wave formulation of quantum mechanics and hence its empirical predictions may be recovered via the dynamics of a density function on the configuration space of a system, without reference to a physical wave function. We label such formulations `CSD frameworks.' But this result only holds if a particular, apparently ad hoc condition, broadly speaking equivalent to the single-valuedness of the wave function in standard quantum mechanics, is imposed. Here we relax this condition. We describe the types of scenarios in which this would lead to deviations from quantum mechanics. Using computational models we ask how the degree of `non-quantumness' of a state, suitably defined, changes with time. We find that it remains constant in time even under non-trivial dynamics, and argue that this implies that a dynamical justification of the Wallstrom condition is unlikely to be successful. However, we also make certain observations about stationary states in CSD frameworks, which may offer a way forward in justifying the Wallstrom condition.Comment: 8 pages, forthcoming in "Advances in Pilot Wave Theory - From Experiments to Foundations", fixed references in this versio

Understanding the Goals of Everyday Instrumental Actions Is Primarily Linked to Object, Not Motor-Kinematic, Information: Evidence from fMRI

Prior research conceptualised action understanding primarily as a kinematic matching of observed actions to own motor representations but has ignored the role of object information. The current study utilized fMRI to identify (a) regions uniquely involved in encoding the goal of others' actions, and (b) to test whether these goal understanding processes draw more strongly on regions involved in encoding object semantics or movement kinematics. Participants watched sequences of instrumental actions while attending to either the actions' goal (goal task), the movements performed (movement task) or the objects used (object task). The results confirmed, first, a unique role of the inferior frontal gyrus, middle temporal gyrus and medial frontal gyrus in action goal understanding. Second, they show for the first time that activation in the goal task overlaps directly with object- but not movement-related activation. Moreover, subsequent parametric analyses revealed that movement-related regions become activated only when goals are unclear, or observers have little action experience. In contrast to motor theories of action understanding, these data suggest that objects-rather than movement kinematics-carry the key information about others' actions. Kinematic information is additionally recruited when goals are ambiguous or unfamiliar

Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings

Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic analysis of its electronic and magnetic properties. By means of a non-trivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.Comment: 5 pages, 5 figures. Phys. Rev. Lett. (in press

Proposed Orbital Ordering in MnV2O4 from First-principles Calculations

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation (LSDA)+U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favours a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.Comment: Revised version. To appear in Phys. Rev. Let