Interplay of Chemical Bonding and Magnetism in Fe_4N, Fe_3N, Fe_2N

Using spin density functional theory we have carried out a comparative study of chemical bonding and magnetism in Fe_4N, Fe_3N and Fe_2N. All of these compounds form close packed Fe lattices, while N occupies octahedral interstitial positions. High spin fcc Fe and hypothetical FeN with rock salt structure have been included in our study as reference systems. We find strong, covalent Fe-N bonds as a result of a substantial \sigma-type p-d hybridisation, with some charge transfer to N. Those Fe d orbitals which contribute to the p-d bonds, do no longer participate in the exchange splitting of the Fe d bands. Because of the large exchange fields, the majority spin d bands are always fully occupied, while the minority spin d bands are close to half-filling, thus optimizing the Fe d-d covalent bonding. As a consequence, in good approximation the individual Fe moments decrease in steps of 0.5 \mu_B from fcc iron (2.7 \mu_B) via Fe_4N (2.7 and 1.97 mu_B}), \chem{Fe_3N} (1.99 \mu_B) to \zeta - Fe_2N (1.43 \mu_B).Comment: 16 pages, 15 figure

Optical Conductivity of the Trellis-Lattice t-J Model: Charge Fluctuations in NaV_2O_5

Optical conductivity of the trellis lattice t-J model at quarter filling is calculated by an exact-diagonalization technique on small clusters, whereby the valence state of V ions of NaV_2O_5 is considered. We show that the experimental features at \sim 1 eV, including peak positions, presence of shoulders, and anisotropic spectral weight, can be reproduced in reasonable range of parameter values, only by assuming that the system is in the charge disproportionated ground state. Possible reconciliation with experimental data suggesting the presence of uniform ladders at T>T_c is discussed.Comment: 4 pages, 4 gif figures. Minor revisions have been made. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to [email protected]

Charge Ordering and Spin gap in NaV_2O_5

A possible ground state of NaV_2O_5 is proposed based on the Hartree approximation for both on-site and intersite Coulomb interactions. The results indicate that the intersite Coulomb interaction induces a zigzag type of charge disproportionation (i.e. charge ordering) along the ladders of V-ions resulting in the localized spins between neighboring ladders to form a spin gap. This new state, which is different from the spin-Peierls state so far believed, seems to be consistent with the existing experimental results.Comment: 3 pages, 4 figures, submitted to J. Phys. Soc. Jp

NaV_2O_5 as an Anisotropic t-J Ladder at Quarter Filling

Based on recent experimental evidences that the electronic charge degrees of freedom plays an essential role in the spin-Peierls--like phase transition of NaV$_2$O$_5$, we first make the mapping of low-energy electronic states of the $d$$-$$p$ model for NaV$_2$O$_5$ to the quarter-filled $t$$-$$J$ ladder with anisotropic parameter values between legs and rungs, and then show that this anisotropic $t$$-$$J$ ladder is in the Mott insulating state, of which lowest-energy states can be modeled by the one-dimensional Heisenberg antiferromagnet with the effective exchange interaction $J_{eff}$ whose value is consistent with experimental estimates. We furthermore examine the coupling between the ladders as the trellis lattice model and show that the nearest-neighbor Coulomb repulsion on the zigzag-chain bonds can lead to the instability in the charge degrees of freedom of the ladders.Comment: 4 pages, 5 gif figures. Fig.3 corrected. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to [email protected]

The C-type lectin receptor SIGNR3 binds to fungi present in commensal microbiota and influences immune regulation in experimental colitis

Inflammatory bowel disease is a condition of acute and chronic inflammation of the gut. An important factor contributing to pathogenesis is a dysregulated mucosal immunity against commensal bacteria and fungi. Host pattern- recognition receptors (PRRs) sense commensals in the gut and are involved in maintaining the balance between controlled responses to pathogens and overwhelming innate immune activation. C-type lectin receptors (CLRs) are PRRs recognizing glycan structures on pathogens and self-antigens. Here we examined the role of the murine CLR specific intracellular adhesion molecule-3 grabbing non-integrin homolog-related 3 (SIGNR3) in the recognition of commensals and its involvement in intestinal immunity. SIGNR3 is the closest murine homolog of the human dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) receptor recognizing similar carbohydrate ligands such as terminal fucose or high-mannose glycans. We discovered that SIGNR3 recognizes fungi present in the commensal microbiota. To analyze whether this interaction impacts the intestinal immunity against microbiota, the dextran sulfate sodium-induced colitis model was employed. SIGNR3(-/-) mice exhibited an increased weight loss associated with more severe colitis symptoms compared to wild-type control mice. The increased inflammation in SIGNR3(-/-) mice was accompanied by a higher level of TNF-α in colon. Our findings demonstrate for the first time that SIGNR3 recognizes intestinal fungi and has an immune regulatory role in colitis

A Model Study of the Low-Energy Charge Dynamics of NaV_2O_5

An exact-diagonalization technique on small clusters is used to calculate the dynamical density correlation functions of the dimerized t-J chain and coupled anisotropic t-J ladders (trellis lattice) at quarter filling, i.e., the systems regarded as a network of pairs (dimers or rungs) of sites coupled weakly via the hopping and exchange interactions. We thereby demonstrate that the intersite Coulomb repulsions between the pairs induce a low-energy collective mode in the charge excitations of the systems where the internal charge degrees of freedom of the pairs play an essential role. Implications to the electronic states of NaV_2O_5, i.e., fluctuations of the valence state of V ions and phase transition as a charge ordering, are discussed.Comment: 4 pages, 4 gif figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to [email protected]

Entanglement and Tensor Product Decomposition for Two Fermions

The problem of the choice of tensor product decomposition in a system of two fermions with the help of Bogoliubov transformations of creation and annihilation operators is discussed. The set of physical states of the composite system is restricted by the superselection rule forbidding the superposition of fermions and bosons. It is shown that the Wootters concurrence is not proper entanglement measure in this case. The explicit formula for the entanglement of formation is found and its dependence on tensor product decompositions of the Hilbert space is discussed. It is shown that the set of separable states is narrower than in two-qubit case. Moreover, there exist states which are separable with respect to all tensor product decompositions of the Hilbert space.Comment: 8pp, published versio

Zigzag Charge Ordering in alpha'-NaV2O5

23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure including both atomic displacement and charge disproportionation is of lower symmetry. It is suggested that zigzag ordering is the most probable. The temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions. It is thus concluded that the charge ordering transition is first-order.Comment: 4 pages, 5 eps figures, submitted to J. Phys. Soc. Jp

Superexchange in the quarter- filled two- leg ladder system NaV2O5

A theory of superexchange in the mixed valent layer compound NaV2O5 is presented which provides a consistent description of exchange both in the disordered and charge ordered state. Starting from results of band structure calculations for NaV2O5 first an underlying electronic model for a ladder unit in the Trellis lattice is formulated. By using the molecular orbital representation for intra-rung electronic states a second-order perturbation procedure is developed and an effective spin-chain model for a ladder is derived. Variation of the resulting superexchange integral J is examined numerically as the ladder system evolves from a charge disordered to the extreme ('zig-zag') charge ordered state. It is found that the effective intra- ladder superexchange is always antiferromagnetic.Comment: 18 pages Revtex, 7 Postscript figure

Exact diagonalisation study of charge order in the quarter-filled two-leg ladder system NaV2O5

The charge ordering transition in the layer compound NaV2O5 is studied by means of exact diagonalization methods for finite systems. The 2-leg ladders of the V-Trellis lattice are associated with one spin variable of the vanadium 3d-electron in the rung and a pseudospin variable that describes its positional degree of freedom. The charge ordering (CO) due to intersite Coulomb interactions is described by an effective Ising-like Hamiltonian for the pseudo-spins that are coupled to the spin fluctuations along the ladder. We employ a Lanczos algortihm on 2D lattice to compute charge (pseudo-spin) and spin-correlation functions and the energies of the low lying excited states. A CO-phase diagram is constructed and the effect of intra-ladder exchange on the CO transition is studied. It is shown that a phase with no-longe range order (no-LRO) exists between the in-line and zig-zag ordered structures. We provide a finite-size scaling analysis for the spin excitation gap and also discuss the type of excitations. In addition we studied the effect of bond-alternation of spin exchange and derived a scaling form for the spin gap in terms of the dimerization parameter.Comment: 9 pages with 9 EPS figures and 1 table, To be appeared in Phys. Rev. B (2001