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

Ab initio evaluation of the charge-ordering in α′NaV2O5\alpha^\prime NaV_2O_5

We report {\it ab initio} calculations of the charge ordering in $\alpha^\prime NaV_2O_5$ using large configurations interaction methods on embedded fragments. Our major result is that the $2p_y$ electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.Comment: 4 page

Interstellar Gas and X-rays toward the Young Supernova Remnant RCW 86; Pursuit of the Origin of the Thermal and Non-Thermal X-ray

We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3 mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to identify the interstellar medium with which the shock waves of the SNR interact. We have found an HI intensity depression in the velocity range between $-46$ and $-28$ km s$^{-1}$ toward the SNR, suggesting a cavity in the interstellar medium. The HI cavity coincides with the thermal and non-thermal emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI distribution, which indicates a strong density gradient, while the non-thermal X-rays are found toward the less dense, inner part of the HI cavity. The most significant non-thermal X-rays are seen toward the southwestern part of the shell where the HI gas traces the dense and cold component. We also identified CO clouds which are likely interacting with the SNR shock waves in the same velocity range as the HI, although the CO clouds are distributed only in a limited part of the SNR shell. The most massive cloud is located in the southeastern part of the shell, showing detailed correspondence with the thermal X-rays. These CO clouds show an enhanced CO $J$ = 2-1/1-0 intensity ratio, suggesting heating/compression by the shock front. We interpret that the shock-cloud interaction enhances non-thermal X-rays in the southwest and the thermal X-rays are emitted by the shock-heated gas of density 10-100 cm$^{-3}$. Moreover, we can clearly see an HI envelope around the CO cloud, suggesting that the progenitor had a weaker wind than the massive progenitor of the core-collapse SNR RX J1713.7$-$3949. It seems likely that the progenitor of RCW 86 was a system consisting of a white dwarf and a low-mass star with low-velocity accretion winds.Comment: 19 pages, 15 figures, 4 tables, accepted for publication in Journal of High Energy Astrophysics (JHEAp

Charge Order Driven spin-Peierls Transition in NaV2O5

We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge ordering transition starting at T=37 K and preceding the lattice distortion and the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single Na site is observed in agreement with the Pmmn space group of this first 1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight distinct 23Na quadrupolar split lines, which evidences a lattice distortion with, at least, a doubling of the unit cell in the (a,b) plane. A model for this unique transition implying both charge density wave and spin-Peierls order is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

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]

X-ray anomalous scattering investigations on the charge order in α′\alpha^\prime-NaV2_2O5_5

Anomalous x-ray diffraction studies show that the charge ordering in $\alpha^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the tex

Low-Temperature Structure of the Quarter-Filled Ladder Compound alpha'-NaV2O5

The low-temperature (LT) superstructure of $\alpha'$-NaV$_2$O$_5$ was determined by synchrotron radiation x-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34K, we observed the Bragg peak splittings, which evidence that the LT structure is monoclinic. It was determined that the LT structure is $(a-b)\times 2b \times 4c$ with the space group $A112$ where $a, b$ and $c$ represent the high temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V$^{4+}$ and V$^{5+}$ with a $zigzag$ charge ordering pattern. This LT structure is consistent with resonant x-ray and NMR measurements, and strikingly contrasts to the LT structure previously reported, which includes V$^{4.5+}$ sites.Comment: 4 pages, 3 figures, 1 tabl

Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3

A huge increase of thermal conductivity k is observed at the phase transition in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04). This behavior is compared with that of pure and Zn-doped CuGeO3 where only modest kinks in the k(T) curves are observed at the spin-Peierls transition. The change of k at critical temperature Tc could be partially attributed to the opening of an energy gap in the magnetic excitation spectrum excluding the scattering of thermal phonons on spin fluctuations. However, the reason for such a strong anomaly in the k(T) may lie not only in the different energy scales of CuGeO3 and NaV2O5, but also in the different character of the phase transition in NaV2O5 which can have largely a structural origin, e.g. connected with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical Review Letter