166 research outputs found
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
Low-Temperature Structure of the Quarter-Filled Ladder Compound alpha'-NaV2O5
The low-temperature (LT) superstructure of -NaVO 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
with the space group where and
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 and V with a
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 sites.Comment: 4 pages, 3 figures, 1 tabl
Magnetic bound states in the quarter-filled ladder system }
Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5
shows in the dimerized singlet ground state () an unexpected
sequence of three magnetic bound states. Our results suggest that the recently
proposed mapping onto an effective spin chain for has to be given
up in favor of the full topology and exchange paths of a ladder in the
dimerized phase for . As the new ground state we propose a dynamic
superposition of energetically nearly degenerate dimer configurations on the
ladder.Comment: 5 pages, 4 figures, to be published in PRB, brief reports, Dec. 199
Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5
The specific heat, magnetic susceptibility and ESR signals of a Na-deficient
vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07
- 10 K, well below the transition point to a spin-gap state. The contribution
of defects provided by sodium vacancies to the specific heat was observed. It
has a low temperature part which does not tend to zero till at least 0.3 K and
a high temperature power-like tail appears above 2 K. Such dependence may
correspond to the existence of local modes and correlations between defects in
V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of
freedom for the defects, with their effective number increasing in temperature
and under magnetic field. The latter results in the nonsaturating magnetization
at low temperature. No long-range magnetic ordering in the system of defects
was found. A model for the defects based on electron jumps near vacancies is
proposed to explain the observed effects. The concept of a frustrated
two-dimensional correlated magnet induced by the defects is considered to be
responsible for the absence of magnetic ordering.Comment: 6 pages, 8 figure
Spin gap behavior and charge ordering in \alpha^{\prime}-NaV_2O_5 probed by light scattering
We present a detailed analysis of light scattering experiments performed on
the quarter-filled spin ladder compound -NaVO for
the temperature range 5 KT300 K. This system undergoes a phase
transition into a singlet ground state at T=34 K accompanied by the formation
of a super structure. For T34 K several new modes were detected. Three of
these modes are identified as magnetic bound states. Experimental evidence for
charge ordering on the V sites is detected as an anomalous shift and splitting
of a V-O vibration at 422 cm for temperatures above 34 K. The smooth and
crossover-like onset of this ordering at T= 80 K is accompanied by
pretransitional fluctuations both in magnetic and phononic Raman scattering. It
resembles the effect of stripe order on the super structure intensities in
LaNiO.Comment: 36 pages, 11 figures, accepted for publication in PRB (sept.99
Charge-ordering and optical transitions of LiV2O5 and NaV2O5
We present the measurements of the polarized optical spectra of NaV2O5 and
LiV2O5. In an energy range from 0.5 to 5.5 eV we observe similar peaks in the E
parallel a spectra of LiV2O5 and NaV2O5, which suggests similar electronic
structure along the a axis in both materials. On the other hand, we find an
almost complete suppression of the peaks in sigma_b of LiV2O5 around 1 and 5
eV. We attribute this suppression to the charge localization originating from
the existence of double-chain charge-ordering patterin in LiV2O5.Comment: 7 pages, 3 figures final version, to appear in PR
Can a frustrated spin-cluster model describe the low-temperature physics of NaV_2O_5 ?
Recent experimental evidence suggest the existence of three distinct
V-valence states (V^{+4}, V^{+4.5} and V^{+5}) in the low-temperature phase of
NaV_2O_5 in apparent discrepancy with the observed spin-gap. We investigate a
novel spin cluster model, consisting of weakly coupled, frustrated four-spin
clusters aligned along the crystallographic b-axis that was recently proposed
to reconcile these experimental observations. We have studied the phase diagram
and the magnon dispersion relation of this model using DMRG, exact
diagonalization and a novel cluster-operator theory. We find a spin-gap for all
parameter values and two distinct phases, a cluster phase and a Haldane phase.
We evaluate the size of the gap and the magnon dispersion and find no parameter
regime which would reproduce the experimental results. We conclude that this
model is inappropriate for the low-temperature regime of NaV_2O_5
One-dimensional dynamics of the d-electrons in -NaVO
We have studied the electronic properties of the ladder compound
-NaVO, adopting a joint experimental and theoretical
approach. The momentum-dependent loss function was measured using electron
energy-loss spectroscopy in transmission. The optical conductivity derived from
the loss function by a Kramers-Kronig analysis agrees well with our results
from LSDA+U band-structure calculations upon application of an
antiferromagnetic alignment of the V~3 spins along the legs and an
on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the
calculated optical conductivity into contributions from transitions between
selected energy regions of the DOS reveals the origin of the observed
anisotropy of the optical conductivity. In addition, we have investigated the
plasmon excitations related to transitions between the vanadium states within
an effective 16 site vanadium cluster model. Good agreement between the
theoretical and experimental loss function was obtained using the hopping
parameters derived from the tight binding fit to the band-structure and
moderate Coulomb interactions between the electrons within the ab plane.Comment: 23 pages, 8 figures; submitted to PR
Ab initio evaluation of the charge-ordering in
We report {\it ab initio} calculations of the charge ordering in
using large configurations interaction methods on
embedded fragments. Our major result is that the 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
Low temperature ellipsometry of NaV2O5
The dielectric function of alpha'NaV2O5 was measured with electric field
along the a and b axes in the photon energy range 0.8-4.5 eV for temperatures
down to 4K. We observe a pronounced decrease of the intensity of the 1 eV peak
upon increasing temperature with an activation energy of about 25meV,
indicating that a finite fraction of the rungs becomes occupied with two
electrons while others are emptied as temperature increases. No appreciable
shifts of peaks were found s in the valence state of individual V atoms at the
phase transition is very small. A remarkable inflection of this temperature
dependence at the phase transition at 34 K indicates that charge ordering is
associated with the low temperature phase.Comment: Revisions in style and order of presentation. One new figure. In
press in Physical Review B. REVTeX, 4 pages with 4 postscript figure
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