47 research outputs found
The symmetry problem in NaV2O5
We discuss the symmetry of NaV2O5 in the high temperature phase on the basis
of optical conductivity data. Conclusive information cannot be obtained by
studying the optically allowed lattice vibrations. However, intensity and
polarization of the electronic excitations give a direct indication for a
broken-parity electronic ground-state. This is responsible for the detection of
charged bi-magnons in the optical spectrum.Comment: Revtex, 2 pages, 1 postscript picture embedded in the tex
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
NaVO, we first make the mapping of low-energy electronic states of the
model for NaVO to the quarter-filled ladder with
anisotropic parameter values between legs and rungs, and then show that this
anisotropic 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 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]
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]
Optical properties and electronic structure of Ca-doped alpha'-NaV2O5
The dielectric function of alpha'-Na(1-x)Ca(x)V2O5 (0 < x < 20%) was measured
for the a and b axes in the photon energy range 0.8-4.5 eV at room temperature.
By varying the Ca-concentration we control the relative abundancy of V4+ and
V5+. We observe that the intensity of the main optical absorption peak at 1 eV
is proportional to the number of V5+ ions. This rules out the interpretation as
a V4+ d-d excitation, and it establishes that this is the on-rung
bonding-antibonding transition.Comment: 6 pages, ReVTeX, 5 figures in encapsulated postscript forma
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
Temperature dependence of optical spectral weights in quarter-filled ladder systems
The temperature dependence of the integrated optical conductivity I(T)
reflects the changes of the kinetic energy as spin and charge correlations
develop. It provides a unique way to explore experimentally the kinetic
properties of strongly correlated systems. We calculated I(T) in the frame of a
t-J-V model at quarter-filling for ladder systems, like NaV_2O_5, and show that
the measured strong T dependence of I(T) for NaV_2O_5 can be explained by the
destruction of short range antiferromagnetic correlations. Thus I(T) provides
detailed information about super-exchange and magnetic energy scales.Comment: 4 pages, 5 figure
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
Dzyaloshinskii-Moriya interaction in NaVO: a microscopic study
We present a unified account of magnetic exchange and Raman scattering in the
quasi-one-dimensional transition-metal oxide NaVO. Based on a
cluster-model approach explicit expressions for the exchange integral and the
Raman-operator are given. It is demonstrated that a combination of the
electronic-structure and the Dzyaloshinskii-Moriya interaction, allowed by
symmetry in this material, are responsible for the finite Raman cross-section
giving rise to both, one- and two-magnon scattering amplitudes.Comment: 7 pages, 1 figur
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
High frequency dielectric and magnetic anomaly at the phase transition in NaV2O5
We found anomalies in the temperature dependence of the dielectric and the
magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency
ranges. The anomalies occur at the phase transition temperature T_c, at which
the spin gap opens. The real parts of the dielectric constants epsilon_a and
epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow
region close to T_c) is proportional to the intensity of the x-ray reflection
appearing at T_c. The dielectric constant anomaly can be explained by the
zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the
microwave magnetic losses is probably related to the coupling between the spin
and charge degrees of freedom in vanadium ladders.Comment: 3 PS-figures, LATEX-text, new experimental data added, typos
correcte