252 research outputs found
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]
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]
Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?
We present results of high-resolution thermal-expansion and specific-heat
measurements on single crystalline alpha'-NaV2O5. We find clear evidence for
two almost degenerate phase transitions associated with the formation of the
dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K
slightly below the onset of a second-order transition at T2onset around
(34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our
results are consistent with a structural transformation at T1 induced by the
incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure
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
Low-energy excitations in NaV2O5
In the (ab) polarized Raman scattering spectra of NaV2O5 single crystals,
measured with 647.1 nm laser line at T < Tc, we found two modes at 86, and 126
cm-1 not previously reported. These two modes, together with 66, and 106 cm-1
modes, make an array of four low-energy equidistant modes below the energy
onset of the continuum at about 132 cm-1. All four modes are strongly
suppressed by increasing Na deficiency, indicating their nonvibrational origin
and the existence of a quantum phase transition at critical Na deficiency
between 3 and 4%. These results question current understanding of NaV2O5 as
quasi one-dimensional Heisenberg antiferromagnet.Comment: 6 pages, 3 figure
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 a quarter-filled ladder compound
A new X-ray diffraction study of the one-dimensional spin-Peierls compound
\alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one
type of V site, contrary to the previously postulated non-centrosymmetric
P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density
functional calculations indicate that NaV_2O_5 is a quarter-filled ladder
compound with the spins carried by V-O-V molecular orbitals on the rungs of the
ladder. Estimates of the charge-transfer gap and the exchange coupling agree
well with experiment and explain the insulating behavior of NaV_2O_5 and its
magnetic properties.Comment: Final version for PRL, value of U correcte
High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV_2O_5
A significant frequency dependence of the ESR line width is found in NaV_2O_5
between 34-100 K and the line width increases as the resonance frequency is
increased from 95 GHz to 760 GHz. The observed frequency dependence is
qualitatively explained in terms of the dynamical charge disproportionation.
The present results show the essential role of the internal charge degree of
freedom in a V-O-V bond. We have also proposed the existence of the
Dzyaloshinsky-Moriya interaction in the low temperature charge ordered phase
considering the breaking of the selection rule of ESR realized as the direct
observation of the spin gap excitation.Comment: 9 figures submitted to J. Phys.Soc. Jp
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