Magnetic excitations in charge ordered

Thalmeier, Peter; Yaresko, A. N.

doi:10.1007/pl00011054

Cited by 9 publications

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“…The superstructure was found to have symmetry F mm2, but it showed two peculiar features [8,9]: (i) In one layer ladders with zigzag charge order alternate with ladders with vanadium in the mixed-valence state, (ii) Each of the two consecutive layers contains half of the six crystallographically independent vanadium atoms, but their structures were nearly equal. This crystal structure was found to be in agreement with two other X-ray diffraction measurements [10,11].Theoretical analyses have produced models that show zigzag charge order on all ladders [3,4,[12][13][14][15][16]. However, most approaches did not consider the true supercell, and therefore they cannot be expected to reveal all aspects of the mechanism of the phase transition.…”

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Orthorhombic versus monoclinic symmetry of the charge-ordered state ofNaV2O5

et al. 2002

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High-resolution X-ray diffraction data show that the lowtemperature superstructure of α ′ -NaV2O5 has an F −centered orthorhombic 2a × 2b × 4c superlattice. A structure model is proposed, that is characterized by layers with zigzag charge order on all ladders and stacking disorder, such that the averaged structure has space group Fmm2. This model is in accordance with both X-ray scattering and NMR data. Variations in the stacking order and disorder offer an explanation for the recently observed devils staircase of the superlattice period along c. 61.50. Ks; 61.44.Fw; 61.66.Fn; 75.30.Fv The low dimensional transition metal oxide α ′ -NaV 2 O 5 undergoes a phase transition at a temperature of T c = 34 K. The transition is characterized by the development of both a nonmagnetic groundstate and a superstructure [1,2]. General agreement exists that the phase transition is associated with the development of charge order on the vanadium sublattice [3,4], but the mechanism of the transition has not been revealed yet.At room temperature α ′ -NaV 2 O 5 crystallizes in space group Pmmn [5][6][7]. There is one crystallographically independent vanadium atom, that is in the mixed-valence state 4.5+. The structure can be considered as built of layers of two-leg ladders V 2 O 3 , that are stacked along c, alternating with sodium atoms and additional oxygen. The lattice parameters of the basic structure at 15 K are a = 11.294Å, b = 3.604Å, and c = 4.755Å [8]. The superlattice below T c can be described by an F −centered orthorhombic 2a × 2b × 4c supercell. The superstructure was found to have symmetry F mm2, but it showed two peculiar features [8,9]: (i) In one layer ladders with zigzag charge order alternate with ladders with vanadium in the mixed-valence state, (ii) Each of the two consecutive layers contains half of the six crystallographically independent vanadium atoms, but their structures were nearly equal. This crystal structure was found to be in agreement with two other X-ray diffraction measurements [10,11].Theoretical analyses have produced models that show zigzag charge order on all ladders [3,4,[12][13][14][15][16]. However, most approaches did not consider the true supercell, and therefore they cannot be expected to reveal all aspects of the mechanism of the phase transition.Various experiments, including anomalous X-ray scat- In order to determine the true superstructure of NaV 2 O 5 , we have measured high-resolution, highsensitive synchrotron radiation X-ray diffraction. The experiment indicates that the true superlattice is F −centered on the 2a × 2b × 4c supercell. We show that an all zigzag charge order model with orthorhombic symmetry is possible assuming stacking disorder. This model is in agreement with both X-ray diffraction and NMR.X-ray diffraction experiments were performed at beamline ID10A of the ESRF in Grenoble, France. Monochromatic radiation of a wavelength of λ = 0.66057Å was selected by the 220 reflection of diamond. Bragg reflections were measured by ω−scans using a scintillation detector.N...

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confidence: 81%

“…Theoretical analyses have produced models that show zigzag charge order on all ladders [3,4,[12][13][14][15][16]. However, most approaches did not consider the true supercell, and therefore they cannot be expected to reveal all aspects of the mechanism of the phase transition.…”

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confidence: 99%

Orthorhombic versus monoclinic symmetry of the charge-ordered state ofNaV2O5

et al. 2002

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“…Indeed, NMR measurements indicate two inequivalent vanadium sites below T c , while there exists only one site above [6]. There has been no direct evidence for the connection between CO and a spin gap, nor to distinguish various conjectured spatial distributions of charge [5,7]. In this letter, we present new results of neutron inelastic scattering (NIS) on the spin excitations in the low-T phase that can now resolve these issues.…”

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confidence: 94%

Charge ordering and spin dynamics in NaV2O5

Regnault

Lorenzo

Boucher

et al. 2000

Physica B: Condensed Matter

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We report high-resolution neutron inelastic scattering experiments on the spin excitations of NaV2O5. Below Tc, two branches with distinct energy gaps are identified. From the dispersion and intensity of the spin excitation modes, we deduce the precise zig-zag charge distribution on the ladder rungs and the corresponding charge order: ∆c ≈ 0.6. We argue that the spin gaps observed in the low-T phase of this compound are primarily due to the charge transfer.PACS numbers: 71.45. Lr, 75.10.Jm, 75.40.Gb The low dimensional inorganic compound NaV 2 O 5 undergoes a phase transition at T c = 34 K [1] associated with both a lattice distortion [2] and the opening of an energy gap to the lowest triplet spin excitations [3]. While the nature of the low-T phase in NaV 2 O 5 is not fully understood it is clear that, unlike CuGeO 3 , the spin-Peierls model does not apply simply to this compound [4]. The spin gap may result from charge-order (CO) rather than the lattice distortion [5]. Indeed, NMR measurements indicate two inequivalent vanadium sites below T c , while there exists only one site above [6]. There has been no direct evidence for the connection between CO and a spin gap, nor to distinguish various conjectured spatial distributions of charge [5,7]. In this letter, we present new results of neutron inelastic scattering (NIS) on the spin excitations in the low-T phase that can now resolve these issues.In NaV 2 O 5 , the vanadium ions have a formal valence of 4.5+. Initially, this was proposed to correspond to an alternation of V 4+ ions, with a spin value S = 1/2, and V 5+ ions with S = 0 [8]. At room temperature, NaV 2 O 5 is well described by a quarter-filled two-leg ladder system, with only one type of vanadium site V 4.5+ . From calculations of electronic structure [9,10], the strongest orbital overlaps are on the ladder rungs. One expects that the S = 1/2 spins are carried by the V-O-V molecular bonding orbitals, with charge fully delocalized on two sites. As the energy of the anti-bonding orbital is much higher, it can be projected out, and above T c , these spins, as they interact in the leg direction ( b axis), form an effective uniform quantum Heisenberg spin chain with interactions between chains that are both weaker and frustrated.At low temperatures, NMR shows this can no longer be so. On each rung, a charge transfer ∆ c may occur. Taking the average charge on vanadium sites to be 1/2, the charges on the two vanadium sites on a rung are defined through n ± = (1 ± ∆ c )/2. Two forms of CO can be considered [5], the in-line, with the same charge transfer on each rung, and the zig-zag with alternation in the charge along the ladders as shown in fig. 1b. Recent X-ray diffraction measurements [11] established that the lattice structure below T c consists of a succession of distorted and non-distorted ladders of vanadium ions (see fig. 1a). Neglecting inter-ladder diagonal couplings J ⊥ , the ladders would behave magnetically as independent spin chains. For one ladder (chain 2 in fig. 1a) distortions in the ...

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“…Moreover, not all regimes of parameters possibly relevant to α ′ -NaV 2 O 5 have yet been considered in the literature. In particular, the case J ′ < 0 remains to be investigated as some LDA+U calculations seem to indicate a negative J ′ [17]. Apart from its possible relevance to α ′ -NaV 2 O 5 , the quadrumer model is closely related to several other interesting spin-chain Hamiltonians [18][19][20][21].…”

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confidence: 99%

“…In contrast, the physical regime for J ′ is less clear. Previous investigations of the model (1) have assumed J ′ > 0 [15,16] while, e.g., in [17] a ferromagnetic J ′ of approximately −18meV has been used, focusing however on a different, i.e. zig-zag, charge order.…”

Section: Preliminariesmentioning

confidence: 99%

Magnetic properties of a spin-12quadrumer chain

Honecker

Brenig

2001

Phys. Rev. B

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We study a novel S = 1/2 cluster chain Hamiltonian which has recently been proposed in the context of the charge ordered low-temperature phase of α ′ -NaV2O5. We perform a detailed investigation of this model within a large range of parameters using perturbation theory and Lanczos diagonalization. Using model-specific local conservation laws and parameter-dependent mappings to various effective low-energy Hamiltonians we uncover a rich phase diagram and several regimes of gapful spin-excitations. We find that the overall features of recent neutron scattering data on α ′ -NaV2O5 can be fitted within this model, however using a set of parameters which seems unlikely.

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Magnetic excitations in charge ordered

Cited by 9 publications

References 0 publications

Orthorhombic versus monoclinic symmetry of the charge-ordered state ofNaV2O5

Orthorhombic versus monoclinic symmetry of the charge-ordered state ofNaV2O5

Charge ordering and spin dynamics in NaV2O5

Magnetic properties of a spin-12quadrumer chain

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