Mixed valency and charge ordering in α′-NaV2O5

Ohama, Tetsuo; Yaśuoka, Hiroshi; Isobe, Masahiko; Ueda, Yutaka

doi:10.1103/physrevb.59.3299

Cited by 185 publications

(190 citation statements)

References 18 publications

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“…The opening of a spin gap, acompanying this transition, is just one of the consequences of the charge ordering. The appearance below T c of two ineqivalent V sites, which were crudely identified as V 4+ and V 5+ , was indeed observed in the recent NMR experiment [14]. …”

supporting

confidence: 56%

Charge ordering and opening of spin gap in NaV2O5

Mostovoy

Khomskii

1999

Solid State Communications

101

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We suggest that the phase transition observed in NaV2O5 at T = 34K is not a spin-Peierls transition, but a charge ordering transition, related to the formal presence in this system of equal number of V 4+ and V 5+ ions. Below Tc, V 4+ ions form a zigzag structure, which is consistent with the experimentally observed doubling of the lattice period in a and b directions. We show that this charge ordering also results in the alternation of spin exchange constants along the b-direction, which opens a gap in the spin excitation spectrum. We emphasize the role of lattice distortions around V ions both in the formation of the charged ordered state and in the spin-gap opening.PACS numbers: 64.70Kb, 75.10Jm In this paper we discuss the interplay between charge and spin degrees of freedom in NaV 2 O 5 , which initially was identified as an inorganic spin-Peierls (SP) material similar to CuGeO 3 . Indeed, according to the first Xray studies of this compound [1] V 4+ ions form spin-1 2 chains, which are separated from each other by nonmagnetic V 5+ -chains. Below T c = 34K, magnetic susceptibility, X-ray and neutron scaterring measurements indicate the opening of a spin gap accompanied by doubling of the lattice period in the chain direction [2][3][4].However, the situation in NaV 2 O 5 is evidently more complicated and more interesting than in conventional SP systems. First of all, the original crystal structure was recently questioned [5][6][7]. According to the new Xray and neutron data, the structure of NaV 2 O 5 was identified as a centrocymmetric group P mmn, in contrast to the earlier assignement P 2 1 mn [1]. The new structure implies that all V sites are equivalent. As an average valence of V in NaV 2 O 5 is 4.5+, there is one d-electron per two equivalent V ions, which makes the description of this compound as a spin-1 2 chain material not obvious. Other indications that the physics of NaV 2 O 5 may be different from that of, e.g., CuGeO 3 come from thermodynamic data. In particular, the study of specific heat [8] shows that the entropy of the transition in NaV 2 O 5 is larger than the entropy of a pure SP system. The ratio 2∆0Tc , where ∆ 0 is the value of the spin gap at T = 0, is for NaV 2 O 5 ∼ 6 -much larger than for the known SP materials, where it is close to 3.5, which follows from the weakcoupling mean field theory of SP transition [10]. Another evidence that the phase transition in NaV 2 O 5 may be of different nature comes from a rather weak dependence of T c on magnetic field: the shift of T c is ∼ 5 times smaller [8,9] than the theoretical predictions [10,11]. Yet another spectacular difference in the behavior of NaV 2 O 5 and CuGeO 3 is the temperature-dependence of the thermal conductivity [12]: below T c the thermal conductivity of NaV 2 O 5 increases by a factor of 5, while in CuGeO 3 the corresponding anomaly is very weak. All these facts show that the phase transition in NaV 2 O 5 is not an ordinary SP transition. We suggest below that the main phenomenon responsible for the transition is, in...

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supporting

confidence: 56%

Charge ordering and opening of spin gap in NaV2O5

Mostovoy

Khomskii

1999

Solid State Communications

101

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“…Another example is the transition-metal oxide NaV 2 O 5 which is reported to be a quarter-filled ladder system, exhibiting a spin-Peierls-like phase transition accompanied by a charge ordering [4]. This system may also be regarded as a dimerized system at quarter filling if we may assume that two V ions on each rung of the ladder form a dimer molecule [5].…”

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confidence: 99%

Ground-state phase diagram of the one-dimensional dimerizedt−Jmodel at quarter filling

Nishimoto

Ohta

1999

Phys. Rev. B

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The ground state of the one-dimensional dimerized t−J model at quarter filling is studied by a Lanczos exact-diagonalization technique on small clusters. We calculate the charge gap, spin gap, binding energy, Tomonaga-Luttinger-liquid parameter, Drude weight, anomalous flux quantization, etc. We thereby show that the two types of dimerization, i.e., a dimerization of hopping integral and a dimerization of exchange interaction play a mutually competing role in controlling the ground state of the system and this leads to the emergence of various phases including the Mott insulating, Tomonaga-Luttinger-liquid, and Luther-Emery-liquid phases. The ground-state phase diagram of the model is given on the parameter space of the dimerizations. 71.27.+a, 71.30.+h, 74.70.Kn, 75.10.Jm

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“…Up to now, accumulated experimental data [4,[6][7][8][9] suggested that NaV 2 O 5 exhibits the CO phase transition at T=34 K into a gapped spin-liquid ground state. The arguments are mostly based on the insensitivity of the phase-transition effects associated with magnetic fields.…”

mentioning

confidence: 99%

Charge ordering and optical transitions ofLiV2O5and
Konstantinović
¹
,
Dong
²
,
Ziaei
³

et al. 2001
Phys. Rev. B
12
1
13
0
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We present measurements of the polarized optical spectra of NaV 2 O 5 and LiV 2 O 5 . In an energy range from 0.5 to 5.5 eV, we observe similar peaks in the E a spectra of NaV 2 O 5 and LiV 2 O 5 , which suggests similar electronic structures along the a axis in both materials. On the other hand, we find an almost complete suppression of the peaks in σ b of LiV 2 O 5 around 1 and 5 eV. We attribute this suppression to the charge localization effect originating from the existence of a double-chain charge-ordering pattern in LiV 2 O 5 . : 78.40.-q, 71.35.-y, 75.50.-y In the past several years, quantum phenomena resulting from the low dimensionality of effective electron interactions in solids have been investigated with increasing intensity from both experimental and theoretical points of view. The increase in interest was partially motivated by the discovery of inorganic materials which exhibit quantum effects, such as the PACS, and by a common belief that these studies would give us a better understanding of electron correlations in general.The vanadate family of AV 2 O 5 oxides have demonstrated a variety of the low-dimensional phenomena which originate from their peculiar crystal structures [3]. These oxides are quasi two-dimensional (2D) materials with layers formed by V O 5 square pyramids. The A atoms are situated between layers as intercalants, but in fact they determine the valence state of vanadium atoms (acting as charge reservoirs). If the A atoms belong to the first column in the periodic table, such as A = Li, Na, each valence electron is shared between two vanadium atoms. As a result the V ions are in a mixed valence-state with an average valence of +4.5. The common consequence of mixed valence in these structures is the appearance of a quasi-1D magnetic interaction, since chains carrying the spin (made of V 4+ , S=1/2) are separated from each other by nonmagnetic chains (V 5+ ). In both LiV 2 O 5 and NaV 2 O 5 the 1D character of the magnetic ordering was confirmed [4,5]. In addition, there is a possibility of the existence of strong valence fluctuations, and eventually charge ordering (CO) effects.

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Mixed valency and charge ordering in α′-NaV2O5

Cited by 185 publications

References 18 publications

Charge ordering and opening of spin gap in NaV2O5

Charge ordering and opening of spin gap in NaV2O5

Ground-state phase diagram of the one-dimensional dimerizedt−Jmodel at quarter filling

Charge ordering and optical transitions ofLiV2O5and
Konstantinović
¹
,
Dong
²
,
Ziaei
³

et al. 2001
Phys. Rev. B
12
1
13
0
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Mixed valency and charge ordering in α′-NaV2O5

Cited by 185 publications

References 18 publications

Charge ordering and opening of spin gap in NaV2O5

Charge ordering and opening of spin gap in NaV2O5

Ground-state phase diagram of the one-dimensional dimerizedt−Jmodel at quarter filling

Charge ordering and optical transitions ofLiV2O5andKonstantinović1, Dong2, Ziaei3 et al. 2001Phys. Rev. B121130View full textAdd to dashboardCite

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Charge ordering and optical transitions ofLiV2O5and
Konstantinović
¹
,
Dong
²
,
Ziaei
³

et al. 2001
Phys. Rev. B
12
1
13
0
View full text Add to dashboard Cite