Magnetic Susceptibility of Quasi-One-Dimensional Compoundα'-NaV2O5–Possible Spin-Peierls Compound with High Critical Temperature of 34 K–

Isobe, Masahiko; Ueda, Yutaka

doi:10.1143/jpsj.65.1178

Cited by 496 publications

(483 citation statements)

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“…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].…”

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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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confidence: 99%

“…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].…”

mentioning

confidence: 99%

Charge ordering and opening of spin gap in NaV2O5

Mostovoy

Khomskii

1999

Solid State Communications

101

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“…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].…”

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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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“…[167][168][169][170][171] However, because the Coulomb potential diverges in one-dimension, and a soft-Coulomb potential shall be used instead, [172][173][174][175] the real physical meaning of this density scaling is rather ambiguous. For this reason, it has never been considered in meta-GGA development; despite it is a possible constraint at this level.…”

Section: Two-dimensional Density Scalingmentioning

confidence: 99%

Kinetic‐energy‐density dependent semilocal exchange‐correlation functionals

Sala

Fabiano

Constantin

2016

Int J of Quantum Chemistry

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We present the theory of semilocal exchange-correlation (XC) energy functionals which depend on the Kohn-Sham kinetic energy density (KED), including the relevant class of meta-generalized gradient approximation (meta-GGA) functionals. Thanks to the KED ingredient, meta-GGA functionals can satisfy different exact constraints for XC energy and can be made one-electron selfcorrelation free. This leads to a better accuracy over a wider range of properties with respect to GGAs, often reaching the accuracy of hybrid functionals, but at much reduced computational cost.An extensive survey of the relevant literature on existing KED dependent XC functionals is provided, considering nonempirical, semi-empirical, and fully empirical ones. A deeper analysis and a wide benchmark are presented for functionals derived considering only exact constraints and parameters obtained from model and/or atomic systems. K E Y W O R D Sdensity functional theory, kinetic energy, meta-GGA | I N T R O D U C T I O NKohn-Sham (KS)-density functional theory (DFT) [1][2][3][4][5] is nowadays one of the most popular computational approaches in quantum chemistry, solid-state physics, and materials science. [6][7][8][9][10][11] While the ground-state description of a real system of interacting electrons requires a complicate N-electron wavefunction, in KS-DFT this is mapped onto an auxiliary system of noninteracting electrons having the same ground-state density, which can be easily described using N single-particle KS orbitals. [1,2] The correspondence between the many-electron problem and the KS system is in principle exact [1,3] and it is based on the so called exchange-correlation (XC) energy functional, which collects all the quantum electron-electron interaction terms beyond the Hartree one. [12] Within the constrained-search formalism, [13] the XC energy functional is:E xc ½q 5 min W!q hWjT 1V ee jWi2T s ½q 2J½q 5E x ½q 1U c ½q 1T c ½q ;(1) where W is an N-electron ground-state wavefunction yielding the electron density q,T is the kinetic energy (KE) operator,V ee is the electron-electron interaction operator, T s is the KE of the noninteracting system, J is the Coulomb energy, E x is the exact exchange energy, and U c and T c represent, respectively, the potential and the kinetic contributions to the DFT correlation energy E c 5U c 1T c . The important point of Equation1 is that it is an universal functional of the electron density. However, its explicit functional form is not known, and any practical realization of KS-DFT is based on an approximated XC functional. The latter determines in practice the overall accuracy of the KS-DFT calculation.The study of the exact properties of the XC energy functional as well as the development of new and improved approximations have been hot research topics in DFT for more than three decades, and a large number of XC approximations has been proposed. [12,14,15] The different XC functionals are usually classified, according to their complexity, on the so called Jacob's ladder of DFT [14,16] whic...

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Magnetic Susceptibility of Quasi-One-Dimensional Compoundα'-NaV₂O₅–Possible Spin-Peierls Compound with High Critical Temperature of 34 K–

Cited by 496 publications

References 11 publications

Charge ordering and opening of spin gap in NaV2O5

Charge ordering and opening of spin gap in NaV2O5

Charge ordering and spin dynamics in NaV2O5

Kinetic‐energy‐density dependent semilocal exchange‐correlation functionals

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