Charge ordering and opening of spin gap in NaV2O5

Abstract

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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Lattice vibrations ofα′−NaV2O5Popova1, Sushkov2, Климин3 et al. 2002Phys. Rev. B215120View full textAdd to dashboardCite

“…As each electron carries the spin (S=1/2), charge and spin degrees of freedom are closely connected. To describe this situation, Mostovoy and Khomskii proposed a spin-isospin model and used it to show that the antiferroelectric charge ordering in the system of vanadium ladders opens a spin gap [42,35]. They argued that in α ′ -NaV 2 O 5 the quasi-one-dimensional spin system is strongly coupled to a low-energy antiferroelectric mode (isospin mode) of the excitonic type and the observed charge ordering corresponds to the softening of this mode.…”

Nature of insulating state inNaV2O5above charge-ordering transition: A cluster dynamical mean-field study

“…[1][2][3][4][5][6][7][8][9][10][11][12] It exhibits a remarkable phase transition at T c ϭ34 K, now identified as chargeordering of the zigzag type. 2,4 -8 The first interpretation of this phenomenon was related to a model of different V 4ϩ and V 5ϩ chains and associated with the spin-Peierls transition in V 4ϩ chains.…”