Charge excitations inLiV2O5and

Abstract

We calculate the optical conductivity of LiV 2 O 5 and NaV 2 O 5 using exact numerical diagonalization of a quarter-filled extended Hubbard model on a system of coupled ladders. In particular, electronic correlations are treated exactly, and a quantitative agreement between calculated and experimentally observed optical conductivity of these two vanadium oxides is presented. Furthermore, it is found that LiV 2 O 5 differs from NaV 2 O 5 not only in the charge ordering pattern but also in the nature of the interladder coupling: In contrast to LiV 2 O 5 , in NaV 2 O 5 neighboring ladders are coupled by a strong Coulomb repulsion, and not by interladder hopping.In the past years, low-dimensional transition-metal compounds have been intensively investigated because of their unconventional spin and charge excitation spectra. 1 In this respect, the vanadate ␣Ј-NaV 2 O 5 has attracted particular interest as a ladder system at quarter filling, containing only one equivalent V site with a formal valence ϩ4.5. 2,3 The magnetic susceptibility of NaV 2 O 5 can be well described by a Sϭ1/2 antiferromagnetic Heisenberg chain with exchange interactions of Jϭ440 and 560 K for temperatures below 4 and above 5 the transition temperature T C Ϸ34 K. The lowtemperature phase is found to be charge ordered, 6 but the nature of this transition is still under discussion. 7-9 The much less studied ␥-LiV 2 O 5 belongs to the same family of vanadium oxides and exhibits a one dimensional Sϭ1/2 Heisenberg like behavior with an exchange interaction of J ϭ308 K. 10,11 In contrast to NaV 2 O 5 , there is no indication of a phase transition at lower temperature. Since both compounds are structurally related, it is interesting to clarify the microscopic origin of the different physical properties of both vanadates. This will be discussed in this paper on the basis of the optical conductivity.Recently, the optical conductivity of LiV 2 O 5 and NaV 2 O 5 has been measured. 12 In the energy range from 0 to 3 eV similar peaks in the Eʈa spectra of both materials were found. On the other hand, a complete suppression of the peaks in the Eʈb spectrum of LiV 2 O 5 was observed. In this paper, we show that this suppression results not only from the double-chain charge ordering pattern but also from the strong interladder hopping in LiV 2 O 5 . Consequently, it is found that both vanadates differ not only in the charge ordering pattern but also in the nature of the interladder coupling. Recently, we studied 13 the optical conductivity of NaV 2 O 5 so that in the present paper we concentrate on LiV 2 O 5 and on the comparison of both vanadium oxides.At room temperature LiV 2 O 5 and NaV 2 O 5 have orthorhombic crystal structures that are described by space groups Pnma and Pmmn, respectively. 14,15 Both compounds consist of layers of VO 5 square pyramids ͑see Fig. 1͒. In contrast to NaV 2 O 5 , a structural analysis 14 for LiV 2 O 5 shows two inequivalent vanadium sites that were also found by NMR experiments. 16 These two vanadium sites were assigned a valen...

Electronic and magnetic structure ofCsV2O5

“…Recent investigations based on an ab initio analysis of the electronic structure 23 show that charge fluctuations in this system are still significant and the experimental observations 22,24 can be well described by a spin-1 2 asymmetric quarter-filled ladder model. Such proposal has also found support by a numerical analysis of optical conductivity experiments 25 . The third member of this vanadate family, CsV 2 O 5 , has been given much less attention.…”

Raman scattering study of charge ordering in -Ca0.33V2O5

“…3 in Ref. [19]) in the absorption spectra of β-Ca 0.33 V 2 O 5 along the b-axis, should confirm the "zigzag" (or double chain) charge ordering pattern. At the moment such measurements are not possible because of lack of single crystal samples.…”

Charge ordering and phase separation in the infinite dimensional extended Hubbard model

“…6 ), the quasi-one dimensional material N aV 2 O 5 (Ref. 7,8 ), and the superconducting layered organic molecular crystal κ−(BEDT −T T F ) 2 X (Ref. 9 ), charge-ordering is closely related to the specific properties of the system.…”