Inversion symmetry in the spin-Peierls compound NaV2O5

At room-temperature NaV2O5 was found to have the centrosymmetric space group Pmmn. This space group implies the presence of only one kind of V site in contrast with previous reports of the non-centrosymmetric counterpart P21mn. This indicates a non-integer valence state of vanadium. Furthermore, this symmetry has consequences for the interpretation of the transition at 34 K, which was ascribed to a spin-Peierls transition of one dimensional chains of V4+.Comment: Revtex, 3 pages, 2 postscript pictures embedded in the text. Corrected a mistake in one pictur

Charge Ordering and Spin Dynamics in NaV2O5

We report high-resolution neutron inelastic scattering experiments on the spin excitations of NaV2O5. Below Tc, two branches associated 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 (about 0.6). We argue that the spin gaps observed in the low-T phase of this compound are primarily due to the charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

Infrared study of spin-Peierls compound alpha'-NaV2O5

Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.Comment: 3 pages, 2 figures, 1 table. Contributed paper for the SCES'98 (15-18 July 1998, Paris). To be published in Physica

The symmetry problem in NaV2O5

We discuss the symmetry of NaV2O5 in the high temperature phase on the basis of optical conductivity data. Conclusive information cannot be obtained by studying the optically allowed lattice vibrations. However, intensity and polarization of the electronic excitations give a direct indication for a broken-parity electronic ground-state. This is responsible for the detection of charged bi-magnons in the optical spectrum.Comment: Revtex, 2 pages, 1 postscript picture embedded in the tex