C. Presura

Direct Two-Magnon Optical Absorption in α'-NaV2O5

We investigated the temperature-dependent optical conductivity of NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground-state and for a non-centrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed in this Letter, is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.

Charge-ordering signatures in the optical properties of beta-Na0.33V2O5

Temperature dependent optical spectra are reported for beta-Na0.33V2O5. The sodium ordering transition at T(Na)=240 K and, in particular, the charge ordering transition at T(MI)=136 K strongly influence the optical spectra. The metal-insulator transition at T(MI) leads to the opening of a pseudogap ( variant Plancks over 2pi omega=1700 cm(-1)) and to the appearance of a large number of optical phonons. These observations and the presence of a midinfrared band (typical for low dimensional metals) strongly suggest that the charge carriers in beta-Na0.33V2O5 are small polarons.

Superconductivity by Kinetic Energy Saving

A brief introduction is given in the generic microscopic framework of superconductivity. The consequences for the temperature dependence of the kinetic energy, and the correlation energy are discussed for two cases: The BCS scenario and the non-Fermi liquid scenario. A quantitative comparison is made between the BCS-prediction for d-wave pairing in a band with nearest neighbor and next-nearest neighbor hoppping and the experimental specific heat and the optical intraband spectral weight along the plane. We show that the BCS-prediction produces the wrong sign for the kink at T c of the intraband spectral weight, even though the model calculation agrees well with the specific heat.

Charge and Sodium Ordering in β-Na0.33V2O3

Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate β-Na0.33V2O3 are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.