We found anomalies in the temperature dependence of dielectric and magnetic susceptibility of NaV2O5 in the microwave and far infrared frequency ranges. The anomalies occur at the phase transition temperature Tc, at which the spin gap opens. The real parts of the dielectric constants ǫa and ǫc decrease below Tc. The decrease of ǫa (except for the narrow region close to Tc) is proportional to the intensity of the x-ray reflection appearing at Tc. The dielectric constant anomaly can be explained by the zigzag charge ordering in the ab-plane appearing below Tc. The anomaly of the microwave magnetic losses is probably related to the coupling between the spin and charge degrees of freedom in vanadium ladders.PACS numbers: 64.70.Kb 75.10.Jm 77.22Ch 75.40.Cx
Polarized far-infrared transmittance spectra of CuGeO3 single crystals were measured at different temperatures (6 K< T < 300 K). Two spectral lines, at 284.2 cm −1 in E||c polarization and at 311.7 cm −1 in E||b polarization, appear at the temperature of the spin-Peierls transition and grow in intensity with decreasing temperature. We assign these spectral features to the folded modes of the dimerized lattice. We discuss a possible role of the spin-phonon interaction in the formation of the 311.7 cm −1 feature.
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