High-frequency dielectric and magnetic anomaly at the phase transition in NaV2O5

We found anomalies in the temperature dependence of the dielectric and magnetic susceptibility of NaV2O5 in the microwave and far-infrared frequency ranges. The anomalies occur at the phase transition temperature T-c, at which the spin gap opens. The real parts of the dielectric constants epsilon(a) and epsilon(c) decrease below T-c, while epsilon(b) remains constant. The change in epsilon(a) is proportional to the square of the ordering parameter below T-c. The dielectric anomalies found indicate a redistribution of charges at the phase transition. The character of the dielectric constant anomalies is of the antiferroelectric type, which is in agreement with the models assuming the zigzag charge ordering in thr ab plane below T-c. The anomaly of the microwave magnetic losses is probably related to the coupling between the spin and charge degrees of freedom in vanadium ladders.