High frequency dielectric and magnetic anomaly at the phase transition   in NaV2O5
  We found anomalies in the temperature dependence of the dielectric and the magnetic susceptibiliy of NaV_2O_5 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. The decrease of epsilon_a (except for the narrow region close to T_c) is proportional to the intensity of the x-ray reflection appearing at T_c. The dielectric constant anomaly can be explained by the zigzag charge ordering in the ab-plane appearing 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. 
A.I.Smirnov, M.N.Popova, A.B.Sushkov, S.A.Golubchik, D.I.Khomskii,   M.V.Mostovoy, A.N.Vasil'ev, M.Isobe, Y.Ueda