Figure: Charge order in NaV2O5. Black circles - localization of the electrons below the transition temperature. Arrows - electric dipole momentum on the rungs arising due to the localization of electrons. The zig-zag order presented on this figure leads to the formation of antisegnetoelectric order.
The magnetic structure of a quasi 1D magnet NaV2O5 is based on a ladder structure with one electron per rung. The opening of the spin gap at 35.5 K was interpreted at first as a spin-Peierls transition, later this kind of transition was argued and the charge ordering transition resulting in the alternating of the exchange integral along the spin ladder was suggested. This alternating of the exchange integral should provide the opening of the spin gap and the nonmagnetic singlet ground state.
In our microwave measurements of the dielectric properties we found a dielectric anomaly revealing the antisegnetoelectric type of the charge ordering. This observation is in the agreement with the zigzag type of the charge and spin ordering on the spin-ladder structure (see Figure)