Two-dimensional charge fluctuation in β-Na0.33V2O5

We have investigated the critical phenomena of the metal-to-insulator (MI) transition of beta-Na0.33V2O5. The critical exponent beta of 0.226(3) and anisotropic diffuse scattering suggest that the MI transition of beta-Na0.33V2O5 is two dimensional in nature and that the dimension of the order parameter is not Ising. The non-Ising-like order parameter indicates that the MI transition is not a simple order-disorder type such as that of alpha'-NaV2O5 but charge-density-wave like, where the valence at each V site is continuous and the arrangement is modulated. This closely corresponds to the situation predicted by neutron diffraction measurements [Nagai et al., J. Phys. Soc. Jpn. 74, 1297 (2005)] and NMR measurements [Itoh et al., Phys. Rev. B 74, 054434 (2006)].