Dimensional crossover and anomalous magnetoresistivity of superconducting NaxCoO2 single crystals

In-plane (rho(ab)) and c-axis (rho(c)) resistivities and magnetoresistivity of NaxCoO2 with x=0.7, 0.5, and 0.3 and superconducting Na(0.3)CoO(2)center dot 1.3H(2)O single crystals were studied. A dimensional crossover from 2 to 3 occurs with decreasing Na concentration from 0.7 to 0.3. A striking feature is that intercalation of water leads to a reentrance of two dimensions (2D) in superconducting crystal. It suggests that the 2D plays an important role in superconductivity as the case of cuprates. The anisotropic magnetoresistivity with a d-wave-like symmetry at 2 K gives evidence for the existence of a special spin ordering like the stripes below 20 K in Na0.5CoO2.