Four-fold symmetry of 90 K YBCO single crystals in magnetic fields

We have measured the angular dependence of the resistivity ρ(θ) in 90 K-YBCO single crystals, where θ is the angle between the direction of a magnetic field H and the direction of [0 1 0] which is perpendicular to the current I in the single crystal. The obtained ρ(θ) shows the four-fold symmetry. On the other hand, Kwok et al. had observed only the two-fold symmetry in 90 K YBCO single crystals. The origin of the difference between the symmetry of each case is speculated that it comes from the difference of anisotropy that depends on the concentration of oxygen in the CuO-chains