Fourfold symmetric anisotropy in the CuO₂ planes of 60-K YBa₂Cu₃O_{7 − δ} single crystals

To study the anisotropy in the CuO₂ planes, the in-plane resistivity ρ has been measured on 60 K YBa₂Cu₃O_{7 − δ} YBCO) single crystal as a function of the angle θ between the directions of a magnetic field H and of the b axis under magnetic fields precisely parallel to the CuO₂ planes. Angular dependence of the resistivity ρ(θ) is mainly described by the fourfold symmetry, and a dip structure appears in ρ(θ) when the magnetic field is applied parallel to the a and b axes. The fourfold symmetry of the in-plane anisotropy of the resistivity is first observed in YBCO system. It is shown that the obtained results have a very close relation to the anisotropy which comes from the superconducting energy gap of the d-wave pairing with d_{x² − y²} symmetry and the effect of the directional vortex pinning.