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Four-fold symmetry of resistivity and flux pinning energy in Ba0.67K0.33BiO3+δ single crystal

Ba0.67K0.33BiO3+δ single crystal with Tc ∼ 26.4 K has been prepared by the molten salt electrochemical method. We observed a four-fold symmetry in the angular dependence of ab-plane resistance R (θ) at H = 0.5 T and T = 25.5 K for the first time. The data can be scaled by R = [R1sin2(θ + 45°) + R2cos2(θ + 45°) − R(90°)]|sin2θ| + R(90°). Although dip structure appears in R(θ) when the angles between the magnetic field direction and c-axis are 0°, 90°, 180° and 270°, respectively, the maximal position of resistance tilted away θ = 45° and θ = 135°, namely the angle difference between two peaks is 79° or 101°. This result indicates the superconducting Ba0.67K0.33BiO3+δ single crystal is probably an orthorhombic structure with distorted Bi−O octahedron. We consider that the angular dependence of resistivity comes from the superconducting energy gap with d x 2 − y 2 pairing symmetry. The flux pinning energy for θ = 90° is higher than that for θ = 135° proves the existence of the anisotropic vortex pinning effect. The field dependence of flux pinning energy displays a power law, U ∝ H−α. The irreversibility line was also discussed.