Vortex phase diagram of underdoped YBa₂Cu₃O _y single crystals in the magnetic field parallel to the ab-plane

To study the phase diagram and dynamics of interlayer vortices in underdoped YBa₂Cu₃O_y single crystals (T _c - 47 K), we measured the c-axis resistivity ρ_c in magnetic fields H parallel to the ab-plane (H || ab). We find that the field dependence of the resistivity ρ_c(H) shows the oscillations under high current densities above J 4 A/cm². In low fields below 3 T, the peaks in ρ_c(H) agree with the commensurate condition between the intervortex spacing perpendicular to the CuO₂ layers and the layer period. In high fields above 3 T, however, peaks in ρ_c(H) deviate from the commensurate condition. The angular dependence of ρ _c(θ) suggests that the deviation originates from the strong pinning effect for interlayer vortices in high field region.