Fractional matching of vortices in Bi2Sr2CaCu2O8+y with triangular array of antidots

To explore the fractional matching effect in vortex states of high-T-c superconductors with lattice of artificially-introduced pinning centers, we have measured the in-plane vortex-flow resistance and the critical current as a function of magnetic field in Bi2Sr2CaCu2O8+y single-crystal films having a triangular array of antidots. At the temperature very close to T-c (T < T-c), fine dip structures in vortex-flow resistance and peaks in the critical current were observed in fractions 1/4, 1/3, 1/2, 2/3, 3/4, and 6/7 of the first matching field H-1. While most of fractional numbers are understood from a simple geometrical consideration on the arrangements of triangular lattices of vortices on the underlying array of antidots, the matching effect in 1/2 of H1 implies a presence of interstitial vortices in comparison with simulations [Reichhardt C and Gronbech-Jensen N, 2001 Phys. Rev. B 63 054510].