Visualization of vortex chains inBi2Sr2CaCu2O8+yby magneto-optical imaging

Abstract

We visualized vortex chains in the crossing-lattices state in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+y}$ by using magneto-optical imaging technique. From the spacing of vortex chains, we could determine the anisotropy parameter quantitatively, from $\ensuremath{\gamma}=800\ifmmode\pm\else\textpm\fi{}80$ for a slightly underdoped sample to $\ensuremath{\gamma}=490\ifmmode\pm\else\textpm\fi{}50$ for slightly overdoped samples. Vortex chains can be rotated by changing the direction of in-plane field in the ${\mathrm{CuO}}_{2}$ plane. Hysteretic motion and the forking of vortex chains are discussed in relation to the indirect pinning of Josephson vortices through pinning of pancake vortices.