Observations of Vortex Chains in Bi2Sr2CaCu2O8+y

Vortex states in Bi₂Sr₂CaCu₂O_8+y under tilted fields were studied by means of magneto-optical imaging and Bitter decoration. One-dimensional arrangements of pancake vortices (vortex chains) interacting with underlying Josephson vortices are visualized. From the spacing of vortex chains we directly determined the variation of the anisotropy parameter of the electronic mass () from 800 in slightly underdoped to 180 in highly overdoped samples. These results indicate that the crossing-lattices model for highly anisotropic superconductors remains relevant at least for 180, contrary to what was predicted theoretically by Dodgson [Matthew J. W. Dodgson, Physica C369, 182 (2002).].