single crystals*

Vortex structures in the highly anisotropic superconductor Bi₂Sr₂CaCu₂O_8+δ depend on the angle the applied field makes with the CuO₂ layers; stacks of pancake vortices form when the field is perpendicular to these and highly elliptical Josephson vortices form when it is parallel to them. For tilted magnetic fields, pancake and Josephson vortices coexist and interact in complex ways to form vortex chains and composite vortex lattices, reflecting the delicate balance between attractive and repulsive interactions. Scanning Hall microscopy has been used to map the rich tilted-field vortex phase diagram in an underdoped Bi₂Sr₂CaCu₂O_8+δ single crystal. We find that the Josephson vortex lattice spacing has an unexpected non-monotonic dependence on pancake vortex density linked to a field-driven structural transformation with increasing out-of-plane fields. We establish the exact evolution of vortex-chain phases as the out-of-plane field is increased and identify especially stable structures spaced by an integer number of rows of interstitial pancake vortex stacks. Our experimental results are in good semi-quantitative agreement with a theoretical model.