Composite to Tilted Vortex Lattice Transition in Bi₂Sr₂CaCu₂O_8+δ in Oblique Fields

Precision measurements of the vortex phase diagram in single crystals of the layered superconductor Bi₂Sr₂CaCu₂O_8+δ in oblique magnetic fields confirm the existence of a second phase transition, in addition to the usual first-order vortex-lattice melting line H_m(T). The transition has a strong first-order character, is accompanied by strong hysteresis, and intersects the melting line in a tricritical point (H_m^⊥, H_cr^∥). Its field dependence and the changing character of the melting line at the tricritical point strongly suggest that the ground state for magnetic fields closely aligned with the superconducting layers is a lattice of uniformly tilted vortex lines.