Angular dependence of the first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8

The paramagnetic peak in the local ac susceptibility χ' is used to identify the first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8 at various tilt angles θ of the dc magnetic field with respect to the c axis. The transition field Hm follows roughly the two-dimensional scaling function Hdccos(θ). The scaling fails for field orientations close to the ab plane. The amplitude of the paramagnetic peak, which is proportional to the jump in magnetization ΔB, does not depend on the tilt angle up to configurations very close to the ab plane (±1 degree). From this we conclude that the entropy jump at the transition, Δs, is insensitive to the presence of the in-plane field. copyright 1997 The American Physical Society