Angular dependence of the first-order vortex-lattice phase transition in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

The paramagnetic peak in the local ac susceptibility ${\mathrm{\chi }}^{|\mathrm{I}\mathrm{H}}$ is used to identify the first-order vortex-lattice phase transition in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ at various tilt angles θ of the dc magnetic field with respect to the c axis. The transition field ${\mathrm{H}}_{\mathrm{m}}$ follows roughly the two-dimensional scaling function ${\mathrm{H}}_{\mathrm{dc}}$cos (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°). From this we conclude that the entropy jump at the transition, Δs, is insensitive to the presence of the in-plane field.