Excitation of the josephson plasma mode in Bi₂Sr₂CaCu₂O₈+δ in an oblique field

We present evidence identifying the resonance in microwave absorption observed in [Formula presented] in a field H as the Josephson plasma mode. The resonance field displays an unusual reentrant cusp when H is very close to alignment with the layers. By fitting to a model of Bulaevskii et al., we obtain an anisotropy parameter [Formula presented] at 5 K, and a Josephson plasma frequency [Formula presented] equal to 5.3 c[Formula presented]. We also describe the dependence of [Formula presented] on H and T when the applied field [Formula presented].