Excitation of the Josephson Plasma Mode inBi2Sr2CaCu2O8+δin an Oblique Field

We present evidence identifying the resonance in microwave absorption observed in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ in a field $\mathbf{H}$ as the Josephson plasma mode. The resonance field displays an unusual reentrant cusp when $\mathbf{H}$ is very close to alignment with the layers. By fitting to a model of Bulaevskii et al., we obtain an anisotropy parameter $\gamma =400\mathrm{}–420$ at 5 K, and a Josephson plasma frequency ${\omega }_J$ equal to 5.3 c${\mathrm{m}}^{–1}$ (at $H=0\mathrm{}$). We also describe the dependence of ${\omega }_J$ on $H$ and $T$ when the applied field $\mathbf{H}\parallel \mathbf{c}$.