We consider the Josephson plasma mode in the mixed state of a single Josephson junction and of a layered superconductor. We focus on the frequencies and optical weights of plasma modes which can be excited by an external spatially homogeneous ac electric field in the mixed state. Our consideration is based on the sum rules for the dielectric function which describe the response of a Josephson coupled system to an ac electric field. We show that for a single Josephson junction in a parallel field the plasma frequency increases with a magnetic field applied parallel to the junction. For layered superconductors we discuss qualitatively the angular dependence of the plasma frequency for a tilted magnetic field using an interpolation between results known for perpendicular and parallel fields. We predict that two plasma peaks may be observed in magnetic fields of the order ${\mathrm{H}}_0$=${\mathrm{\Phi }}_0$/γ${\mathrm{s}}^2$, orientated close to the ab plane, and that the plasma frequency increases with increasing magnetic field B⩽${\mathrm{H}}_0$ applied parallel to the layers.

• Received 23 September 1996

DOI:https://doi.org/10.1103/PhysRevB.55.8482