Spontaneous Decay of a Quadrupole between Two Parallel Plates, One or Both Infinitely Permeable

We present the explicit expression of the spontaneous decay rate of a quadrupole placed between two parallel plates (one or both the plates have infinite permeability, i.e. μ→∞). It is found that the spontaneous decay rate displays oscillations with the position of the quadrupole, and the oscillations exhibit a symmetric profile with respect to the central plane of the plates for the symmetric models and lack the symmetry for the asymmetric model.