Unusual low-frequency electromagnetic response in anisotropic superconductors: Application toUPt3

We calculate the electromagnetic response of an anisotropic superconductor, and show that the temperature dependence of the magnetic penetration depth is strongly suppressed by an ac field even for frequencies very small compared to the gap magnitude. The effect arises as a result of virtual excitations of Bogoliubov quasiparticle-quasihole pairs in the neighborhood of the gap nodes. We propose that this phenomenon is responsible for apparent discrepancies among various recent penetration-depth experiments on superconducting ${\mathrm{UPt}}_3$. The available data are found to be consistent with a state of ${\mathit{E}}_{1\mathit{g}}$ symmetry.