Effect of magnetic criticality and Fermi-surface topology on the magnetic penetration depth.

We investigate the effect of antiferromagnetic (AF) quantum criticality on the magnetic penetration depth λ(T) in line-nodal superconductors, including the cuprates, the iron pnictides, and the heavy-fermion superconductors. The critical magnetic fluctuation renormalizes the current vertex and drastically enhances the zero-temperature penetration depth λ(0), which is more remarkable in the iron-pnictide case due to the Fermi-surface topology. Additional temperature (T) dependence of the current renormalization makes the expected T-linear behavior at low temperatures approach T(1.5) asymptotically. These anomalous behaviors are consistent with experimental observations. We stress that λ(T) is a good probe to detect the AF quantum critical point in the superconducting state.