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

We investigate the effect of anti-ferromagnetic (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 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 approaching to $T^{1.5}$ asymptotically. These anomalous behaviors are well consistent with experimental observations. We stress that $λ(T)$ is a good probe to detect the AF quantum critical point in the superconducting state.