Nonexponential London Penetration Depth of FeAs-Based Superconducting RFeAsO_0.9F_0.1 (R=La, Nd) Single Crystals

The superconducting penetration depth λ(T) has been measured in RFeAsO_0.9F_0.1 (R=La, Nd) single crystals (R-1111). In Nd-1111, we find an upturn in λ(T) upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, Δλ_L(T)∝T² at low temperatures. The same T² variation of λ(T) was found in nonmagnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe₂As₂ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.