Non-exponential London penetration depth in RFeAsO${}_{0.9}$F${}_{0.1}$ (R=La,Nd) single crystals

Abstract

The superconducting penetration depth, $\lambda (T)$, has been measured in RFeAsO${}_{0.9}$F${}_{0.1}$ (R=La,Nd) single crystals (R-1111). In Nd-1111, we find an upturn in $\lambda (T)$ 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, $\Delta {\lambda }_L(T)\propto T^2$ at low temperatures. The same $T^2$ variation of $\lambda (T)$ was found in non-magnetic 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 BaFe${}_2$As${}_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

v2 is the same as v1 - error in the arxiv submission