Nonexponential London Penetration Depth of FeAs-Based Superconducting $R{\mathrm{FeAsO}}_{0.9}{\mathbf{F}}_{0.1}$ ($R=\mathrm{La}$, Nd) Single Crystals

The superconducting penetration depth $\lambda (T)$ has been measured in $R{\mathrm{FeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($R=\mathrm{La}$, Nd) single crystals ($R\mathrm{\text{−}}1111$). 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 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 ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

Figure 1

Transverse scans through the (200) and (220) Bragg reflections of the Nd-1111 single crystal. The lines are fits to Lorentzians yielding a full width half maximum of 0.019 and 0.028°, respectively. Left inset: photographs of Nd-1111 (top) and La-1111 (bottom) single crystals. Right inset: $\lambda (T)$ in the full temperature range for three samples.

Figure 2

(a) Frequency shift $\Delta f(T)$ for a Nd-1111 crystal (symbols), contribution from the sample holder (continuous line) and $\sqrt{\mu }$ (dashed line). Inset: $\Delta f(T)$ for the La-1111 crystal. (b) Low-temperature region of $\lambda (T)$ in the Nd-1111 crystal. The lines are fits to Eq. (1a) (dashed line) and Eq. (1b) (continuous line) Inset: Original data (symbols) and ${\lambda }_L(T)$ obtained after dividing by $\sqrt{\mu }$ from a fit to Eq. (1a) (dots) and to Eq. (1b) (continuous line).

Figure 3

$\Delta {\lambda }_L$ vs $(T/T_c{)}^2$ after dividing the paramagnetic contribution for two Nd-1111 samples and raw data for a La-1111 crystal (continuous lines). The curves have been shifted vertically for clarity.

Figure 4

The superfluid density, $\rho (T)$, for $R\mathrm{\text{−}}1111$ crystals calculated with the two values of 200 and 350 nm for ${\lambda }_L(0)=(\mathrm{\text{symbols}})$. Solid lines are fits to the two-gap model described in the text. Inset: temperature dependent anisotropy of the penetration depth.