High-quality single crystals of NdFeAsO$_{ 0.65}$F$_{ 0.35}$ (the transition temperature $T_{ \rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($\mu$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$\mu$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($\lambda$) were performed in the TF geometry. By applying the external magnetic field $B_{ \rm ex}$ parallel to the crystallographic $c$-axis ($B_{ \rm ex}\| c$) and parallel to the $ab$-plane ($B_{ \rm ex}\| ab$), the temperature dependencies of the in-plane component ($\lambda_{ ab}^{ -2}$) and the combination of the in-plane and the out of plane components ($\lambda_{ ab,c}^{ -2}$) of the superfluid density were determined, respectively. The out-of-plane superfluid density component ($\lambda_{ c}^{ -2}$) was further obtained by combining the results of $B_{ \rm ex} \| c$ and $B_{ \rm ex} \| { ab}$ set of experiments. The temperature dependencies of $\lambda_{ ab}^{ -2}$, $\lambda_{ ab,c}^{ -2}$, and $\lambda_{ c}^{ -2}$ were analyzed within the framework of a self-consistent two-gap model despite of using the traditional $\alpha$-model. Interband coupling was taken into account, instead of assuming it to be zero as it stated in the $\alpha$-model. A relatively small value of the interband coupling constant $\Lambda_{ 12} \simeq 0.01$ was obtained, thus indicating that the energy bands in NdFeAsO$_{ 0.65}$F$_{ 0.35}$ are only weakly coupled. In spite of their small magnitude, the coupling between the bands leads to the single value of the superconducting transition temperature $T_{ \rm c}$.
Categories: cond-mat.supr-con
