Scanning SQUID microscopy studies of the penetration depth and pairing symmetry in Fe-pnictide superconductors

We present scanning SQUID magnetometry and susceptometry data on dense polycrystalline samples of the iron oxypnictide superconductors NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, and on a single crystal of LaFePO. On LaFePO, we demonstrate, in a local measurement requiring no correction for sample geometry,that the magnetic penetration depth is linear in $T$, indicating line nodes in the superconducting order parameter. On polycrystalline NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, we show that the order parameter is $s$-wave: if the order parameter were non-$s$-wave there would be direction-dependent phase shifts in the intergrain tunnelling, which in a dense polycrystalline sample would result in spontaneous orbital currents below $T_c$. Orbital currents would give a complex magnetization, and moments polarizable by a weak field during cooling. Paramagnetism against weak cooling fields is a well-established phenomenon in polycrystalline cuprate samples, consistent with $d$-wave superconductivity. In NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, we observe neither the complex magnetization nor weak-field paramagnetism, ruling out non-$s$-wave order parameters.