Limits on the Superconducting Order Parameter in NdFeAsO$_{1-x}$F$_y$ and SmFeAsO$_{1-x}$ from Scanning SQUID Microscopy

As a test of the symmetry of the order parameter of the ferric oxyarsenide family of superconductors RFeAsO$_{1-x}$F$_y$, where R is a rare earth, we perform scanning SQUID microscopy on dense polycrystalline samples of NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$. Dominant $d$- or $p$-wave orders, for example, would result in direction-dependent phase shifts in tunneling. In well-coupled polycrystalline samples, these phase shifts would result in spontaneous orbital currents and magnetization in the superconducting state. We do not find any spontaneous currents in NdFeAsO$_{0.94}$F$_{0.06}$ or SmFeAsO$_{0.85}$, ruling out order parameters with direction-dependent phase shifts in tunneling.