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Identifying the symmetry of the superconducting order parameter in therecently-discovered ferro-oxypnictide family of superconductors,RFeAsO$_{1-x}$F$_{y}$, where $R$ is a rare earth, is a high priority. Many ofthe proposed order parameters have internal $\pi$ phase shifts, like the d-waveorder found in the cuprates, which would result in direction-dependent phaseshifts in tunnelling. In dense polycrystalline samples, these phase shifts inturn would result in spontaneous orbital currents and magnetization in thesuperconducting state. We perform scanning SQUID microscopy on a densepolycrystalline sample of \NdFeAsO$_{0.94}$F$_{0.06}$ with $T_c=48$ K and findno such spontaneous currents, ruling out many of the proposed order parameters.

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