Limits on the Superconducting Order Parameter in NdFeAsO1-xFy from Scanning SQUID Microscopy

Identifying the symmetry of the superconducting order parameter in the recently-discovered ferro-oxypnictide family of superconductors, RFeAsO1-xFy, where R is a rare earth, is a high priority. Many of the proposed order parameters have internal pi phase shifts, like the d-wave. order found in the cuprates. which would result in direction-dependent phase shifts in tunnelling. In dense polycrystalline samples, these phase shifts in turn would result in spontaneous orbital currents and magnetization in the. superconducting state. We perform scanning SQUID microscopy on a dense polycrystalline sample of NdFeASO(0.94)F(0.06) with T-c = 48 K and find no such spontaneous currents, ruling out many of the proposed order parameters.