Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La_2-xCe_xCuO_4-y and Pr_2-xCe_xCuO_4-y

We present measurements of the magnetic penetration depth, λ^-2(T), in Pr_2-xCe_xCuO_4-y and La_2-xCe_xCuO_4-y films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, λ^-2(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, λ^-2(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.