Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La_(2-x)Ce_xCuO_(4-y) and Pr_(2-x)Ce_xCuO_(4-y)

We present measurements of the magnetic penetration depth, \lambda^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{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, \lambda^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, \lambda^{-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 nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.