Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: evidence for a nodeless gap.

We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T(c), and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124< or =x < or =0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.

Evidence for a nodeless gap from the superfluid density of optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films.

We present measurements of the ab-plane magnetic penetration depth, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)'s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(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 nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.