Magnetic Penetration Depth Measurements of Pr_2-xCe_xCuO_4-δ Films on Buffered Substrates: Evidence for a Nodeless Gap

We report measurements of the inverse squared magnetic penetration depth, λ^-2(T), in Pr_2-xCe_xCuO_4-δ (0.115≤x≤0.152) superconducting films grown on SrTiO₃ (001) substrates coated with a buffer layer of insulating Pr₂CuO₄. λ^-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≤x≤0.144, λ^-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 λ^-2(T) produce values of the minimum superconducting gap in the range of 0.29≤Δ_min/k_BT_c≤1.01.