Magnetic penetration depth in electron-doped cuprates - evidence for gap nodes

The in-plane penetration depth \lambda(T) is measured in electron-doped single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a 11 MHz LC resonator. In NCCO, \lambda(T) exhibits a minimum at 3.8 K and a pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+ ions. The London penetration depth contribution is linear in T. The paramagnetic contribution is absent in PCCO, where \lambda(T)~T^2 at low temperatures. Our results indicate the presence of nodes in the superconducting gap, i.e., non s-wave symmetry of the order parameter in electron-doped cuprates.