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

The in-plane penetration depth lambda (T) is measured in electron-doped sin gle crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15O4-x (PCCO) using an 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 Nd 3+ ions. The London penetration depth contribution is linear in T. The para magnetic contribution is absent in PCCO, where lambda (T)similar toT(2) at low temperatures. Our results indicate the presence of nodes in the superco nducting gap, i.e., non s-wave symmetry of the order parameter in electron- doped cuprates.