Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, lambda>(*) over bar * (T), was mea sured down to 0.4 K in single crystals of electron-doped superconductors, P r1.85Ce0.15CuO4-delta (PCCO) and Nd1.85Ce0.15CuO4-delta (NCCO). In PCCO, th e superfluid density varies as T-2 from 0.025 up to roughly 0.3T/T-c sugges tive of a d-wave state with impurities. In NCCO, lambda>(*) over bar * (T) shows a pronounced upturn for T < 4 K due to the paramagnetic contribution of Nd3+ ions. Fits to an s-wave order parameter over the standard BCS range (T/T-c = 0.32) limit any gap to less than <Delta>(min)(0)/T-c = 0.57 in NC CO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of Delta (min)(0)/T-c = 0.2.