Phase-Sensitive Evidence ford-Wave Pairing Symmetry in Electron-Doped Cuprate Superconductors

We present phase-sensitive evidence that the electron-doped cuprates ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$ (NCCO) and ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$ (PCCO) have $d$-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in $c$-axis-oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] ${\mathrm{SrTiO}}_3$ substrates designed to be frustrated for a $d_{x^2{-y}^2}$ order parameter. Samples with two other configurations, designed to be unfrustrated for a $d$-wave superconductor, do not show the half-flux quantum effect.