Distinct Pairing Symmetries in $N{d}_{1.85}C{e}_{0.15}Cu{O}_{4-y}$ and $L{a}_{1.89}S{r}_{0.11}Cu{O}_4$ Single Crystals: Evidence from Comparative Tunnelling Measurements

Abstract

We used point-contact tunnelling spectroscopy to study the superconducting pairing symmetry of electron-doped $N{d}_{1.85}C{e}_{0.15}Cu{O}_{4-y}$ (NCCO) and hole-doped $L{a}_{1.89}S{r}_{0.11}Cu{O}_4$ (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with $\Delta /k_B{T}_c=1.66$, a value remarkable close to that of a weakly coupled BCS superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at ($\pm \pi$,0) and (0,$\pm \pi$) although a d-wave component may also exist.

4 pages, 4 figures