Direct Observation of a Nonmonotonic $d_{x^2-y^2}$-Wave Superconducting Gap in the Electron-Doped High-$T_c$ Superconductor ${\mathrm{Pr}}_{0.89}{\mathrm{LaCe}}_{0.11}{\mathrm{CuO}}_4$

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-$T_c$ superconductor ${\mathrm{Pr}}_{0.89}{\mathrm{LaCe}}_{0.11}{\mathrm{CuO}}_4$ to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the $d_{x^2-y^2}$-wave symmetry, but obviously deviates from the monotonic $d_{x^2-y^2}$ gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results suggest the spin-mediated pairing mechanism in electron-doped high-$T_c$ superconductors.

Figure 1

Relation between the Fermi surface and the antiferromagnetic Brillouin zone for (a) hole- and (b) electron-doped HTSCs. Thick solid curve and thin straight line show the FS and the AFM-BZ, respectively. The arrow and the open circle show the AFM scattering vector $Q=(\pi ,\pi )$ and the hot spot, respectively.

Figure 2

(a)–(c) ARPES spectra of PLCCO at 30 K along three representative cuts shown by an arrow in each inset. Black-colored spectra are measured at the Fermi momentum. (d)–(f) Corresponding ARPES-intensity plots as a function of the momentum and binding energy, showing the experimental band dispersion. Peak positions in ARPES spectra are shown by bars and dots.

Figure 3

Near-$E_{\mathrm{F}}$ ARPES spectra measured below and above $T_c$ at three $k_{\mathrm{F}}$ points on the FS shown in the inset. The 8 and 30 K spectra are shown by blue and red dots, respectively. Solid curves show the fitting of the spectra.

Figure 4

The energy position of a leading-edge midpoint (${\Delta }_{\mathrm{shift}}$) plotted as a function of the FS angle ($\varphi$). The solid curve is the result of fitting ${\Delta }_{\mathrm{shift}}(\varphi )$ with the nonmonotonic $d_{x^2-y^2}$ gap function, ${\Delta }_0[B\cos (2\varphi )+(1-B)\cos (6\varphi )]$, compared with the monotonic $d_{x^2-y^2}$ gap function (broken line).