Evidence for the coexistence of antiferromagnetism and superconductivity in the electron-doped infinite-layer cuprate Sr$_{1-x}$La$_x$CuO$_2$ from ARPES

The asymmetry between hole doping and electron doping of cuprates has major implications for theories of high-$T_c$ superconductivity, yet the vast majority of our knowledge of electron-doped cuprates originates from the Re$_{2-x}$Ce$_x$CuO$_4$ (RCCO) materials. One salient feature of electron doping is the robustness of antiferromagnetism, but at present it has not been established whether this is intrinsic to the electron-doped CuO$_2$ plane or idiosyncratic to the RCCO family. Here we report high-resolution \emph{in situ} angle-resolved photoemission spectroscopy measurements of superconducting Sr$_{1-x}$La$_x$CuO$_2$ (SLCO) thin films grown by molecular beam epitaxy. The observed electronic structure exhibits many features consistent with ($\pi$,$\pi$) scattering, and the clear observation of such scattering in this material demonstrates that strong antiferromagnetism is generic to the electron-doped CuO$_2$ plane. Furthermore, the ($\pi$,$\pi$) order in SLCO is sufficiently strong to fully gap the nodal portion of the Fermi surface, leaving only electron pockets at ($\pi$,0); these pockets are gapped by the coexisting (presumably \emph{d}-wave) superconductivity. This offers a simple explanation for the many reports of \emph{s}-wave superconductivity in this material.