Electronic structure of ${\mathrm{CuO}}_2$ planes: From insulator to superconductor

Using angle-resolved photoemission and linearly polarized synchrotron radiation, we measured the electronic band structure of electronic states of CuO${}_2$ plane materials ranging from insulators (Sr${}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2)$ to overdoped superconductors (Bi${}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x})$. We report three results: (i) The CuO${}_2$ containing insulator possesses a spin-density-wave (SDW) ground state; (ii) there are precursors of the SDW state for underdoped Bi${}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x};$ (iii) an extended saddle-point-type van Hove singularity is neither a necessary nor a sufficient condition for a high superconducting transition temperature, $T_c.$