Electronic Structure of Doped 1D Cuprate Chains*

1D cuprate chain compounds provide a unique opportunity to reveal the Hamiltonian of cuprates due to the availability of reliable theoretical solutions. However, progress has been hindered by the inability of controlled doping in these materials. We synthesize the 1D cuprate Ba2-xSrxCuO3+δ over a wide range of hole doping. Our angle-resolved photoemission spectroscopy experiments disclose the doping evolution of both the holon and spinon excitations. A prominent folding branch with an intensity far exceeding the prediction of the simple Hubbard model is identified. A quantitative theory-experiment comparison provides concrete evidence for a key missing ingredient in cuprates: a strong near-neighbor attraction.