The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.
@article{arxiv.cond-mat/0312270,
title = {Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates},
author = {K. M. Shen and T. Yoshida and D. H. Lu and F. Ronning and N. P. Armitage and W. S. Lee and X. J. Zhou and A. Damascelli and D. L. Feng and N. J. C. Ingle and H. Eisaki and Y. Kohsaka and H. Takagi and T. Kakeshita and S. Uchida and P. K. Mang and M. Greven and Y. Onose and Y. Taguchi and Y. Tokura and Seiki Komiya and Yoichi Ando and M. Azuma and M. Takano and A. Fujimori and Z. -X. Shen},
journal= {arXiv preprint arXiv:cond-mat/0312270},
year = {2009}
}