Fully gapped single-particle excitations in lightly doped cuprates

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_x²-y² nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_2-xSrₓCuO₄ and Ca_2-xNaₓCuO₂Cl₂ and electron-doped Nd_2-xCeₓCuO₄. 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.