Fully gapped single-particle excitations in lightly doped cuprates

Abstract

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 ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and electron-doped ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}.$ 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.