Luttinger Fermi surface of metallic gap spectral weight in ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$

Angle-resolved photoemission measurements for bulk single crystals of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ with x=0.15 show that the spectral weight which builds in the gap of the x=0 insulator displays a Luttinger Fermi surface, the first such observation for a material which is electron doped and which must be driven metallic by alloying. This finding greatly strengthens our previous ocnclusion that the metal’s chemical potential lies in gap-filling spectral weight, and sharpens the distinction between theories in which this does or does not occur.