Measurement of an Anisotropic Energy Gap in Single Plane Bi₂Sr(2-x)LaₓCuO(6+d)

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We report angle-resolved photoemission spectra both above and below Tc in the single-plane cuprate superconductor Bi₂Sr₂₋ₓLaₓCuO₆₊d. The superconducting state measurements show a highly anisotropic excitation gap with a maximum magnitude smaller than that of the bilayer compound Bi₂Sr₂CaCu₂O₈ by a factor of 3. For a range of doping, the gap persists well above Tc, behavior previously associated with underdoped bilayer cuprates. The anisotropy and magnitude of the normal-state gap are very similar to the superconducting state gap, indicating that the two gaps may have a common origin in a pairing interaction.