Anomalous superconducting state gap size versus T-c behavior in underdoped Bi2Sr2Ca1-xDyxCu2O8+delta

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi2Sr2Ca1-xDyxCu2O8-delta. As T-c is reduced by a factor of 2 by underdoping, the superconducting state gap Delta does not fall proportionally, but instead stays constant or increases slightly, in violation of the BCS mean-field theory result. The different doping dependences of Delta and KT2 indicate that they represent different energy scales. The measurements also show that a is highly anisotropic and consistent with a d(x2-y2) order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above T-c. The existence of a normal state gap is related to the failure of Delta to scale with T-c in theoretical models that predict pairing without phase coherence above T-c.