Anomalous superconducting state gap size versus Tc behavior in underdoped Bi₂Sr₂Ca₁-xDy_xCu₂O₈+d

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc 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 kT_c indicate that they represent different energy scales. The measurements also show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} 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.