ANOMALOUS SUPERCONDUCTING STATE GAP SIZE VERSUS T-C BEHAVIOR IN UNDERDOPED BI2SR2CA1-XDYXCU2O8+DELTA

We report angle-resolved photoemission spectroscopy measurements of th e excitation gap in underdoped superconducting thin films of Bi2Sr2Ca1 -xDyxCu2O8-delta. As T-c is reduced by a factor of 2 by underdoping, t he superconducting state gap Delta does not fall proportionally, but i nstead stays constant or increases slightly, in violation of the BCS m ean-field theory result. The different doping dependences of Delta and KT2 indicate that they represent different energy scales. The measure ments also show that a is highly anisotropic and consistent with a d(x 2-y2) order parameter, as in previous studies of samples with higher d opings. However, in these underdoped samples, the anisotropic gap pers ists 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 pre dict pairing without phase coherence above T-c.