Ivan Bozovic

Anomalous superconducting state gap size versus {ital T}{sub {ital c}} behavior in underdoped Bi{sub 2}Sr{sub 2}Ca{sub 1{minus}{ital x}}Dy{sub {ital x}}Cu{sub 2}O{sub 8+{delta}}

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.