Anomalous superconducting state gap size versus Tc behavior in underdoped Bi2Sr2Ca1−xDyxCu2O8+δ

Abstract

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi2Sr2Ca1−xDyxCu2O8+δ. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap Δ 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 Δ and kTc indicate that they represent different energy scales. The measurements also show that Δ is highly anisotropic and consistent with a dx2−y2 order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above Tc. The existence of a normal state gap is related to the failure of Δ to scale with Tc in theoretical models that predict pairing without phase coherence above Tc.

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