Excitation gap in the normal and superconducting state of underdoped Bi2Sr2Ca1-xDyxCu2O8+delta thin film and single crystals

We summarize photoemission data from underdoped Bi2Sr2Ca1-xDyxCu2O8+delta that revealed anomalous properties: i) We observed an excitation gap in the normal state of underdoped samples, and this normal state gap closes in overdoped samples; ii) The normal state gap has similar magnitude and momentum dependence as the superconducting gap, which is consistent with a d(x2-y2) order parameter; iii) The normal state gap persists to a temperature range much higher than T-c; iv) The superconducting gap in the underdoped regime does not scale with T-c. These results are consistent with theoretical models that suggest the underdoped regime can be characterized by two temperatures, a mean-field temperature below which there is pairing, and a lower superconducting transition temperature at which the pairs become phase coherent.