Excitation gap in the normal state of underdoped Bi2Sr2CaCu2O8+delta

Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+delta. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature T-c is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the d(x2-y2) gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal stale. Some possible theoretical explanations for this effect are reviewed.