Excitation Gap in the Normal State of Underdoped Bi; ₂; Sr; ₂; CaCu; ₂; O; ₈₊; _δ Journal Article

Overview

; Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi; 2; Sr; 2; CaCu; 2; O; 8+; δ; . 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; ; x; ; ; 2; ; −; ; y; ; ; 2; ; 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 state. Some possible theoretical explanations for this effect are reviewed.;