Strong pairing interactions in the underdoped region of Bi2Sr2CaCu2O8+delta

Doping effects on the superconducting gap of Bi2Sr2CaCu2O8+delta have been studied for 0.1 less than or similar to p less than or similar to 0.21 by STM/STS. The gap is consistent with d(x2-y2) symmetry in the p-range examined. The gap magnitude 2 Delta(o) increases with decreasing p even below p(0) similar to 0.17, at which T-c starts to decrease, and seems to correlate with the development of an antiferromagnetic correlation. The in-plane resistivity is reduced just like a superconducting precursor below T*, at which pseudogaps open in the electronic and magnetic energy spectra. Furthermore, the reduced gap 2 Delta(o)/k(B)T* is nearly constant for p less than or similar to 0.21, 4 similar to 5, comparable to the mean field value 2 Delta(0)/k(B)T(c0) similar to 4.3 for d-wave pairing states. These results suggest convincingly that incoherent pairs are preformed well above T-c at low doping levels where the pairing interaction is extremely strong but T-c is largely reduced. (C) 1997 Elsevier Science B.V.