Carrier Concentration Dependence of Superconducting Gap of Bi2(Sr,La)2CuO6+δ

Abstract

Carrier concentration dependence of the superconducting gap observable around the nodal region was investigated in detail for a series of high-Tc cuprate superconductors, Bi2(Sr,La)2CuO6+δ, by means of ultra-high-resolution laser-induced angle resolved photoemission spectroscopy. We found that the gap can be expressed by the simple d-wave gap form, Δ = Δ0 cos(2θ), at least around the node even in the heavily underdoped samples. The gap size Δ0 increases with decreasing hole concentration in the overdoped region, and starts to decrease after taking a plateau that extends over a relatively wide hole concentration, p, ranging 0.16 ≤ p ≤ 0.26. The critical temperature of superconductivity, Tc, decreases more drastically than Δ0 with decreasing hole concentration. This fact indicates that Tc is not simply determined by Δ0, and we argue that the reduction of Tc with decreasing hole concentration in the underdoped regions is caused by a synergy effect of the decreasing Δ0 and the development of pseudogap.