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

Carrier concentration dependence of the superconducting gap observable around the nodal region was investigated in detail for a series of high-Tc cuprate superconductors, Bi-2(Sr,La)(2)CuO6+delta, 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, Delta = Delta(0) cos(2 theta), at least around the node even in the heavily underdoped samples. The gap size Delta(0) 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, T-c, decreases more drastically than Delta(0) with decreasing hole concentration. This fact indicates that T-c is not simply determined by Delta(0), 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 Delta(0) and the development of pseudogap.