Superconducting gap of heavily underdoped copper oxide superconductor (Bi,Pb)2Sr2(Ca,Y)Cu2O8+δ

The heavily underdoped (Bi,Pb)2Sr2(Ca,Y)Cu2O8+δ single crystals were successfully grown by conventional floating zone technique with a very slow growth rate of 0.20 mm/h. The superconducting gap of prepared single crystals was investigated by means of laser induced ARPES measurement. We found that the superconducting gap was kept almost unchanged from optimally doped region, Tc = 92 K, to heavily underdoped region, Tc ∼ 12 K. The result suggest that the pair-formation-energy in underdoped region is similar to that in optimally doped region, and the superconducting temperature is presumably suppressed by the pseudogap which increase with decreasing hole concentration.