Temperature dependence of the bulk energy gap in underdoped Bi2Sr2CaCu2O8+δ: Evidence for the mean-field superconducting transition

Understanding of the puzzling phenomenon of high-temperature superconductivity requires reliable spectroscopic information about temperature dependence of the bulk electronic density of states. Here I present a detailed analysis of the T evolution of bulk electronic spectra in Bi2Sr2CaCu2O8+delta obtained by intrinsic tunneling spectroscopy on small mesa structures. Unambiguous spectroscopic information is obtained by obviation of self-heating problem and by improving the spectroscopic resolution. The obtained data allow accurate determination of the superconducting transition temperature and indicate that (i) the superconducting transition maintains the mean-field character down to moderate underdoping and is associated with a rapid opening of the superconducting gap, which is well described by the conventional BCS T dependence. (ii) The mean-field critical temperature reaches maximum at the optimal doping and decreases with underdoping. Such behavior is inconsistent with theories assuming intimate connection between superconducting and antiferromagnetic spin gaps and supports proposals associating high-temperature superconductivity with the presence of competing ground states and a quantum critical point near optimal doping.