We present a systematic analysis of the energy gap in underdoped Bi2212superconductor as a function of temperature and hole doping level. Within theframework of the theoretical model containing the electron-phonon andelectron-electron-phonon pairing mechanism, we reproduced the measurementresults of modern ARPES experiments with very high accuracy. We showed that theenergy-gap amplitude is very weakly dependent on the temperature but clearlydependent on the level of doping. The evidence for a non-zero energy gap abovethe critical temperature, referred to as a pseudogap, was also obtained.
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