Energy Gap Evolution over Wide Temperature and Doping Ranges in Bi2Sr2CaCu2O8+δ

Abstract

In this article we report the energy gap evolution in Bi 2 Sr 2 CaCu 2 O 8+δ( Bi2212 ) on the basis of recent tunneling and Raman scattering experiments over wide temperature and hole-doping (p) ranges. It has been demonstrated that in the normal-state electronic excitations, there exist two kinds of pseudogaps (LPG and SPG) with different characteristic energies. The LPG, which is 3 to 4 times larger than the superconducting (SC) gap at T ≪ T c , develops below ~ T max , where the magnetic susceptibility starts to decrease because of the gradual development of antiferromagnetic spin fluctuations. On the other hand, the SPG, whose magnitude is comparable to the SC gap, develops progressively, in addition to the LPG, below the mean-field characteristic temperature T co for d-wave superconductors, and then evolves into the SC gap below T c , suggesting that it will be some kind of precursor of superconductivity. It has also been demonstrated that the maximal gap Δ0 or the gap at the positions near (±π, 0) and (0,±π), determined at T ≪ T c by tunneling technique, does not scale with T c , while the gap around (±π/2,±π/2), determined at T ≪ T c from the coherence peak energy in B 2g Raman spectra, scales with T c , suggesting that the latter gap will function as an effective SC gap Δeff in determining T c . Furthermore, we will report that Δeff is given by the product of p and Δ0; Δ eff ~ 5.3pΔ0 in Bi2212, and discuss some scenarios for the SC transition which is consistent with the present result on the effective SC gap.