Tunneling studies on the energy gap evolution in Bi2Sr2CaCu2O8+δ

We report, on the basis of our recent tunneling experiments over wide temperature (7) and hole-doping (p) ranges in Bi2212/vaccum/Bi2212 junctions fabricated using STM, that in the electronic excitation spectrum, 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 magnitude 2Delta(0), develops below similar toT(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 2Delta(0), 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. Furthermore, in accordance with the SC transition, the high-energy feature of quasiparticle spectrum outside the SC-gap, changes from a broad hump to a clear dip and hump accompanied by a shift of the hump position toward lower energies, in addition to the rapid growth of SC-gap from SPG. We also report that T-c nearly scales with the product of Delta(0) and p, k(B)T(c)similar topDelta(0); the effective SC gap is -pDelta(0) in high-T-c cuprates, instead of Delta(0) in conventional BCS superconductors.