Tunneling studies on the energy gap evolution of Bi 2 Sr 2 CaCu 2 O 8+δ

Abstract

We report, on the basis of our recent tunneling experiments over wide temperatures (T) and hole-doping (p) ranges in Bi2212/vaccum/Bi2212 junctions fabricated using STM, that in the electronic excitation spectrum, there exists 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 2Δ 0 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 2Δ 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 superconditivity. 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 Δ 0 and p, k B T c ~pΔ 0 ' the effective SC gap is ~pΔ 0 in high T c cuprates, instead of Δ 0 in conventional BCS superconductors.