Energy gap evolution in the tunneling spectra of Bi2Sr2CaCu2O8+δ

On the basis of the tunneling spectra in Bi2212/vaccum/Bi2212 junctions fabricated using STM, we report that, in the electronic excitations, there exist two kinds of pseudogaps; one with a characteristic energy comparable to the superconducting (SC) gap and another one that is 3 to 4 times larger. The smaller energy-scale pseudogap (SPG) develops progressively below temperature T*, which nearly scales with the SC gap amplitude Delta(0) at T much less than T-c, in addition to the larger energy-scale pseudogap (LPG), which already exists above T*. The SPG smoothly develops into the SC-state gap with no tendency to close at T-c.