Tunneling Spectroscopy Study on the Origin of Coherence of Election-Pans m High-Tc Superconductor

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In the present study, it was confirmed in the tunneling spectroscopy measurements on Bi2212 that the small pseudogap (SPG) starts to grow around the mean-field critical temperature of d-wave superconductors, and connects smoothly to the energy gap of the superconducting (SC) state. The development of SPG reduces the Fermi line into the so-called nodal Fermi arc centered near (π/2,π/2) at T〜T_c, as was revealed by ARPES measurements on Bi2212. In high T_c-cuprates, mobile holes on the nodal Fermi arcs have high in-plane mobility, and are expected to drive the superconductivity in the SPG regime when they start to form pairs. This is because the pairing of mobile carriers with high in-plane mobility will play a crucial role in causing the superconductivity. In fact, it was demonstrated in the present study that the remarkable reduction of U(0) in the SPG regime of La214 can be explained quantitatively on condition that only holes on the nodal Fermi arcs will be associated with the superconducutivity. This is consistent with the experimental result that the coherence peaks of both tunneling and ARPES spectra, reflecting the nature of quasi-particles over the antinodal parts of the Fermi line, are rather smeared in the SPG regime. By scanning tunneling microscopy and spectroscopy (STM/STS) measurements on Bi2212, we also ruled out the possibility that no phase separation causes the remarkable suppression of U(0), at least, in the SPG regime of interest. Thus, we concluded that mobile holes on the nodal Fermi arcs cause the superconductivity in the SPG regime.