Scanning tunneling microscopy studies on Andreev reflection and pseudogap in high-T_c cuprate superconductors

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In the present study, we have performed tunneling spectroscopy and Andreev reflection experiments using scanning tunneling microscope (STM), and obtained the following results.1) In tunneling spectroscopy experiments, it has been found that in the lectronic excitations, there exist two kinds of pseudogaps (LPG and SPG) ; one with a characteristic energy comparable to the superconducting (SC) gap Δ and another one that is 3 to 4 times larger.2) The larger energy-scale pseudogap (LPG) develops progressively below higher crossover temperature T_<max>. On the other hand, the smaller energy-scale pseudogap (SPG) develops progressively below lower crossover temperature T* in addition to the LPG, and evolves smoothly into the SC gap below T_c. The SC gap amplitude Δ scales with T* in the entire carrier (hole) concentration region examined.3) In Andreev reflection experiments, it has been suggested that the Andreev gap, which has been considered to reflect the effective energy gapΔ_<eff>^ due to coherent pairing, scales with T_c.The above findings, combined with the results in angle-resolved photoemission experiments on the pseudogap evolution in the k-space, suggest that an energy gap around the so-called "cold spots", which opens below T_c, functions as the effective energy gap Δ_<eff>, and the carriers around me cold spots play an crucial role in establishing the long-range phase coherence in the collective motion of pairs.