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

在本研究中，我们利用扫描隧道显微镜（STM）进行了隧道光谱和Andreev反射实验，得到了以下结果：1)在隧道光谱实验中，发现在电子激发中存在两种赝隙（LPG和SPG）；一种具有与超导（SC）隙Δ相当的特征能量，另一种具有比超导隙大3到4倍的特征能量。2)在较高的交叉温度T_<max>以下，较大的能量尺度赝隙（LPG）逐渐形成。另一方面，在较低的交叉温度T*以下，除了LPG之外，较小的能量尺度赝隙（SPG）逐渐发展，并在T_c以下顺利演变为SC隙。SC间隙振幅Δ随整个载流子（空穴）浓度区T*的变化而变化。3)在Andreev反射实验中，已经提出Andreev隙（由于相干配对而被认为反映有效能量gapΔ_<eff>^）与T_c的尺度。上述发现，结合角分辨光发射实验中k空间赝隙演化的结果表明，在T_c以下打开的所谓“冷点”周围的能隙为有效能隙Δ_<eff>，冷点周围的载流子在建立对集体运动的远程相相干中起着至关重要的作用。

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.