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

本文利用扫描隧道显微镜（STM）进行了隧道谱和Andreev反射实验，得到了以下结果：1）在隧道谱实验中，发现在电子激发下，存在两种赝能隙（LPG和SPG）;一个具有与超导（SC）间隙Δ相当的特征能量，另一个是3到4倍大。2）较大的能量标度赝间隙（LPG）在较高的交叉温度T_以下逐渐发展<max>。另一方面，在较低的交叉温度T* 以下，除了LPG外，较小的能标赝能隙（SPG）逐渐发展，并在T_c以下平滑地演化为SC能隙。在Andreev反射实验中，Andreev能隙被认为反映了由于相干配对而产生的有效能隙Δ ^<eff>，它与T_c成比例关系。上述结果结合角分辨光电子能谱实验中k空间赝能隙演化的结果，提出在T_c以下打开的所谓“冷点”周围的能隙作为有效能隙Δ_<eff>，并且冷点周围的载流子在建立对集体运动的长程相位相干性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.