An Overview of Current Research on the High-Temperature Superconductivity Using SI-STM

Throughout the history of superconductivity, tunneling has been one of the most important experimental tools of choice. Examples include the first tunneling experiment on the superconductor-insulator-superconductor (SIS) junction made of superconducting aluminum, experiments on Josephson tunneling, and many others. In 1981, scanning tunneling microscopy, which can visualize atomic corrugations, was invented. Meanwhile, in 1986 copper based high-temperature superconductors, known as cuprates with perovskite structures, were discovered and so began the era of high-temperature superconductivity (HTSC). Twenty years after the discovery of superconducting cuprates, yet another new superconductor has come to light in the ferropnictide family, which contains FeAs layers. Experimentally, HTSC phenomena have revealed many physical properties different from those of conventional superconductors, and theory-wise, none of the theoretical models has successfully explained all the experimental results for HTSC as of the time this review was written. Iron based superconductors including ferropnictides have many similarities to cuprates, such as their phase diagram, 2D properties, and the anti-ferromagnetic (AF) nature of the parent compounds. Distinguishing characteristics, such as gap symmetry, conducting parent compounds, and multi-band characters, are clearly observed. In this review, we will focused on the contributions of the spectroscopic imaging scanning tunneling microscopy (SI-STM) to HTSC research. With the development of SI-STM, sub-atomic distributions of various physical quantities can be surveyed in the form of “maps” because one can acquire spectroscopic information at each and every topographic point. Moreover, implementing a Fourier-transform of the spectroscopic map, one can also explore k-space information in great detail. Recently, the scanning Josephson tunneling microscopy (SJTM) technique, which is related to both SI-STM and the Josephson effect, has succeeded in verifying the existence of a pair density wave (PDW) in the Bi2Sr2CaCu2O8+d cuprate. A theoretical explanation of the PDW can provide an important insight into the mechanism responsible for the HTSC.