High Energy Resolution Angle-Resolved Photoemission Studies of Electronic Properties of Bismuth-Based High Temperature Superconductors.

High energy resolution angle-resolved photoemission spectroscopy (ARPES) has been applied to the studies of both the superconducting state and normal state electronic properties of the Bi-based high temperature superconductors, especially of the Bi_2Sr _2CaCuO_{8+x} (Bi-2212) single crystals and samples oxygenated differently or doped with cations. A temperature dependent superconducting gap anisotropy between the Gamma -| M and Gamma -X directions was found for oxygen-overdoped Bi-2212, which is very suggestive of a two component superconducting order parameter for Bi-2212. ARPES studies of the electronic properties of normal states of oxygen-overdoped, iodine -intercalated, cobalt-doped, and lead-doped Bi-2212 have strongly enhanced our knowledge of the fundamental microscopic mechanisms that might lead to the formation of the high temperature superconductivity.