Abstract
In the last two decades, the resolution of photoemission spectroscopy has been dramatically improved nearly 100 times and is approaching 1 meV very recently. Consequently, photoemission spectroscopy can now directly measure fine electronic structures (having an energy scale of a few meV) very close to the Fermi energy, which are responsible for physical properties of solids. In this article, we would like to describe how ultrahigh-resolution photoemission spectroscopy performed at low temperatures is powerful in studying the fine electronic structures, using an example of our recent results on the superconducting gap of the MgB superconductor.
