Monochromatic field electron emission from a Nb superconductor

To observe a monochromatic electron beam with energy width much narrower than that of a conventional electron beam, we have constructed an apparatus consisting of a low-temperature field-emission electron gun cooled to 4.2 K and a high-resolution electron spectrometer operating at high vacuum. With this system, we have observed spectra of the field electron emission from a niobium superconducting emitter. After tip cleaning by repeated treatments of field evaporation in vacuum less than 10−10 Pa, we have detected a sharp peak at the Fermi level in the spectra with an energy width of ∼20 meV at 10 nA.