  Modern electron microscopy and spectroscopy is a key technology for studying
the structure and composition of quantum and biological materials in
fundamental and applied sciences. High-resolution spectroscopic techniques and
aberration-corrected microscopes are often limited by the relatively large
energy distribution of currently available beam sources. This can be improved
by a monochromator, with the significant drawback of losing most of the beam
current. Here, we study the field emission properties of a monocrystalline
niobium tip electron field emitter at 5.2 K, well below the superconducting
transition temperature. The emitter fabrication process can generate two tip
configurations, with or without a nano-protrusion at the apex, strongly
influencing the field-emission energy distribution. The geometry without the
nano-protrusion has a high beam current, long-term stability, and an energy
width of around 100 meV. The beam current can be increased by two orders of
magnitude by xenon gas adsorption. We also studied the emitter performance up
to 82 K and demonstrated the beam's energy width can be below 40 meV even at
liquid nitrogen cooling temperatures when an apex nano-protrusion is present.
Furthermore, the spatial and temporal electron-electron correlations of the
field emission are studied at normal and superconducting temperatures and the
influence of Nottingham heating is discussed. This new monochromatic source
will allow unprecedented accuracy and resolution in electron microscopy,
spectroscopy, and high-coherence quantum applications.
