Electron-Beam Source with a Superconducting Niobium Tip

Modern electron microscopy and spectroscopy are key technologies 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 nanoprotrusion at the apex, strongly influencing the field-emission energy distribution. The geometry without the nanoprotrusion has nanoampere beam currents, long-term stability, and an energy width of around 100 meV. The beam current can be increased by 2 orders of magnitude by xenon-gas adsorption. We also study the emitter performance up to 82 K and demonstrate that the energy width of the beam can be below 40 meV with high emitter brightness even at liquid-nitrogen cooling temperatures when an apex nanoprotrusion 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 monochromatic source will allow exceptional accuracy and resolution in electron microscopy, spectroscopy, and high-coherence quantum applications.