K. Nagaoka

Field emission spectroscopy from field-enhanced diffusion-growth nano-tips

We have found a new formation procedure of nano-tips, namely field-enhanced diffusion-growth (FDG) method. This method has the advantage of forming the nano-protrusion at room temperature, which is quite different from the conventional method by Binh et al. With field ion microscope, we confirmed that the tip was terminated with either a few atoms or the less in the end. The field emission (FE) electrons from the nano-tips converged to a single spot, and the FE current was very stable. The FE spectra consist solely of well-separated peaks. These features are similar to the conventional ones. However, we have found that the FE current and the shape of FE spectrum were so sensitive to the atomic arrangement of the nano-protrusion that the site-shift of only a few atoms caused drastic change in emission characteristics.

Monochromatic electron emission from the macroscopic quantum state of a superconductor

The ground state of a superconductor is a macroscopic quantum state that can extend coherently over substantial distances. As a result, electrons tunnelling from two different points (separated by macroscopic length) on the surface of a superconductor remain coherent in phase and so are able to interfere: this property forms the basis of superconducting quantum interference devices (SQUIDs). Another characteristic of electrons tunnelling from a superconductor is that they are monochromatic, their energy being determined by the ground-state energy of the superconducting state. Monochromatic electrons have been observed tunnelling from a superconductor to a normal metal, and the resulting currents have been used to probe the dynamics of atoms and molecules at interfaces. Here we report the results of field-emission experiments that confirm the prediction that monochromatic electrons can similarly be emitted from a superconductor into vacuum. Monochromatic emissions of this type might find application as the sources in a range of electron-based spectroscopies.

Energy spectra of field emission electrons from a W〈310〉 tip

Abstract Total energy distributions of field emission electrons from a single crystal 〈310〉-oriented tungsten tip have been measured at temperatures of 80 and 300 K and in the emission current region from 10 −8 to 10 −6 A. We have used a high resolution field emission spectrometer computer-controlled by a low-noise power supply developed in this experiment. The main part of the observed distributions agree with the theoretical ones calculated on the basis of the Fowler-Nordheim theory, while discrepancies have been observed in two regions.

Angular resolved field emission spectra from Nb tips

Abstract Angular resolved field emission spectra from Nb tips have been investigated around the 〈111〉 orientation in detail. We have observed humps and a low-energy tail in the spectra, depending on the crystal orientation. Their intensities have shown a three-fold symmetry around the 〈111〉 orientation.

Field emission energy spectra from superconducting and normal states of a niobium tip

We have measured field emission (FE) electron energy distributions from superconducting and normal states of a niobium tip, and we succeeded in observing an extra peak correlated to the superconducting state. The energy width of the peak was much larger than expected on the basis of simple theory.

Energy distribution of field emission electrons from a niobium 〈111〉 tip

Abstract We have fabricated Nb〈111〉 tips by electrochemical polishing from the single crystal wires along the 〈111〉 orientation, which has been prepared by means of floating zone, spark erosion, and mechanical polishing techniques. The energy distribution of the field emission electrons from the Nb〈111〉 tip has been measured at room temperature using a high resolution electron spectrometer recently developed. The experiments were carried out in vacuum of ∼ 3 × 10 −9 Pa, and the tip surface was cleaned by field evaporation. The main part of the energy distribution is in agreement with the theoretical curves calculated on the basis of the Fowler-Nordheim theory. Two discrepancies are observed on low and high energy sides.

Construction of a low-temperature gun and high-resolution field emission spectra from a Nb superconductor

Abstract To investigate the fine structures in high-resolution field electron emission (FE) spectra from superconductors, we have constructed new apparatuses including a low-temperature FE gun, which can be operated at about 2.2 K. The FE patterns have been observed with an optical-fiber measuring system and the electrons emitted in the direction of the tip axis are analyzed with the resolution of ∼1 meV. By using this experimental apparatus, we have observed a sharp peak at the Fermi level in FE spectra from a niobium superconducting emitter.