Katsuyoshi Kobayashi

First-principles theory of scanning tunneling microscopy

Abstract Scanning tunneling microscopy/spectroscopy (STM/STS), which has been so epoch-making in surface science experiments introduced many challenging problems also to the theory of condensed matter physics. Recent progress in theories of STM/STS contributed to revealing the relation between the atomic structure of the tip and the STM/STS data, and to clarify various strange phenomena observed. The present article reviews various important issues of the fundamentals of STM/STS from theoretical view points. After surveying the so far presented theoretical approaches, the first-principles simulation method based on the microscopic electronic state of both the sample surface and the tip is introduced. Several examples of the simulation such as graphite and Si surfaces, are described. Some novel phenomena of the microscopic tunnel system of STM such as the negative differential resistance in STS and single electron tunneling through fine supported particles are also discussed, as well as the many-body effect or electron-phonon coupling effect on STM/STS.

Spin‐polarized electronic structure of cobalt cluster anions studied by photoelectron spectroscopy

The photoelectron spectra of Co−n (3≤n≤70) were measured at the photon energy of 4.025 eV by use of a XeCl excimer laser. For Co−n with n=3, 4, and 6, the geometric and electronic structures were obtained from the spectra in comparison with the calculated spectra by the spin‐polarized DV‐Xα method. The spectra observed are reproduced reasonably well by the calculation with postulating the most probable geometrical structures. It is revealed that the 3d band with the majority spin is separated by 1.0–2.8 eV from that with the minority spin; the former is completely filled while the latter is partly filled and extends above Fermi level. The magnetic moments and the average exchange energies of these cluster anions were estimated. For Co−n with n≥7, the observed electron affinity depends linearly on the reciprocal of the cluster radius and approach the work function of a cobalt metal, as n increases. Below n=6, the electron affinity deviates from the linear dependence. This finding indicates that a size‐depe...

Effect of electronic states of the tip on the STM image of graphite

Abstract A first-principles calculation of the STM images of graphite is performed for three different shapes of a tungsten tip. In the calculation the tip is approximated by a small cluster. One of the tips, with more than one atom at the top, produces an abnormal image, while the others, with only one atom at the top, produce normal trigonal images. The abnormal image is caused by current interference through the multi peaks of the squared amplitude of the tunnel-active orbital at the tip.

Effect of tip atomic and electronic structure on scanning tunneling microscopy/spectroscopy

Abstract The current image and the tunnel conductance in the scanning tunneling microscopy/spectroscopy (STM/STS) of graphite are simulated by the first-principles local density functional calculation of the electronic states. As models of the tip, W 10 [111], W 14 [110], W 13 [110] and Pt 10 [111] clusters are used. For the tips with a single apex atom, normal STM images are obtained. However, abnormal images are generated by the removal of the apex atom. STS spectra are changed in a systematic way by the apex angle of the tip. The negative differential resistance observed in the STS of the Si(111) 3 × 3 -B/defect surface is reproduced by the theoretical simulation with the cluster models of the tip.

Microscopic theory of light emission from the scanning tunneling microscope

Abstract A microscopic theory of light emission due to surface plasmon excitation by inelastic tunneling is developed. It is shown that the excited plasmon power spectrum function is composed of a macroscopic part and a microscopic part. The former represents the plasmon field distribution reflecting the coarse-grained shape of the tip and the surface, while the latter includes the information of the microscopic electronic states of the surface. Performing a realistic calculation by the first-principles electronic states theory, we can evaluate the intensity of the plasmon excitation. We performed a numerical study for the case of a Ag surface and a W tip, and predicted how the atomistic image can be obtained from the emission intensity. Further, we discuss the bias dependence of the excitation spectra.

Theoretical simulation of atomic-scale friction in atomic force microscopy

Abstract We theoretically reproduce a two-dimensional map of the lateral force appeared in AFM, by the simulation based on a simple atomistic model. Various features of microscopic non-conservative lateral force are clarified and their dependence on the cantilever spring constant is discussed. It is also revealed that positions of the tip atom exhibit distributions of “stick regions” peculiar to the graphite surface.

Theory of STM images of monolayer graphite on transition-metal surface

Abstract A first-principles band calculation is performed on monolayer graphite on a TiC(111) surface. Occupied π bands of the monolayer graphite are similar to those of a bulk graphite and are lowered by 2 eV. Unoccupied π* bands are largely affected and split into bonding and antibonding bands due to a hybridization effect with d-bands of top-layer Ti. A calculated local density of states shows a 2 × 2 superstructure of graphite lattice, which explains the shorter period of the moirepattern observed in scanning tunneling microscopy.

Connected vacuum tail method and its application to scanning tunneling microscopy

Abstract A new improved method, the so-called connected vacuum tail (CVT) method, is proposed for the first-principles theoretical simulation of the STM/STS. The CVT method is especially powerful in reproducing the tail part of the surface electronic states extending far into vacuum. In this method, the natural asymptotic decay functions are used as the basis for the wave function in the far-vacuum tail. The expansion coefficients are obtained by solving the Schrodinger equation in the intermediate region by the Laue representation, and connecting these to the solutions of the wave function inside the surface obtained by the standard LDA method. The CVT method is utilized for the theoretical simulation of the STM/STS of Si(100) reconstructed surfaces. In this simulation, the dependence of the STM images on the tip-surface distance shows an interesting feature which corresponds well with experiment.

Theoretical approach to the microscopic mechanism of light emission from a scanning tunneling microscope

Abstract A microscopic theory of the light emission due to the surface plasmon excitation by inelastic tunneling is developed. It is shown that the excited plasmon power spectrum function consists of a macroscopic and a microscopic part. The former represents the plasmon field distribution reflecting the coarse-grained shape of the tip and the surface, while the latter includes the information on the microscopic electronic states of the surface and the tip. Performing a realistic calculation by the first-principles electronic states theory, we can simulate the light emission from an STM. We have performed a numerical study for the case of a silver surface with a tungsten tip, and predict how an atomic image can be obtained from the emission intensity. Further we discuss the bias and photon energy dependence of the microscopic part of emitted light spectra.

Ab initio study of Si(001)2 × 1C chemisorption surface

The optimized structure of Si(001)2 × 1C with monolayer coverage, is obtained by the first-principles molecular dynamics method with a plane-wave basis set. It is revealed that the symmetric dimer structure is stable, although the Si(001)2 × 1 surface prefers an asymmetric dimer structure, and that this surface shows a semiconducting feature with an indirect gap. Although the optimized dimer structure is rather flat, it is slightly raised from the substrate. The Cue5f8C dimer bond is much shorter than the diamond single bond. These features are mainly due to the strong interaction between the carbon atoms.