Vacuum tunneling

Many different methods have been developed for detecting the minute deflection of the cantilever (Sarid, 1991). In this. section, we present several important ones, including vacuum tunneling (Binnig, Quate, and Gerber, 1986), mechanical resonance (Diirig, Gimzewski, and Pohl 1986), optical interferometry (Martin et al., 1988 Erlandson et al., 1988), and optical beam deflection (Meyer and Amer, 1988). 

The tunneling current between two metal electrodes separated by a vacuum gap varies about one order of magnitude per A. Therefore, vacuum tunneling provides an extremely sensitive method for detecting minute displacements. The first AFM, demonstrated by Binnig, Quate, and Gerber (1986), utilized vacuum tunneling to detect the cantilever deflection. 

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Fig. 1. Principle of scanning tunneling microscopy. A sharp needlelike tip probes the surface atomic structure ofaspecimenby closely scanningthe surface, utilizing extreme sensitivity of the vacuum tunneling current to the tunneling gap. From Sakurai et al. (10) with permission.

Fig. 1.5. Apparatus for demonstrating vacuum tunneling. The tunneling unit consists of a W tip and a Pt plate. Vibration isolation is achieved by magnetic levitation on a superconducting bowl. (Reproduced from Binnig et al., 1982a, with permission.)...

Field emission is a tunneling phenomenon in solids and is quantitatively explained by quantum mechanics. Also, field emission is often used as an auxiliary technique in STM experiments (see Part II). Furthermore, field-emission spectroscopy, as a vacuum-tunneling spectroscopy method (Plummer et al., 1975a), provides information about the electronic states of the tunneling tip. Details will be discussed in Chapter 4. For an understanding of the field-emission phenomenon, the article of Good and Muller (1956) in Handhuch der Physik is still useful. The following is a simplified analysis of the field-emission phenomenon based on a semiclassical method, or the Wentzel-Kramers-Brillouin (WKB) approximation (see Landau and Lifshitz, 1977). 

Fig. 1.39. Transition from field emission to vacuum tunneling. (Reproduced from Young, Ward, and Scire, 1971a, with permission.)...

Binnig, G., Garcia, N., Rohrer, H., Soler, J. M., and Flores, F. (1984). Electron-metal-surface interaction potential with vacuum tunneling Observation of the image force. Phys. Rev. B 30, 4816-4818. 

Lang, N. D. (1985). Vacuum tunneling current from an adsorbed atom. Phys. Rev. Lett. 55, 230-233. 

Plummer, E. W., Gadzuk, J. W., and Penn, D. R. (1975a). Vacuum-tunneling spectroscopy. Physics Today, April 1975, 63-71. 

Quate, C. F. (1986). Vacuum tunneling A new technique for microseopy. Physics Today August 1986, 26-33. 

Stoll, E., Baratoff, A., Selloni, A., and Camevali, P. (1984). Current distribution in the scanning vacuum tunneling microscope a free-electron model. J. Phys. C 17, 3073-3086. 

Wiesendanger, R., Giintherodt, H. J., Giintherodt, G., Gambino, R. J., and Ruf, R. (1990). Observation of vacuum tunneling of spin-polarized electrons with the scanning tunneling microscope. Phys. Rev. Lett. 65, 247—250. 

Deflection detection methods 317—323 detection limit 322 mechanical resonance 318 optical beam deflection 321 optical interferometry 319 vacuum tunneling 317 Density of states 23... 

G. Binning, H. Rohrer, Scanning apparatus for surface investigation using vacuum-tunnel effect at cryogenic temperatures, Eur. Pat. Appl. IBM, USA, 1981, p. 37. 

Figure 14. CO gap in NdojSro.iMnOj as revealed by vacuum tunneling measurements (from Biswas et al.19).

A solid sample surface (under vacuum) is scanned by an ultrafine stylus tip with a very small voltage applied across the gap, which is just a few angstrom units wide. Vacuum tunneling of electrons occurs across the gap, and this effect is very sensitive to the width of the gap, providing a means of mapping the surface at a horizontal resolution of A... 

The STM s roots lie in electron vacuum tunneling spectroscopy. In the context of measuring electronic properties, it is more correctly described as a spectrometer, for it is the electronic properties of the surface that are being probed in the STM experiment. The correspondence between the electronic and topographic properties is responsible for the microscope label. 

This chapter will start with a very brief background on electron vacuum tunneling and a description of the STM s conceptual basis and historical evolution, particularly as it relates to the study of high Tc superconductors... 

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