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The scanning vacuum tunnelling microscope is a device capable of providing information on an atomic scale about the charge distribution of a conducting surface at the Fermi energy. Simple WKB theory explains the I-V-s tunnelling characteristics quite well, including the origin of anomalously low values for the height of the potential barrier, OБ. Limiting factors in the resolution are the lateral spread of the current and the rapid decay of the structural information contained in high k∥ Fourier components of the surface wavefunctions. Both effects produce a fall-off in resolution of the form ~(r+s)1/2/φ b 1/4 and appear to be similar in magnitude. The best lateral resolution achieved to date is ≈4 A, for separation s≈4 A, tip radius r≈5 A, and barrier height φb≈4 eV. The tunnel current IT is proportional to the surface local density of states at the position of the centre of the probe tip, which is proportional to the charge density. The relation between the tunnel current and the actual atomic structure is less well-defined, and some care is necessary in the detailed interpretation of STM images. |
