Formation in an STM

The control parameter in an STM, the current in the tunneling junction, is always due to the same physical process. An electron in one lead of the junction has a nonvanishing probability to pass the potential barrier between the two sides and to tunnel into the other lead. However, this process is highly influenced by (i) the distance between the leads, (ii) the chemical composition of the surface and tip, (iii) the electronic structure of both the systems, (iv) the chemical interactions between the surface and the tip atoms, (v) the electrostatic interactions of the sample and tip. The main problem, from a theoretical point of view, is that the order of importance of all these effects depends generally on the distance and therefore on the tunneling conditions [5-8]. 

Unlike electrostatic forces, chemical forces between the probing tip and the probed surface have been shown to profoundly affect the tunneling current from a certain onset. Owing to the advent of first-principle methods and powerful computers, it could finally be resolved by a calculation of the combined tip-sample system [ 15 ]. The point of onset for chemical bonding on metals was found to be at a distance of 4—5 A. As the tip approaches the surface, chemical forces rapidly become large enough to 

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