Pull-off force spectroscopy pfs

In contrast to pfm, pfs measurements do not allow the two contributions Pz and a to be separated from their sign, which therefore results in summing up over the two signals rather than building the difference as shown in Equation (12.1) [24], Since pull-off force experiments 

The positive sign directly results from electrostatic forces always being attractive between tip and (any) sample independent of whether electrons or holes are the majority carriers. The proportionality factor 5 is larger than one since the surface charge density a clearly dominates the polarization induced force on the tip. 

In order to separate Pz and a we therefore have to determine 7 and 5 either theoretically or in our experiment. 

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