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The NaCl(001) surface was investigated by friction force microscopy in ultra-high vacuum conditions at room temperature. A homemade atomic force microscope was used which allows an in situ sample rotation. With this ability, it is not only possible to measure friction along arbitrary orientations of the NaCl crystal, but also the symmetry directions of the sample can be precisely tuned parallel to the scan orientation which is fixed orthogonal to the cantilever axis for a calibrated friction measurement. With such a perfect alignment, the tip moves over identical crystallographic positions along the whole scanned line of a couple of nanometers. A relative shift along the slow scan direction was observed between forward and backward scanned force maps. By reconstructing the tip path, we identified five distinguishable modes of tip motions, and found that the asymmetric friction loops are predominant. Prandtl-Tomlinson simulations based on a sinusoidal corrugation potential cannot reproduce the experimental observation. Instead a very good agreement is achieved using an ab initio calculated interaction potential. Measurements along arbitrary orientations show a monotonic decrease of the friction coefficient towards the [110] direction in agreement with the simulation results.
