The combination of the high spatial resolution of scanning tunneling microscopy with the chemical and magnetic contrast provided by synchrotron X-rays has the potential to allow a unique characterization of advanced functional materials. While the scanning probe provides the high spatial resolution, synchrotron X-rays that produce photo-excitations of core electrons add chemical and magnetic contrast. However, in order to realize the method's full potential it is essential to maintain tunneling conditions, even while high brilliance X-rays irradiate the sample surface. Different from conventional scanning tunneling microscopy, X-rays can cause a transition of the tip out of the tunneling regime. Monitoring the reaction of the z-piezo (the element that controls the tip to sample separation) alone is not sufficient, because a continuous tip current is obtained. As a solution, an unambiguous and direct way of fingerprinting such near to far field transitions of the tip that relies on the simultaneous analysis of the X-ray-induced tip and sample current is presented. This result is of considerable importance because it opens the path to the ultimate resolution in X-ray enhanced scanning tunneling microscopy.
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