content_image/368318.jpg
the calibrated sensor. 

## B. STM 

<img src='content_image/368318.jpg'>

The STM consists of a scan-head unit with a commercial five-electrode piezo tube scanner (EBL #2, Staveley NDT Technologies, East Hartford, USA) (fig. 2 B). The inner electrode of the piezo tube is used for the z movement and the outer electrode is split into four parts, for ± x and ± y movement, respectively. The way of assembling the scan head from the parts of fig. 2 is displayed in fig. 3. Almost all parts are fixed with metallic screws to ensure good thermal coupling, only the piezo is glued with Torr Seal epoxy (Varian) to a macor [8] panel and the sapphire rods. 

The coarse approach of the STM tip to the sample is done by a vertical slip-stick mechanism using the z movement. The tip is attached to a non-magnetic CuBe slider which is supported by two sapphire rods (fig. 2 C). The coarse approach of the tip, or rather the coarse approach of the slider, is based on inertia movement introduced by Pohl [9]. The cross-section of the slider is sketched in figure 4. The upper part (fig. 4 B) consists of a CuBe cube with a cylindrical hole and three perpendicular drilling holes. One hole is used to fix the tip, another one is used for the connections of the current lead. The lower part (fig. 4 C) is connected through the middle hole to the upper part by a screw (fig. 4 D) and pre-loaded by a spring which allows to apply a variable pressure to the sapphire rods (fig. 4 A). This changes the friction between the slider and the sapphire rods and therefore permits the control of the speed, in particular the step width, of the coarse approach. The step width 

