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atomic resolution a higher spatial sensitivity for the scanning unit is needed. To increase the resolution at room temperature a voltage divider could be used optionally. The applied voltage to the scan piezo is divided by a factor of 10 to avoid the digitization artifacts. These steps appear in the control voltage of the piezo amplifier, which has a limited resolution of 12 bit. Figure 6 shows the raw data of the hexagonal atomic structure of the HOPG sample at room temperature at ambient conditions. 

## IV. CALIBRATION AT LOW TEMPERATURE AND IN MAGNETIC FIELD 

<img src='content_image/351308.jpg'>

Besides HOPG, the layered type-II superconductor NbSe 2 with a critical temperature of 7 . 2 K was investigated. This material is a favorite object for STM and STS studies [11–14] because it has a layered structure similar to HOPG and the sample preparation can be done in a similar way. Figure 7 displays a topographic scan on NbSe 2 taken at T ≈ 50 mK. The in-plane lattice constant is 0 . 345 nm [15]. From the comparison to the lattice constant obtained at room temperature and at low temperature the conversion factor between the room-temperature and the low-temperature calibration of the scan head can be calculated. 

