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. 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. 11 FIG. 7: Atomic resolution on NbSe 2 at T ≈ 50 mK, with V = 322 . 3 mV and I = 1 . 7 nA. The rombus indicates the unit cell of the hexagonal lattice. With this data the scan head was calibrated for low temperature and small scanning areas. IV. CALIBRATION AT LOW TEMPERATURE AND IN MAGNETIC FIELD