(a) Sectional view of the UHV system. The cryostat is mounted below the central microscope chamber. The 3D inset represents schematically the superconducting magnets consisting of two split coil pairs and a solenoid coil. The microscope can be vertically moved from the cryostat to the central microscope chamber by the motor controlled microscope elevator exhibiting a 1.7 m transfer path. The thermal coupling to the liquid helium is realized by a movable copper cone above the microscope and a counter cone fixed within the He cryostat. (b) Sectional view of the central chamber containing the STM in sample transfer position. An additional MBE evaporator allows evaporation of atoms directly into the microscope at temperatures of about 30 K. The optical microscope outside the UHV chamber provides a resolution of at the sample position.
(a) Sectional view of the STM consisting of the walker unit with the piezotube scanner and the new sample positioning unit. (b) Image of the tip and a graphene flake contacted by Indium wires as displayed through the optical long distance microscope. In combination with the sample positioning unit, the tip can be positioned with an accuracy of . (c) Photograph of the actuator of the positioning unit. The sample stage is placed on four shear piezos mounted on a titanium drawer, enabling a sample movement of about 2 mm in - and -directions. Individual steps of about 30 nm are achieved by applying an appropriate sawtooth voltage pulse with an amplitude of 40 V.
Schematic circuit diagram of the electronics. In order to prevent ground loops, the ground potential of the STM electronics is decoupled from the microscope ground potential by two differential amplifiers. The current-to-voltage converter is a commercial low-noise current amplifier. The inset diagram shows the current noise of the electronics as a function of the frequency.
(a) unfiltered constant-current image of a reconstructed surface at and (scan velocity of 40 nm/s). The bright spots are residual gas adsorbates. (b) Zoom into the reconstruction (image size: ) at and (scan velocity of 23 nm/s). (c) Height profiles taken along the white lines in image (b) across the scan lines (i) and along the scan direction (ii). The analysis of the height profiles leads to a -noise of (i) and (ii), respectively. Corresponding analysis of height profiles for the maximum magnetic in-plane field of (f), and out-of-plane field of (g).
Characterization of the sample positioning unit. The STM images show charged residual gas adsorbates on a surface . From image (a) and (b), we applied a voltage pulse of to the -piezos of the stage. From image (d) and (e), we applied a -pulse to the -piezos. To recognize the displacement, two bright adsorbates are marked in the images. (c) Cross correlation of images (a) and (b). (f) Cross correlation of images (d) and (e). The arrows in (c) and (f) indicate the displacement vectors.
Spectra of differential conductivity of a surface. The inset represents an atomically resolved STM image of the surface. The sharp peaks in the spectra stem from tip-induced quantum dot states which merge into Landau levels at high external magnetic fields. The energy evolutions of the first five Landau levels are indicated by the dashed lines as marked in the top part of the image. If the external field exceeds , spin splitting becomes clearly visible (peaks marked by and ). The measured energy width of the first spin splitted peak is 6.5 meV at .
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