Tuning-fork-based AFM for Scanning Gate Microscopy
in top-loading dilution refrigerator
- Bluefors dry dilution refrigerator
- top-loading insert
- superconducting magnet
- attoAFM III microscope for mK
- tuning fork with AFM tip
- chip carrier sample holder with thermalization
- positioners and scanners for sample motion
For this project, we have developed a mK tuning-fork AFM for a dry toploading dilution refrigerator (DR) in close collaboration with BlueFors. The DR has a cooling power of typically 300 µW at 100 mK and a base temperature of 8 mK. The cooling power at the sample location on the toploading insert is 100 µW at 100 mK. Thanks to the toploading probe, the turnaround time is typically 9-11 hours, and hence tip and sample exchange can be achieved within a reasonable time frame without warming up the whole DR including the superconducting magnet. The customized attoAFM III has been carefully adapted for the mK environment in terms of cabling and thermalization, and the whole configuration has been tested and optimized thoroughly at mK temperatures. The microscope is intended to be used for scanning gate microscopy (SGM), where the wire-type AFM tip on the tuning fork serves as a mobile local gate. This allows to characterized electrical transport properties in mesoscopic samples on the nanoscale as a function of gate position and tip potential.
Previous implementations of such mK microscopes  have involved heavy spring isolation of the microscope inside the DR, which yields good results. However, it makes the design and practical use much more complicated, since any material shows a finite susceptibility and hence undesired motion in strong magnetic fields. Despite the toploading probe design, and despite having no spring isolation on the microscope module, the tuning fork AFM system reaches 2.9 nm rms z noise (5.12 ms). This is roughly on the same order of magnitude as previously reported in literature  including a heavy 6 kg damping stage inside the DR. At typical tip-sample distances of a few tens of nanometers, this is well suitable for scanning gate microscopy experiments.
 M. Pelliccione et al., Rev. Sci. Instrum. 84, 033703 (2013)