Rastersondenmikroskop in einem Mischkryostat
Final Report Abstract
The dilution refrigerator and associated instruments have been the basis for developing a novel type of SQUID microscope based on dispersive readout for magnetic scanning measurements at sub-Kelvin temperatures. Superconducting Quantum Interference Devices (SQUIDs) are very sensitive magnetometers. When used for scanning microscopy, they enable a broad range of applications in meso- and nanoscale as well as material physics. Compared to other magnetic imaging techniques, the strengths of SQUID sensors lie in the almost non-invasive measurements and a high sensitivity. Furthermore, field coils integrated into the sensors can be used for local magnetic response measurements, including nonlinear and dynamic effects. The innovative microscope concept of the present instrument enables an improved sensitivity and bandwidth, two key figures of merits for scanning SQUID microscopes that are fundamentally limited for conventional DC-SQUIDs. A major activity in our group was the design and characterization of the SQUID sensors with dispersive readout to be used. We demonstrated about an order of magnitude improvement, reaching a noise level of 80 nΦ0 /√Hz at 4 K and a bandwidth of 200 MHz. At 100 mK as provided by the present setup, our noise model validated by these experiments predicts a noise levels of 30 nΦ0 /√Hz. In parallel, we have integrated our scanner and the associated microwave components into the setup, which involved major construction work beyond the commercially available components. The setup is now operational. The expected sensitivity has not been demonstrated yet due to limitations of the available sensors, but can likely be verified in the near future by changing the operating frequency range. The first science project relying on the instrument is progressing in collaboration with the group of Prof. Yoichi Ando (Cologne University). Our goal is to study superconducting rings that incorporate Josephson junctions based on topological insulators, taking advantage of the specific strength of the instrument. In its first year of operation, the refrigerator has been used for the characterization of gated, doped Si/SiGe quantum dots. This work has been important for the ramp-up of our activity in Si-based spin qubits in collaboration with Dominique Bougeard (Uni Regensburg), now funded by DFG.
Publications
- A micro-SQUID with dispersive readout for magnetic scanning microscopy, Appl. Phys. Lett. 112, 252601 (2018)
F. Foroughi, J.-M. Mol, T. Müller, J. Kirtley, K.A. Moler, and H. Bluhm
(See online at https://doi.org/10.1063/1.5030489)
