On-and-off chip magnetic cooling for semiconductor nanostructures below 1 mK*

Magnetic cooling has the potential to make the regime beyond dilution refrigerators and below 1 mK accessible to experiments, to investigate interesting novel physics such as exotic quantum phases, new (topological) quasi particles and unprecedented quantum coherence, e.g. in quantum transport. In Basel, the strategy is to cool each of the sample wires with its own, separate demagnetization refrigerator and to simultaneously provide on-chip cooling. This approach reduces external heat leaks (off-chip cooling) and provides local cooling (on-chip) where it’s most effective.
This strategy has proven highly successful in metallic Coulomb blockade thermometers, where temperatures below 1 mK were recently demonstrated. Efforts in semiconductors such as 2D gases and nanostructures are underway, integrating ideas such as large metallic reservoirs on low-resistance ohmic contacts and metallic back gates for on-chip cooling. Semiconductor noise of typical micro-eV scale (corresponding to 10s of mK) presents a serious obstacle for on-chip thermometry. We are exploring methods which are insensitive to charge-noise and employ these as integrated on-chip microkelvin thermometers.