Towards single spin magnetometry at mK temperatures

The liquefaction of helium and the dilution refrigerator have enabled cooling down objects to 4K and even mK temperatures. Fascinating effects, such as superconductivity and strongly correlated electron systems (SCES) emerge at these temperatures and can be studied in appropriate cryogenics experiments. A powerful tool to study magnetic phenomena in such systems is a point lattice defect in diamond, the nitrogen-vacancy (NV) center. The NV center contains an electron spin that offers exceptional properties in terms of coherence time and optical addressability, and can thereby be employed for high-performance magnetic field sensing. Due to its atom-like size, the NV center can be used for nanoscale magnetometry, particularly in a scanning probe configuration where a single NV is located in a tip. This allows for nanometric spatial resolution combined with sensitivities of approximately 1 µT/Hz^0.5. In this work, we pave the way towards NV magnetometry at mK temperature by implementing NV magnetometry in a dilution refrigerator, so far at 4 K, and by conducting transport experiments on a SCES at mK temperature.
At first, two phenomena in the type-II superconductor YBCO are examined at 4K in a liquid helium bath cryostat. On the one hand, the Meissner effect is measured over a thin YBCO disk by directly imaging the penetration of magnetic fields into the superconductor. On the other hand, stray magnetic fields emerging from vortices in the same superconductor are imaged with approximately 30nm resolution. In both cases, we benchmark our findings against existing theoretical models and use this analysis to extract quantitative values for the London penetration depth.
Additionally, we examine out-of-plane (OOP)-oriented NV centers with respect to the scanning plane, which offer benefits such as improved sensitivity and data interpretability. OOP-oriented NV centers in our fabricated scanning probes are uniquely identified and used for nanoscale magnetic imaging for the first time.
The oxide interface LAO/STO, which hosts a two-dimensional electron gas that exhibits SCES physics, is examined in the dilution refrigerator. Transport measurements show signs of superconductivity and laser illumination is found to significantly increase the conductance. In terms of NV magnetometry in a dilution refrigerator at 4 K, current imaging reveals an inhomogeneous current ow through the interface. In contrast to previous findings, no magnetic signatures are found.
Lastly, longitudinal T1 relaxation is studied in a high-density NV ensemble. Relaxation rates at mK are found to be lower than at 4 K, on the order of 1 Hz. However, they are still much higher than in previous findings, possibly explainable by spin diffusion out of the laser focus. At mK temperature, a shift of the thermal spin population is observed, corresponding to the Boltzmann distribution.
Overall, in this thesis we can present meaningful results in several areas, some of which have already been published. In the future, we will use the increased sensitivity of OOP-oriented NV magnetometers and explore magnetism and superconductivity in various exotic materials at mK temperatures.