Milli-Kelvin Microwave Microscopy of Quantum Anomalous Hall States

Near-field microwave impedance microscopy (MIM) is a powerful technique that can visualize

sub-surface conductivity distribution with a spatial resolution on the order of 100 nm. In this

work, we report the implementation of a dilution-refrigerator-based MIM with a base

temperature of ~ 100 mK. The vibration noise of our apparatus with tuning-fork feedback control

is as low as 1 nm. Using this setup, we have successfully performed nanoscale imaging of

quantum anomalous Hall (QAH) states in magnetically (Cr and V) doped (Bi,Sb) 2 Te 3 thin films

grown on mica substrates. At high magnetic fields, the conductive chiral edge modes are clearly

observed in the MIM images, consistent with the quantization of Hall conductance in the

transport data. The two topological phase transitions near the coercive fields of Cr and V are also

vividly visualized in the field-dependent results. Our work establishes the experimental platform

for the investigation of nanoscale quantum phenomena under ultralow temperatures.