Direct Real Space Imaging of Quantum Spin Hall Edge States in HgTe Quantum Well*

Microscopic real space imaging of the helical edge states is an important milestone to fully elucidate quantum spin Hall effect as a new state of quantum matter. By employing a unique cryogenic microwave impedance microscope, we directly imaged quantum spin Hall edges in a gapped HgTe quantum well. The edge state size increases monotonically as the Fermi level is tuned from p-type across the Dirac point into n-type. Whereas this result is counter-intuitive within any particle-hole symmetric model, it actually agrees well with the 8-band model of real material. Real space evolution of the edge states shows surprising dependence on the magnetic field which could not be explained by Landau level physics assuming a clean system. Alternative scenarios will be discussed.