Single Vanadium Atom Magnetism on the Topological Insulator Bi2Te3 Surface.

Magnetically doped topological insulators host emergent quantum phases such as the quantum anomalous Hall effect and the topological magnetoelectric effect, whose realization depends critically on the magnitude and orientation of the dopant magnetic moments. Here, we use low-temperature scanning tunneling microscopy and inelastic tunneling spectroscopy to probe the magnetism of individual vanadium atoms adsorbed on Bi2Te3. Spin-flip excitations resolved on single V atoms provide direct evidence of local magnetic moments. The excitation energy evolves nonmonotonically with an out-of-plane magnetic field, revealing an in-plane magnetic easy axis imposed by the local crystal field. At higher fields, the ground-state moment is driven from an in-plane to an out-of-plane orientation, in quantitative agreement with spin-Hamiltonian analysis and first-principles calculations. Quasiparticle interference measurements further show no detectable backscattering of the topological surface states around V atoms. Our results offer microscopic insight into the magnetically doped topological insulators and provide guidance for engineering magnetic topological phases at the single-atom limit.