Nickel: The time-reversal symmetry conserving partner of iron on a chalcogenide topological insulator

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We report on the quenching of single Ni adatom moments on Te-terminated Bi₂Te₂Se and Bi₂Te₃ topological insulator surfaces. The effect is noted as a missing x-ray magnetic circular dichroism for resonant L3,2 transitions into partially filled Ni $3d$ states of theory-derived occupancy nd=9.2. On the basis of a comparative study of Ni and Fe using scanning tunneling microscopy and ab initio calculations, we are able to relate the element specific moment formation to a local Stoner criterion. Our theory shows that while Fe adatoms form large spin moments of mₛ=2.54μB with out-of-plane anisotropy due to a sufficiently large density of states at the Fermi energy, Ni remains well below an effective Stoner threshold for local moment formation. With the Fermi level remaining in the bulk band gap after adatom deposition, nonmagnetic Ni and preferentially out-of-plane oriented magnetic Fe with similar structural properties on Bi₂Te₂Se surfaces constitute a perfect platform to study the off-on effects of time-reversal symmetry breaking on topological surface states.