Quasiparticle interference on the surface of 3D topological insulator Bi$_{2}$Se$_{3}$ induced by cobalt adatom in the absence of ferromagnetic ordering

Quasiparticle interference induced by cobalt adatoms on the surface of the topological insulator Bi$_{2}$Se$_{3}$ is studied by scanning tunneling microscopy, angle-resolved photoemission spectroscopy and X-ray magnetic circular dichroism. It is found that Co atoms are selectively adsorbed on top of Se sites and act as strong scatterers at the surface, generating anisotropic standing waves. A long-range magnetic order is found to be absent, and the surface state Dirac cone remains gapless. The anisotropy of the standing wave is ascribed to the heavily warped iso-energy contour of unoccupied states, where the scattering is allowed due to a non-zero out-of-plane spin.