Imaging Dirac-Mass Disorder from Magnetic Dopant-Atoms in the Ferromagnetic Topological Insulator Crx(Bi0.1Sb0.9)2-xTe3

Topological insulators (TI) have a gapless surface state of Dirac fermions protected by the time reversal symmetry (TRS). However, TRS can be often broken in the ferromagnetic state induced by magnetic doping, leading to the opening of “mass gap” at the Dirac point. This gap is predicted to involve many exotic phenomena for which understanding the microscopic role of magnetic dopants is important. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale. In this talk I will present the study of Dirac-mass gap ∆(r) in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92Te3 single crystals measured at 4.5 K by spectroscopic imaging scanning tunneling microscopy (SI-STM). ∆(r) identified by spectroscopic analysis in both real and momentum space shows a strong disorder, but has an anticipated relationship ∆(r) ∝ n(r), where n(r) is the areal Cr atom density. These observations reveal how magnetic dopant atoms actually generate the TI mass gap and that control of the resulting Dirac-mass gap disorder will be essential to achieve the novel physics expected of TRS breaking TI materials.