Direct Observation of Broken Time-Reversal Symmetry on the Surface of a Magnetically Doped Topological Insulator Yoshinori Okada, 1 Chetan Dhital, 1 Wenwen Zhou, 1 Erik D. Huemiller, 1 Hsin Lin, 2 S. Basak, 2 A. Bansil, 2 Y.-B. Huang, 3 H. Ding, 3 Z. Wang, 1 Stephen D. Wilson, 1 and V. Madhavan 1 1 Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA 2 Physics Department, Northeastern University, Boston, Massachusetts 02115, USA 3 Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China (Received 20 January 2011; revised manuscript received 4 March 2011; published 17 May 2011) We study interference patterns of a magnetically doped topological insulator Bi 2x Fe x Te 3þd by using Fourier transform scanning tunneling spectroscopy and observe several new scattering channels. A comparison with angle-resolved photoemission spectroscopy allows us to unambiguously ascertain the momentum-space origin of distinct dispersing channels along high-symmetry directions and identify those originating from time-reversal symmetry breaking. Our analysis also reveals that the surface state survives far above the energy where angle-resolved photoemission spectroscopy finds the onset of continuum bulk bands. DOI: 10.1103/PhysRevLett.106.206805 PACS numbers: 73.20.r, 74.55.+v When spin-orbit coupling is strong enough, a band-parity inversion can be generated around the direct conduction- valence band gap of an insulator. Such topological insula- tors (TIs) support gapless surface states that in the low-energy and long-wavelength limit have linear energy- momentum dispersions. The extraordinary properties of these Dirac surface states translate into a unique potential for new types of devices and for realizing novel physical phenomena [1–3]. Since massless Dirac fermions are he- licity eigenstates, the states carry a helical spin texture; the electron’s spin is normal to the direction of its lattice momentum. The constraints of time-reversal (TR) symme- try prohibit direct backscattering between the time-reversed pair of helicity eigenstates, which carry opposite momenta and spin and restore these channels when TR symmetry is violated. Direct experimental determination of scattering processes is important not only for establishing their pre- dicted spin texture and its role within the protected surface phase but also for theoretical modeling of the physical properties of these materials. Intensive STM [4–7] and angle-resolved photoemission spectroscopy (ARPES) [8–10] studies have beautifully demonstrated many of the canonical properties including the absence of backscatter- ing in TR invariant TIs. However, while the effects of magnetic impurities on bulk and surface band structures have been extensively studied [11–13], to date no direct observations of TR violating scattering vectors have been reported. In this work, we probe the scattering processes in Fe-doped Bi 2 Te 3 by the technique of using the Fourier transform of STM dI=dV maps (spectroscopic maps) to obtain q-space information (FT-STS) [14–16] to show the emergence of new scattering channels due to broken TR symmetry. Our Fourier transform STM data combine high momentum resolution [spatial maps with a minimum linear dimension of 1500 A ˚ were used for the fast-Fourier transform (FFT)] with data over a large range of energy (up to 600 meV above the Dirac point). By comparing these high-resolution data to ARPES, we were able to distinguish the momentum-space (k-space) origins of the STM scat- tering vectors and show that the new channels arise from forbidden backscattering. With an unambiguous identifi- cation of the scattering channels, we find a surprising result: The dispersion at higher energies indicates the survival of the surface states well beyond the onset of the conduction band. This suggests that their topological na- ture may play an unexpected role in protection against the mixing with the bulk states beyond the insulating band gap. We focus on Fe-doped Bi 2 Te 3 single crystals with a nominal Fe doping of 0.25%, ðBi 1x Fe x Þ 2 Te 3 (x ¼ 0:0025). Sample preparation methods as well as ARPES and STM experimental details are described in Ref. [17]. Bi 2 Te 3 belongs to a new generation of TR invariant 3D topological insulators of the type Bi 2 X 3 (X ¼ Te; Se, etc.) whose discovery has been pivotal in this field [18–21]. Fe doping acts to locally break TR symmetry, which has many interesting consequences [22–26]. In Bi 2 Te 3 , warping of the surface state due to the threefold symmetric crystal potential and interaction with the bulk bands provides an enhancement of certain scattering chan- nels over others, allowing them to be observed by STM. Bi 2 Te 3 cleaves between the quintuple layers terminating in a Te surface [Fig. 1(a)]. For our studies, we have deliber- ately doped a dilute concentration of Fe atoms, which are expected to enter substitutionally in the Bi plane but may also appear as defects in the Te plane. FFT of the STM topography shows the hexagonal lattice associated with the Te atoms [Fig. 1(a), inset]. dI=dV spectra reveal a sup- pression of density of states near the Fermi energy (E F ) [Fig. 1(b)]. Although it is not straightforward to extract the PRL 106, 206805 (2011) PHYSICAL REVIEW LETTERS week ending 20 MAY 2011 0031-9007= 11=106(20)=206805(4) 206805-1 Ó 2011 American Physical Society