| AVS 61st International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Wednesday Sessions |
| Session SP+AS+BI+NS+SS-WeA |
| Session: | Advances in Scanning Probe Microscopy |
| Presenter: | Fumio Komori, University of Tokyo, Japan |
| Authors: | F. Komori, University of Tokyo, Japan S. Kim, University of Tokyo, Japan S. Yoshizawa, University of Tokyo, Japan Y. Ishida, University of Tokyo, Japan K. Eto, Osaka University, Japan K. Segawa, Osaka University, Japan S. Shin, University of Tokyo, Japan Y. Ando, Osaka University, Japan |
| Correspondent: | Click to Email |
Three-dimensional (3D) topological insulators (TIs) are accompanied by gapless surface states due to a nontrivial Z2 topology of the bulk wave functions. The topological surface state (TSS) of a 3D TI is helically spin polarized, which leads to a suppression of electron scatterings due to spin mismatch between the eigenstates before and after the scattering. The suppression has been inferred from the measurements of quasiparticle interference (QPI) using scanning tunneling microscopy. No QPI was observed for intraband scatterings within unwarped TSSs. However, it has not been clear to what extent the scattering is suppressed within TSS.
In the present study, we have elucidated how the elastic scattering is suppressed as a function of the scattering angle and electron energy in the helically-spin-polarized surface electrons in a single and unwarped upper Dirac cone of Bi1.5Sb0.5Te1.7Se1.3. In this material [1], EF is located very close to the Dirac energy ED. We compared the scattering wave vectors observed at 5 K with the diameters of the constant-energy contours of the unoccupied TSS which was measured by using time-resolved ARPES implementing a pump-probe method. Moreover, the inelastic scattering time of unoccupied TSS was directly obtained by this method.
At the energy above ED, we found that there is a sharp threshold for the length of the scattering vector, above which the observed QPI intensity is abruptly diminished [2]. Such a threshold indicates the existence of a well-defined critical scattering angle beyond which elastic scattering is suddenly suppressed. The observed protection from backscattering in the TSS occurs not only for 180° but also for a wide range of angles between 100° and 180°. Such a wide angle range for the protection from backscattering is found to be essentially independent of the energy up to 300 meV above ED until the Dirac cone becomes warped and/or the bulk scattering events intervene. At energies higher than 300 meV, we found hexagonal patterns in the FT-QPI images that come from warping of the TSS Dirac cone. In this energy range, the critical scattering vector was not clearly observed, indicating a different mechanism of the protection from backscattering in the warped Dirac cone. The observed inelastic scattering lifetime of TSS is longer than 10 psec just above EF. The robust protection from the backscattering and long inelastic scattering in the TSS strongly support the possible applications for electronics and spintronics using weak electron scattering of TSS at EF.
References
1. A. A. Taskin et.al, Phys. Rev. Lett. 109, 066803 (2012).
2. S. Kim, et al, Phys. Rev. Lett. 112, 136802 (2014).