Ultrafast carrier dynamics in the thin films of the topological insulator Bi2Se3

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Date/Time:Thursday, 03 Oct 2013 - Thursday, 03 Oct 2013
Location:PHYSICS Hall Room 3
Channel:Condensed Matter Physics
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Yuri Glinka
Research Assistant Professor
West Virginia University

Topological insulator (Tis) are novel electronic materials that have an insulator-type band gap in the bulk (for Bi2Se3 Eg ~ 0.3 eV) but have protected gapless conducting phase on their surface due to the combination of spin-orbit interactions and time-reversal symmetry. The most effective experimental methods currently used to monitor metallic two-dimensional (2D) Dirac surface states (SS) of TIs are angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (TrARPES). These techniques are equally sensitive to SS and the bulk atoms residing in the close proximity to the surface as a consequence of the extremely small penetration depth (a few nm) of incident energetic photons used for photoemission, combined with the limited escape depth of the electrons (also a few nm).
Reaching a similar sensitivity to SS using traditional optical pump-probe techniques (like transient reflectivity (TR)/transmission), which use less energetic photons in the visible/infrared range, seems problematic since the absorption length of the laser light normally used for these measurements (a few tens of nm) significantly exceeds the range where the effect of SS can actually be monitored. As a result, the transient optical response of Bi2Se3 is dominated by the bulk contribution. The novel experimental techniques recently developed at the Department of Physics of West Virginia University allow us to overcome the problem owing to the surface sensitivity of the methods. One of the methods is based on the acoustic surface waves generation and monitoring within the pump-probe reflectivity transient measurements. The surface state sensitivity in this case is achieved through the traveling of surface acoustic wave in the plane of the sample exclusively through the surface states of TIs. Another surface sensitive technique is based on the centrosymmeric nature of TIs, which governs exclusively surface-related response to the optical second harmonic generation (SHG) process. Transient reflectivity, transient SHG, stationary SHG rotational anisotropy, and transient SHG rotational anisotropy measurements have been applied to study thin films (6 - 40 nm in thickness) of TIs Bi2Se3. The recent experimental results on the ultrafast dynamical properties of TIs will be presented and discussed