Manipulating Coherent Transport in Topologically Protected Surface States

Manipulating Coherent Transport in Topologically Protected Surface States
Ultrafast THz pump and THz probe spectroscopy for Bi2Se3. (Central) Schematic of the surface and bulk structures driven by ultrashort laser pulses.  A 2D false-color plot of pump-induced THz field changes as a function of gate time after 248 meV (5 µm)  photoexcitation at (right panel) T=300 K and (left panel) T=5 K, respectively. Plotted together are the corresponding static THz fields (green curves) and their pump-induced changes at 1 ps (blue curves from the cut positions as marked by the blue dashed lines).

Topology-protected charge transport on surfaces of three-dimensional topological insulators (TIs) is breaking new ground in quantum science and technology. One of the open questionsfor these novel surface states is how to disentangle and ultimately control surface helical spin transport. Here we use the mid-infrared (mid-IR) and terahertz (THz) photoexcitation, tuned specifically to the intraband transitions, to manipulate the surface and bulk THz conductivity in n-type Bi2Se3. We observe frequency-dependent carrier cooling times of photoinduced THz conductivity are clearly differentiating surface from bulk contributions and allow determination of their scattering rates. We show thatthe topological enhancement of surface transport is responsible for suppressing the surface electron scattering rates compared to the bulk by nearly a factor of 4 in equilibrium. This result is consistent with surface helical spin transport in the presence of short-range disorder. The ultrabroadband, wavelength-selective pumping may be applied to emerging topological semimetals for separation and control of the protected transport connected with the Weyl nodes from other bulk bands.

L. Luo, X. Yang, X. Liu, Z. Liu, C. Vaswani, D. Cheng, M. Mootz, X. Zhao, Y. Yao, C.-Z. Wang, K.-M. Ho, I. E. Perakis, M. Dobrowolska, J. K. Furdyna, and J. Wang, “Ultrafast Manipulation of Topologically Enhanced Surface Transport Driven by Mid-Infrared and Terahertz Pulses in Bi2Se3”, Nature Communications, 10:607 (2019), DOI: 10.1038/s41467-019-08559-6.