Resolving the complex interplay between surface and bulk response is a long-standing issue in the topological insulators (TIs). Some studies have reported surface-dominated metallic responses, yet others show semiconducting-like bulk photoconductance. Using ultrafast terahertz spectroscopy with the advent of Fermi-level engineered TIs, we discovered that such difference arises from the time-dependent competing process of two parameters, namely, the Dirac-carrier surface scattering rate and the bulk Drude weight. After infrared femtosecond pulse excitation, we observed a metal-like photoconductance reduction for the prototypical n-type Bi2Se3 and a semiconductor-like increased photoconductance for the p-type Bi2Se3. Surprisingly, the bulk-insulating Bi2Se3, which is presumably similar to graphene, exhibits a semiconducting-to-metallic phase apparent transition at 10 ps. The sign-reversed, yet long-lasting (≥500 ps) metallic photoconductance was observed only in the bulk-insulating Bi2Se3, indicating that such dynamic phase transition is governed by the time-dependent competing interplay between the surface scattering rate and the bulk Drude weight. Our observations illustrate new photophysical phenomena of the photoexcited-phase transition in TIs and demonstrate entirely distinct dynamics compared to graphene and conventional gapped semiconductors.
Bibliographical noteFunding Information:
C.I., J.L., M.N., and H.C. were supported by the National Research Foundation of Korea (NRF) through the government of Korea (MSIP; Grant NRF-2018R1A2A1A05079060), Creative Materials Discovery Program (Grant 2017M3D1A1040828), Scalable Quantum Computer Technology Platform Center (Grant 2019R1A5A1027055), and the Institute for Basic Science (IBS), Korea under Project Code IBS-R014-G1-2018-A1). S.S. was supported by the NRF through the government of Korea (MSIP) (Grant NRF-2019R1F1A1063457) and the Korea Basic Science Institute under the R&D program (Project No. C030440) supervised by the Ministry of Science and ICT. J.M. and S.O. were supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative (Grant No. GBMF4418) and the National Science Foundation (NSF; Grant No. EFMA-1542798). J.K. and D.K. were supported by the Basic Science Research Program through the NRF funded by MSIP (Grant No. NRF- 2015R1C1A1A02037430). S.C., S.Y.S., and M.-H.J. were supported by the Institute for Basic Science (IBS), Korea, under the Project Code No. IBS-R014-A1. W.J., H.J., A.S., and H.L. were supported by NRF through MSIP (Grant No. NRF- 2016R1A4A1012929).
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All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Atomic and Molecular Physics, and Optics
- Electrical and Electronic Engineering