title: Terahertz conductivity of topological surface states in Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂.
abstract: Topological insulators are electronic materials with an insulating bulk and conducting surface. However, due to free carriers in the bulk, the properties of the metallic surface are difficult to detect and characterize in most topological insulator materials. Recently, a new topological insulator Bi₁.₅Sb₀.₅Te₁.₇Se₁.₃ (BSTS) was found, showing high bulk resistivities of 1-10 Ω.cm and greater contrast between the bulk and surface resistivities compared to other Bi-based topological insulators. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we present complex conductivity of BSTS single crystals, disentangling the surface and bulk contributions. We find that the Drude spectral weight is 1-2 orders of magnitude smaller than in other Bi-based topological insulators, and similar to that of Bi₂Se₃ thin films, suggesting a significant contribution of the topological surface states to the conductivity of the BSTS sample. Moreover, an impurity band is present about 30 meV below the Fermi level, and the surface and bulk carrier densities agree with those obtained from transport data. Furthermore, from the surface Drude contribution, we obtain a ~98% transmission through one surface layer--this is consistent with the transmission through single-layer or bilayer graphene, which shares a common Dirac-cone feature in the band structure.
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Chi Sin
authors.initials: CS
authors.lastname: Tang
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Bin
authors.initials: B
authors.lastname: Xia
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Xingquan
authors.initials: X
authors.lastname: Zou
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Shi
authors.initials: S
authors.lastname: Chen
authors.affiliation: NUSNNI-NanoCore, Department of Physics, National University of Singapore, 117542, Singapore.
authors.forename: Hong-Wei
authors.initials: HW
authors.lastname: Ou
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Lan
authors.initials: L
authors.lastname: Wang
authors.affiliation: NUSNNI-NanoCore, Department of Physics, National University of Singapore, 117542, Singapore.
authors.forename: A
authors.initials: A
authors.lastname: Rusydi
authors.affiliation: Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos NM 87545, USA.
authors.forename: Jian-Xin
authors.initials: JX
authors.lastname: Zhu
authors.affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Elbert E M
authors.initials: EE
authors.lastname: Chia