Abstract
Mid- and near-infrared measurements reveal that the optical conductivity of the three-dimensional topological insulator, Bi2Te2Se, is dominated by bulk carriers and shows a linear-in-frequency increase at 0.5 to 0.8 eV. This linearity might be interpreted as a signature of three-dimensional (bulk) Dirac bands; however, band-structure calculations show that transitions between bands with complex dispersion contribute instead to the inter-band optical conductivity at these frequencies and, hence, the observed linearity is accidental. These results warn against the oversimplified interpretations of optical-conductivity measurements in different Dirac materials.
Highlights
Spin-orbit coupling often leads to the formation of linear bands in solids. Electrons in such bands manifest themselves in special ways in different experiments [1,2,3,4,5]. One of these manifestations is in their optical response: the contribution of a d-dimensional Dirac band to the inter-band optical conductivity, which is calculated to follow a simple power–law frequency dependence [6,7]: σ(ω) ∝ ωd−2
The linearity in σ(ω) over a broad frequency range in a 3D electron system is often considered as a ”smoking gun” for Dirac physics
We have experimentally found that the bulk optical conductivity of Bi2 Te2 Se is linear in frequency at 4000 to 7000 cm−1 (~0.5–0.8 eV)
Summary
Spin-orbit coupling often leads to the formation of linear bands in solids Electrons in such bands (the Dirac electrons) manifest themselves in special ways in different experiments [1,2,3,4,5]. One of these manifestations is in their optical response: the contribution of a d-dimensional Dirac band to the inter-band optical conductivity, which is calculated to follow a simple power–law frequency dependence [6,7]: σ(ω) ∝ ωd−2 . Timusk et al [14] suggested the presence of 3D Dirac fermions in a number of quasicrystals, based entirely on the observation of a linear σ(ω) in these materials
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