Abstract-Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

R. M. Ireland, Liang Wu, M. Salehi, S. Oh, N. P. Armitage, and H. E. Katz

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.044003

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by utilizing a nonvolatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is shifted toward the Dirac point with this method. Using terahertz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (approximately 50 meV above the Dirac point and 170 meV below the conduction-band minimum), and it is stabilized in the intrinsic regime while enhancing electron mobility. The mobility of surface state electrons is enhanced to a value as high as approximately $1600{\mathrm{cm}}^2/Vs$ at 5 K.

• 
• 
• 
• 
• 
• 

Abstract- Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

 Liang Wu, M. Salehi, N. Koirala, J. Moon, S. Oh, N. P. Armitage

http://esciencenews.com/sources/science.now/2016/12/02/report.quantized.faraday.and.kerr.rotation.and.axion.electrodynamics.a.3d.topological.insulator

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here, we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to ~30 meV above the Dirac point and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 tesla, we observed quantized Faraday and Kerr rotations, whereas the dc transport is still semiclassical. A nontrivial Berry’s phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine-structure constant based on a topological invariant of a solid-state system