Terahertz Faraday and Kerr rotation spectroscopy of Bi${}_{1-x}$Sb${}_x$ films in high magnetic fields up to 30 Tesla

Abstract

We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of ${\text{Bi}}_{1-x}{\text{Sb}}_x$, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition $x$ increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed mini-coil magnet, we conducted measurements in magnetic fields up to 30~T, observing distinctly different behaviors between semimetallic ($x<0.07$) and topological insulator ($x>0.07$) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blue-shifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields ($>$20~T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi${}_{1-x}$Sb${}_x$.

arXiv comment17 pages, 22 figures

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• Chiral Phonons with Giant Magnetic Moments in a Topological Crystalline Insulator
• Time-Domain Terahertz Spectroscopy in High Magnetic Fields
• Multimode Ultrastrong Coupling in Three-Dimensional Photonic-Crystal Cavities
• Shaping terahertz waves using anisotropic shear modes in a van der Waals mineral
• Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet
• Observation of Colossal Terahertz Magnetoresistance and Magnetocapacitance in a Perovskite Manganite
• Ultrafast scattering dynamics of coherent phonons in Bi${}_{1-x}$Sb${}_x$ in the Weyl semimetal phase