Terahertz Faraday and Kerr rotation spectroscopy of Bi1−xSbx 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 Bi1−xSbx, 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 Bi1−xSbx. arXiv:1907.00137v1 [cond-mat.mes-hall]

Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet

“…It should be noted that both Faraday and Kerr rotation signals can be extracted in a transmission configuration. [34][35][36][37] The THz pulse that directly transmits through the sample contains only the Faraday rotation signal. Nevertheless, the back reflection pulse experiences both Faraday and Kerr rotation due to additional reflection events at the interfaces.…”

“…In addition, polarization-dependent THz spectroscopy allows one to determine both the diagonal and off-diagonal elements of the conductivity tensor through Faraday and Kerr rotations. 34,37,[56][57][58][59][60][61]…”

“…sion onstrated optical-pump/THz-probe measurements in bulk, intrinsic silicon in ds up to 30 T by developing a single-shot THz-TDS system around a minicoil parameters, the authors concluded that the feature is mainly caused by bulk hole cyclotron resonance. 37,77,78) Figures 12a and 12b show, respectively, θ F (ω, B) and θ K (ω, B) of the Bi 0.9 Sb 0.1 film versus frequency and B up to 30 T at a temperature of 21 K. At a fixed B, both θ F and θ K spectra are featureless. Nevertheless, their values increase with increasing B and finally saturate when B is above 15 T. In contrast to the semimetallic film, a surface band model was used to analyze the experimental data.…”

Optical and Galvanomagnetic Properties of Bi1-xSbx Thin Films in the Terahertz Frequency Range

“…Mobility and concentration of electrons and holes in the trigonal plane (the film plane) were calculated using Equations (7)- (9). Calculation takes into account that solid solutions of bismuth-antimony are intrinsic semimetals which means that they have equal concentrations of electrons and holes.…”

“…Many studies are dedicated to investigation of dependency of Bi 1−x Sb x sheet DC conductivity on antimony content, film thickness, and temperature [1][2][3][4][5]. In two recent works it was shown that thin bismuth-antimony films have tunable optical response in the THz frequency range [8,9]. In the work of Katayama et al, the optical response of charge carriers was investigated in thin films of Bi and Bi 1−x Sb x (x from 0 to 16 %) in a frequency range up to 50 THz, but the measurements were performed only for film thickness of 40 nm [8].…”