Magnetic order influenced phonon and electron dynamics inMnBi2⁢Te4and Sb-dopedMnBi2⁢Te4investigated by terahertz time-domain spectroscopy

MnBi2⁢Te4, the first topological insulator with inherent magnetic ordering, has recently attracted significant attention for providing a platform to realize several exotic quantum phenomena at relatively higher temperatures. In this work, we have carried out an exhaustive investigation of MnBi2⁢Te4 and Sb-doped MnBi2⁢Te4 thin films using THz time-domain spectroscopy. The extracted real THz conductivity displays a strong IR active E𝑢 phonon absorption peak (at ∼1.5 THz) merged on top of the Drude-like contributions from bulk and surface electrons. The extracted parameters from the THz conductivity data fitted to the Drude-Fano-Lorentz model show significant changes in their temperature dependence around the magnetic ordering Néel temperature, which is suggestive of the coupling between magnetic ordering and electronic band structure. The frequency of the 𝐸𝑢 phonon displays an anomalous blueshift with increasing temperatures by ∼0.1 THz (∼7%) for MnBi2⁢Te4 and ∼0.2 THz (∼13%) for Sb-doped MnBi2⁢Te4 between 7 and 250 K. The line shape of the 𝐸𝑢 phonon mode in Sb-doped MnBi2⁢Te4 shows significant Fano asymmetry compared to that of MnBi2⁢Te4, indicating that Sb doping plays an important role in the Fano interference between the phonons and the electrons, in this system. These results indicate that the anomalous phonon behavior seen in MnBi2⁢Te4 arises mainly from a positive cubic anharmonicity-induced self-energy parameter, whereas both anharmonicity and the electron phonon coupling are at play in making the relatively higher anomalous blueshift of phonons in Sb-doped MnBi2⁢Te4. Our studies provide a comprehensive understanding of the phonon and electron dynamics of MnBi2⁢Te4 and Sb-doped MnBi2⁢Te4 in the THz range using time-domain THz spectroscopy.