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Investigation of magnetic order influenced phonon and electron dynamics in MnBi 2 Te 4 and Sb doped MnBi 2 Te 4 through terahertz time-domain spectroscopy

Authors: Soumya Mukherjee, Anjan Kumar NM, Subhadip Manna, Sambhu G Nath, Radha Krishna Gopal, Chiranjib Mitra, N. Kamaraju

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The interplay of magnetism and topological properties can provide a unique platform for realization of several novel quantum phenomena such as quantum anomalous Hall effect and axion insulator phases. Conventional methods of introducing magnetic order in topological materials like magnetic impurity doping, and magnetic proximity effects provide limited accessibility due to their very low operating temperature (sub kelvin regime) of investigation. With the recent introduction of the first stoichiometric anti-ferromagnetic (AFM) topological insulator (TI) MnBi 2 Te 4 , which hosts inherent magnetic ordering, the mentioned challenges have been repressed to a significant extent. Along with the non-trivial topological properties, MnBi 2 Te 4 provides an avenue to open an exchange gap in TSS due to magnetic ordering induced time reversal symmetry breaking below temperature of ~25 K. However, the nature and the occurrence of the exchange gap, are found to be highly sample specific and have invited contrasting views based on ARPES measurements and IR spectroscopic techniques. As the appearance of the exchange gap mostly influences the low energy (few to tens of meV) spectroscopic responses of MnBi 2 Te 4 , THz-TDS technique becomes instrumental for their in-depth characterizations. Moreover, the observation of IR active E u phonon mode (~1.5 THz, predicated in lattice dynamical calculations of MnBi 2 Te 4 ) and realization of its potential coupling with continuum of surface electron excitations are achievable through THz-TDS measurements.

Here, we have carried out time domain terahertz (THz) Spectroscopy measurements on PLD (pulsed laser deposition) grown epitaxial thin films of MnBi 2 Te 4 (MBT) and Sb doped MnBi 2 Te 4 (MBST) in a temperature range of 7K to 250K. We find that the extracted THz conductivity displays a strong Eu phonon absorption peak (at ∼1.5 THz) merged on top of the Drude like background contribution of bulk and surface electrons for both samples. 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 Neel temperature of ∼ 25K, which is suggestive of the coupling between magnetic ordering and electronic band structure. The frequency of the Eu phonon displays an anomalous blue-shift with increasing temperatures by ∼ 0.1 THz (∼7%) for MnBi2Te4 and ∼0.2 THz (∼13%) for Sb doped MnBi2Te4. The line-shape of the Eu phonon mode in Sb doped MnBi2Te4 shows significant Fano asymmetry compared to that of MnBi2Te4, indicating that Sb doping plays an important role in the Fano interference between the bulk phonons and the surface electrons, here. A positive value of cubic anharmonicity induced phonon self-energy parameter is attributed to the anomalous blue shift of the Eu phonon mode in MnBi 2 Te 4 . Whereas the relatively higher anomalous shift in the phonon frequency of Sb doped MnBi2Te4 indicates the combined contribution of phonon anharmonicity and electron-phonon coupling, which is consistent with the emergence of Fano asymmetry in it. Our studies suggest THz-TDS is an efficient non-contact technique for probing the behaviour of charge carriers, phonons and their coupling in magnetic TIs. Furthermore, modulation of Fano asymmetry via doping and as a function of temperature has also been realized here. Thus, our investigations pave the way for fabricating a new class of quantum devices using MnBi 2 Te 4 , where both tunable Fano resonance and magnetic order influenced topological properties can be utilized.



#Research Highlight

Posted on: November 2nd, 2024