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Title: Synthesis and physical properties of magnetic Bi₂Se₃-based topological materials
Date & Time: 15:00, Mar 4（Wednesday）, 2026
Host: Prof. Vadim Grinenko
Zoom Meeting link: https://zoom.us/j/99100721966?pwd=khiRJYvcedwKaKJLxhr2lJUgBmARpX.1
Zoom Meeting ID: 99100721966, password: 260304
Abstract:
Magneto-transport, thermoelectric, magnetic, and Hall effect properties of pristine and doped Bi₂Se₃-based topological insulators have been systematically investigated to elucidate the interplay between topology, magnetism, and charge transport. In Co-doped Bi₂Se₃, the undoped sample exhibits a maximum magnetoresistance (MR) governed by destructive quantum interference arising from a π-Berry phase, leading to a reduction in MR. With increasing Co concentration, the MR becomes increasingly linear. While the MR behavior of pristine Bi₂Se₃ can be described using a classical model, the low-temperature MR response of Co-doped samples deviates from this framework and is well explained by the quantum linear MR model. Magnetization measurements reveal the emergence of ferromagnetic ordering upon Co doping, which is further corroborated by Hall effect measurements through the observation of an anomalous Hall effect. When spectral weight suppression is insignificant, Bi₂Se₃ exhibits dilute magnetic semiconductor behavior. Thermoelectric measurements show that the power factor is maximized when time-reversal symmetry (TRS) is preserved, whereas TRS breaking leads to a reduction in the power factor. This reduction is accompanied by enhanced MR linearity and anomalous Hall response, indicating that the upward shift of the Dirac cone relative to the Fermi level plays a crucial role in governing transport properties.
To further explore the influence of non-magnetic disorder on magneto-transport, the properties of Bi₂Se₃₋ᵧSᵧ were examined. The MR progressively decreases with increasing S content and eventually becomes negative at 7% S doping (y = 0.21), an unusual behavior observed even when the magnetic field is applied perpendicular to the sample plane. The appearance of Shubnikov–de Haas (SdH) oscillations confirms the coexistence of surface and bulk conduction channels, and the negative MR is attributed to non-trivial bulk conduction mechanisms.
Extending this investigation to magnetic and non-magnetic co-doping, Bi₂₋ₓFeₓSe₃₋ₓSₓ was studied using dc-magnetization, magneto-transport, and angle-resolved photoemission spectroscopy (ARPES). A giant negative MR is observed under parallel electric and magnetic fields (H ∥ E). At low magnetic fields, the MR behavior follows semi-classical transport theory, whereas at higher fields it is dominated by the axial anomaly, with the system reaching the quantum limit at a relatively low magnetic field (~4.5 T). The observed magnetic ordering originates from the combined effects of surface-mediated RKKY interactions and bulk van Vleck magnetism. ARPES measurements reveal suppression of the surface gap as the magnetic ordering evolves from ferromagnetic to antiferromagnetic, while the persistence of SdH oscillations and clear surface state signatures confirm that the topological surface properties remain robust under Fe and S co-doping.
Biography:
Rahul Singh received his M.Tech. degree from the Department of Ceramic Engineering, IIT (BHU), Varanasi, where his research focused on magnetic and multiferroic materials such as BiFeO₃ and BiMnO₃. He subsequently earned his Ph.D. degree from the Department of Physics, IIT (BHU), Varanasi, India. He worked as a postdoctoral fellow in the Solid-State Physics Division, Bhabha Atomic Research Centre (BARC), Mumbai, from March 2019 to September 2021. In 2021, he was awarded a Dr D. S. Kothari Fellowship in the Department of Physics, Banaras Hindu University (BHU), India. His research interests include magnetic materials, topological insulators, topological semimetals, and the synthesis of various single-crystal cathode materials for rechargeable batteries.