‘Topological materials’ host electrons with wavefunctions that are twisted in a topologically non-trivial manner. The addition of magnetism to such materials profoundly influences these topological electronic states, giving rise to manifestations of quantum mechanics at macroscopic length scales, such as dissipationless, quantized edge currents and magneto-electric effects.
At the heart of these phenomena are exchange couplings - those that induce magnetic order, and those that couple the magnetic order to the topological electronic states. In my doctoral work, I experimentally quantified these exchange couplings and discovered new routes towards manipulating them in the magnetic topological insulator MnBi2Te4.
Phonons as a knob to control the interlayer exchange interaction
MnBi2Te4 is the first instance of a topological insulator with intrinsic magnetism. Being a quasi-two-dimensional layered magnet, long-range magnetic order in this material is highly sensitive to the weak interlayer exchange interaction. Here, using magneto-Raman spectroscopy, we show that selected lattice vibrational modes strongly modulate the interlayer exchange interaction, enabling a new knob to control magnetism, and hence, band topology.
(left) Eigendisplacements of two phonon modes that strongly modulate the interlayer exchange interaction. (right) Change in Raman scattering intensities of the two phonons across magnetic phase transitions.
H. Padmanabhan, M. Poore, P. Kim, N. Z. Koocher, V. A. Stoica, D. Puggioni, H. Wang, X. Shen, A. H. Reid, M. Gu, M. Wetherington, S. H. Lee, R. D. Schaller, Z. Mao, A. M. Lindenberg, X. Wang, J. M. Rondinelli, R. D. Averitt, V. Gopalan. Nature Communications 13, 1929 (2022)
Quantifying the exchange coupling between localized spins and topological bands
Magnetic topological phenomena rely on the presence of a strong exchange interaction between magnetic order and the topological bands. We quantify this exchange interaction in MnBi2Te4, by optically pumping the p-like topological bands, and probing the picosecond-timescale response of the Mn 3d spins using a suite of ultrafast probes. We find that this p-d exchange interaction is a hundred times larger than the weak d-d superexchange interaction between Mn spins.
(left) Schematic of ultrafast energy and angular momentum transfer across different subsystems in MnBi2Te4. (right) Time-resolved resonant soft X-ray scattering at the Mn L-edge to probe the response of Mn 3d spins upon optical excitation of the topological bands.
H. Padmanabhan, V. A. Stoica, P. K. Kim, M. Poore, T. Yang, X. Shen, A. H. Reid, M-F. Lin, S. Park, J. Yang, H. Wang, N. Z. Koocher, D. Puggioni, A. B. Georgescu, L. Min, S. H. Lee, Z. Mao, K. M. Rondinelli, A. M. Lindenberg, L-Q. Chen, X. Wang, R. D. Averitt, J. F. Freeland, V. Gopalan. Advanced Materials 34, 49, 2202841 (2022)