Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Stacking layers of two-dimensional materials into heterostructures is an attractive strategy to control the key properties of advanced spin- and optoelectronic devices, including the band gap and the charge transfer efficiency. Thus far, semiconducting transition metal dichalcogenides were the main focus of such approaches, but the recently discovered family of van der Waals heterostructures in the form MnBi2Te4(Bi2Te3)m (m = 1, 2, 3, …) constitutes a novel and interesting target. A layered topological insulator with intrinsic antiferromagnetic ordering, MnBi2Te4, has gained notoriety owing to the realization of exotic phases such as quantum anomalous Hall effect and axion insulator states. The strength of magnetic interaction in this system can be adjusted by introducing non-magnetic quintuple layers (QLs) of Bi2Te3 between magnetic septuple layers (SLs) of the base material MnBi2Te4, but the influence of interlayer interactions on the ultrafast charge carrier dynamics in this system remained unexplored.

Here, we used the time- and angle-resolved photoelectron spectroscopy (TR-ARPES) end-station at T-ReX (FERMI) to investigate the ultrafast response of magnetic heterostructures MnBi2Te4(Bi2Te3)m (m = 0, 1) to infrared optical excitation. We found that in MnBi4Te7 (m = 1), the states derived from Bi2Te3 surface layer are preferentially occupied, and that the carrier extraction into the adjacent MnBi2Te4 layers is fluence-tunable and may take up to 350 fs. The thermal relaxation of the excited carriers is mediated by the intralayer phonon scattering, leading to slower decay rates within the SL.

Figure 1 of the tops-story by Janas et al.

Figure 1: a) Crystal structures of (i) MnBi2Te4, (ii) septuple layer (SL)-terminated MnBi4Te7, (iii) quintuple layer (QL)-terminated MnBi4Te7. b) Difference photoemission spectra taken just after the optical excitation. c) Slab calculations of the surface-projected band structures. Purple (orange) circles denote states derived from SL (QL). d) Photoemission intensity integrated over an energy range of 70 meV above the Fermi level (red boxes shown in panel c), as a function of delay time Δt, for SL (purple) and QL (orange) termination of MnBi4Te7, for (i) high fluence - 300 μJ/cm2, (ii) low fluence - 50 μJ/cm2. e) Fluence dependence of the rise time, τ0, and decay contant, τ1, for the two terminations. Figures are adapted with permission from Nano Lett. 2, 414 (2023). Copyright 2023 American Chemical Society.

Fig. 1a illustrates the crystal structures of the materials under investigation. In the case of MnBi4Te7 (m = 1), cleaving can expose two surface terminations with distinct sets of surface states in addition to the common bulk states. This is manifested in Fig. 1b, where we show the difference in photoemission intensity between the initial moments after the excitation (time delay Δt  = 40 fs) and the equilibrium conditions before the excitation (Δt < 0). The red (blue) regions reflect the gain (loss) of photoemission signal, which corresponds to excited electron (hole) population. The surface states are characterized by higher intensity features, and are well-matched to the slab band structure calculations presented in Fig. 1c. The time dependence of the electron occupation in MnBi4Te7 just above the Fermi level is shown in Fig. 1d. At high fluence, the dynamics are comparable across all the two terminations: the signal rises instantaneously, then decays slowly over several picoseconds. At low fluence, however, the build-up of photoemission intensity in the SL termination is delayed to 0.8 ps after the excitation. We investigate the detailed fluence dependence in Fig. 1e. The width of sigmoid fit to the rising edge provides the growth rate, τ0, of the population just above the Fermi level. While the rise time of QL signal is fast and insensitive to fluence, in the SL, the growth rate becomes slower with decreasing excitation density, reaching the maximum of 0.35 ps at a fluence of ≈50 μJ/cm2. The growth rate is influenced by the filling from states at higher energies which are first populated by the pump. One of those bands, demarcated by red boxes in Fig. 1c, is localized to the QL. At low excitation energy, the intralayer scattering within the QL is more favorable than the interlayer charge transfer to the SL, producing a bottleneck effect. Finally, the exponential decay constant τ1 describes the electron-hole recombination at the Fermi level, which is mediated by electron-phonon interactions in topological insulators. The excited carriers are removed more efficiently in the QL, suggesting that the intralayer phonons play a dominant role in the relaxation.

The above results illustrate how the competition between intra- and interlayer processes shapes up the dynamic response in magnetic topological insulators, and may provide guidelines for optical control of spin-polarized charge carriers in their van der Waals heterostructures. 

This research was conducted by the following research team:

Paulina Ewa Majchrzak1, Yuntian Liu2, Klara Volckaert1, Deepnarayan Biswas1, Chakradhar Sahoo1, Denny Puntel3, Wibke Bronsch4, Manuel Tuniz3, Federico Cilento4, Xing-Chen Pan5, Qihang Liu2, Yong P. Chen1,5,6, and Søren Ulstrup1

1 Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, Aarhus, Denmark
2 Department of Physics and Shenzhen Institute for Quantum Science and Engineering (SIQSE), Southern University of Science and Technology, Shenzhen, China
3 Dipartimento di Fisica, Università degli Studi di Trieste, Trieste, Italy
4 Elettra - Sincrotrone Trieste S.C.p.A., Basovizza, Italy
5 Advanced Institute for Materials Research, Tohoku University, Sendai, Japan
6 Department of Physics and Astronomy and School of Electrical and Computer Engineering and Purdue Quantum Science and Engineering Institute, Purdue University, West Lafayette, USA

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P.E. Majchrzak, Y. Liu, K. Volckaert, D. Biswas, C. Sahoo, D. Puntel, W. Bronsch, M. Tuniz, F. Cilento, X.-C. Pan, Q. Liu, Y.P. Chen, and S. Ulstrup, “Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures”, Nano Lett. 23(2), 414–421 (2023); DOI: 10.1021/acs.nanolett.2c03075

Last Updated on Friday, 03 March 2023 11:44