Crossover from 2D Ferromagnetic Insulator to Wide Band Gap Quantum Anomalous Hall Insulator in Ultrathin MnBi₂Te₄

Abstract

Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi 2 Te 4 , and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi 2 Te 4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from large bandgap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi 2 Te 4 . We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi 2 Te 4 . The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

Even–Odd Layer-Dependent Anomalous Hall Effect in Topological Magnet MnBi2Te4 Thin Films

“…Therefore, the molecular beam epitaxy (MBE) growth of high-quality MnBi 2 Te 4 films is essential. To date, the MBE growth of MnBi 2 Te 4 has been pursued by several groups, ,− but a systematic thickness-dependent on transport study on MnBi 2 Te 4 films is still lacking.…”

Large Magnetic Gap in a Designer Ferromagnet–Topological Insulator–Ferromagnet Heterostructure

“…For 1SL MBT, only a M‐shaped valence band is present below the Fermi level indicating a bandgap greater than 780 meV. [ 22 ] For 4QL BT, the n‐type doping and band dispersion are consistent with previous reports [ 23 ] where the Dirac cone with Fermi velocity along ΓM, v F = 4.1 ± 0.5 × 10 5 m s −1 is buried within the valence band and appears gapless within our experimental resolution, which is consistent with DFT that predicts 4 QL BT is gapless. [ 27 ] Additional high‐resolution spectra, and Fermi surface map along with overlaid DFT data can be found in Figure S4, Supporting Information.…”

“…It is worth noting that the lower bound of the v F in MBT/BT/MBT (4.5 × 10 5 m s −1 ) is nearly the same as the higher bound of the v F of Bi 2 Te 3 (4.6 × 10 5 m s −1 ) and comparable to the Fermi velocity of 5 SL MnBi 2 Te 4 (5.0 × 10 5 m s −1 ). [ 22 ] We also investigate the effect of MnTe on the band structure, and have grown and measured a 4QL BT/MBT sample with lower MnTe concentration which has been assembled along with the high‐resolution scans of 4QL BT and MBT/BT/MBT in Figure S6a–c, Supporting Information. Momentum distribution curve (MDC) analysis to these scans yields a Dirac electron band for the lower concentration of MnTe which is similar to the MBT/BT/MBT sample (i.e., ≈ 5.0 ± 0.5 × 10 5 m s −1 ), yet the sample doping remains at a level with the BT sample.…”

“…1SL MnBi 2 Te 4 was achieved by first growing 1 QL of Bi 2 Te 3 , then a bilayer of MnTe was deposited in order to spontaneously form MnBi 2 Te 4 . [22,32] The 1SL MnBi 2 Te 4 was subsequently annealed in Te flux for 5-10 min to improve crystallinity. Following this, 4QL Bi 2 Te 3 was grown, followed by 1SL MnBi Angle-Resolved Photoelectron Spectroscopy (ARPES) Measurements: Our ARPES measurements were performed at Beamline 10.0.1 with linear polarized synchrotron light and data was taken using a Scienta R4000 analyzer at temperatures between 8 and 50 K. The overall energy resolution is 15 meV.…”

Colloquium: Quantum anomalous Hall effect

“…Atomically thin MnBi 2 Te 4 films can be formed either by MBE growth (Bac et al, 2021;Gong et al, 2019;Kagerer et al, 2020;Lapano et al, 2020;Rienks et al, 2019;Tai et al, 2021;Trang et al, 2021;Zhu et al, 2020b) or by manual exfoliation from bulk crystals (Deng et al, 2020;Gao et al, 2021;Ge et al, 2020;Liu et al, 2020bOvchinnikov et al, 2021;Ying et al, 2022). MBE growth of MnBi 2 Te 4 thin films have been achieved by alternate deposition of Bi 2 Te 3 and MnTe layers (Bac et al, 2021;Gong et al, 2019;Trang et al, 2021;, co-evaporation of Mn, Bi, and Te elements (Lapano et al, 2020;Rienks et al, 2019;Tai et al, 2021;Zhu et al, 2020b) and co-deposition of MnTe and Bi 2 Te 3 sources (Kagerer et al, 2020). Due to unusual growth dynamics, Bi 2 Te 3 , MnTe, and Mn-doped Bi 2 Te 3 inevitably coexist with the dominant MnBi 2 Te 4 phase in all these MBE-grown materials.…”