Feasibility of many emergent phenomena that intrinsic magnetic topological insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi₂Te₄)(Bi₂Te₃)_m with m = 0, ⋯, 6. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi₂Te₄ (m = 0) to MnBi₄Te₇ (m = 1) and MnBi₆Te₁₀ (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, respectively, realized in the m = 0, 1, 2 and m = 3 cases. For large m numbers a hitherto-unknown topologically nontrivial phase can be created, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi₂Te₄ building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi₂Te₄)(Bi₂Te₃)_m allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

We have investigated plasmonic excitations at the surface of ${\mathrm{Bi}}_2{\mathrm{Se}}_3(0001)$ via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer $q_{\parallel }$, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with $q_{\parallel }$. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta ($q_{\parallel }\sim 0.04{\AA }^{-1}$), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators.

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator $${\hbox {MnBi}}_2 {\hbox {Te}}_4$$MnBi 2 Te 4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced ( $$<20~ \hbox {meV}$$< 20 meV ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( $$T_{\mathrm{N}} = 24.5~ \hbox {K}$$T N = 24.5 K ). We propose that the gap above $$T_{\mathrm{N}}$$T N remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.