46 47 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Investigating the electron properties at the surface of novel magnetic topological insulators Related publication: Vidal, R. C., Bentmann, H., Facio, J. I., Heider, T., Kagerer, P., Fornari, C. I., Peixoto, T. R. F., Figgemeier, T., Jung, S., Cacho, C., Büchner, B., van den Brink, J., Schneider, C. M., Plucinski, L., Schwier, E. F., Shimada, K., Richter, M., Isaeva, A., & Reinert, F. Orbital complexity in intrinsic magnetic topological MnBi 4 Te 7 and MnBi 6 Te 10 . Physical Review Letters 126, 176403 (2021). DOI: 10.1103/PhysRevLett.126.176403 Publication keywords: Angle-resolved photoelectron spectroscopy; Topological insulators; Magnetism T he information technologies we currently rely on, used in ( e.g. ) cell phones and laptops, involve a significant waste of energy that is lost as heat. Our ever-growing need to access, process and store information makes it increasingly important that we develop new, more energy-efficient technologies. Current technologies are based on electron charge. One potential avenue for future information technologies is touse theelectronspin - an intrinsicmagneticpropertyof electrons - toachievemoreenergy-efficient informationprocessing. Theoretical predictions suggest that certain magnetic materials exhibit spin polarised electrons at their surface with unusual ‘topological’ properties. In the future, such topological surface electrons might enable information processing without heat loss in electronic devices. An international teamof researchers used Angle-Resolved Photoelectron Spectroscopy (ARPES) measurements on beamline I05 to verify the theoretical predictions. The I05 setup allows ARPES measurements with high energy resolution and a small beam spot. Their experiments confirmed that the compounds MnBi 4 Te 7 and MnBi 6 Te 10 display spin-polarised, topological surface electrons. In the future, further optimisation of the materials could enable applications in electronic high-precision metrology (the scientific study of measurement) without the need for external magnetic fields. However, raising the temperature at which themagnetic order arises to room temperature - a pre-requisite for applications in real-world devices - still poses a significant challenge. Topological insulators (TI) feature spin-polarised, metallic states at their surfaces while their bulk remains insulating. Here, the term “topological” refers to a special kind of “knotting” of the bulk electron wave functions, which evolves differently than in conventional insulators. The unique surface properties of a TI are a result of this non-trivial bulk topology. Over the past decade, the understanding of these materials has matured and quite a significant number of compounds have been identified to be TI 1 . Interestingly, introducing magnetic order in a TI may enable realisation of new topological phenomena 2 . One important example is the quantum anomalous Hall state, where a dissipationless flow of electronic charge is realised in the absence of external magnetic fields 3 . Another example isMajorana fermion quasi-particles, a special type of electronic excitation in a solid, that is discussed in the context of quantum computation. Yet to date, only a few Magnetic Topological Insulators (MTI) are known, i.e. materials that combine the presence of magnetic order and a non-trivial topology. Recently, the compound MnBi 2 Te 4 was discovered as the first MTI that does not require doping with magnetic impurities 4 . In the present study, the compounds MnBi 4 Te 7 and MnBi 6 Te 10 were studied with the goal to directly verify the spin-polarised, metallic states at the surface. To this end, experiments based on Angle-Resolved Photoelectron Spectroscopy (ARPES) were employed using a synchrotron light source (beamline I05 at Diamond Light Source) and a laser source (HiSOR Hiroshima, Japan, and FZ Jülich, Germany). ARPES allows to directly measure the electron dispersion relation in crystalline materials with high surface sensitivity and was therefore the ideal method for the purpose of this study. In MnBi 4 Te 7 and MnBi 6 Te 10 , magnetic layers, containing the element Mn, and non-magnetic layers are stacked on top of each other in a regular fashion. Depending on the stacking sequence the magnetic properties vary 5 , potentially leading to interesting opportunities to manipulate the interplay of magnetic order and topology. The alternating layer stacking in the crystal structure also gives rise to different possible terminations of the surface, i.e. surfaces with different atomic structure. Fig. 1showsARPESmeasurementsof the surface states for all possible surface terminations in MnBi 4 Te 7 and MnBi 6 Te 10 . It is apparent that the dispersion relations are similar for both compounds but strongly depend on the specific surface termination. The latter can be either a magnetic MnBi 2 Te 4 layer or a non-magnetic Bi 2 Te 3 layer. To further verify the topological and spin-polarised properties, more detailed ARPES experiments were carried out. In particular, spin-resolved ARPES and Circular Dichroism in ARPES (CD-ARPES) were used for this purpose. In the first case, the spin polarisation of photoelectrons is directly detected. In the latter case, the difference in photoemission cross section depending on the circular polarisation of the light used to excite the photoelectrons is measured. Fig. 2 shows CD-ARPES data for a MnBi 2 Te 4 -terminated surface. One can discern a sign change in the CD pattern at the so-called Dirac point (DP) of the surface-state dispersion, i.e. at the band crossing of the two surface-state branches at kx = 0. Such a sign change indicates a helicity change in the surface-state wave function across the DP, as expected for a topological surface state. The Bi 2 Te 3 -terminated surface shows a particularly complex electronic structure, because the surface state strongly hybridises with the bulk valence band. To disentangle contributions from the surface and from the bulk to the photoelectron spectra, photon-energy-dependent measurements at beamline I05 turned out to be crucial. Fig. 3 shows ARPES data for a Bi 2 Te 3 -terminated surface measured at various photon energies. One can see that, depending on photon energy, different parts of the measured dispersion relation display high or low intensity. Through a detailed analysis, also involving theoretical calculations (Fig. 3), this effect could be traced back to a varying surface or bulk character of the electron bands as a function of wave vector. This allowed for a precise determination of the surface-state dispersion, including its hybridisation with the bulk valence band. Overall, the study provides comprehensive evidence for the presence of spin-polarised, topological surface states in the magnetic compounds MnBi 4 Te 7 and MnBi 6 Te 10 . In the future, further optimisation of these materials and their synthesis in the form of ultrathin layers could enable interesting opportunities to investigate the interplay of magnetic order and topological electronic properties. References: 1. Bansil, A. et al. Colloquium : topological band theory. Reviews of Modern Physics 88, 021004 (2016). DOI: 10.1103/RevModPhys.88.021004 2. Tokura, Y. et al. Magnetic topological insulators. Nature Reviews Physics 1, 126–143 (2019). DOI : 10.1038/s42254-018-0011-5 3. Chang, C.-Z. et al. Experimental observation of the quantum anomalous Hall effect in a magnetic topological insulator. Science 340, 167–170 (2013). DOI: 10.1126/science.1234414 4. Otrokov, M. M. et al. Prediction and observation of an antiferromagnetic topological insulator. Nature 576, 416–422 (2019). DOI: 10.1038/s41586- 019-1840-9 5. Vidal, R. C. et al. Topological electronic structure and intrinsic magnetization in MnBi 4 Te 7 : A Bi 2 Te 3 derivative with a periodic Mn sublattice. Physical Review X 9, 041065 (2019). DOI: 10.1103/PhysRevX.9.041065 Funding acknowledgement: We acknowledge financial support from the DFG through SFB1170 “Tocotronics”(Project A01), SFB1143“Correlated Magnetism,”RE1469/13-1, SPP 1666“Topological insulators”(IS 250/1-2 and PL 712/2-1), ERA-Chemistry Programm (RU-776/15-1), and theWürzburg-Dresden Cluster of Excellence on Complexity and Topology in QuantumMatter—ct.qmat (EXC 2147, project-id 390858490). Part of this work was carried out with the support of the Diamond Light Source, beamline I05 (Proposal No. SI22468-1). Part of the ARPES measurements were performed with the approval of the Proposal Assessing Committee of the Hiroshima Synchrotron Radiation Center (Proposal No. 19BU010). The work received funding from the DFG under Germany’s Excellence Strategy–Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1-390534769. J. I. F. acknowledges the support from the Alexander von Humboldt Foundation. Corresponding author: Hendrik Bentmann, University ofWürzburg,
[email protected] Structures and Surfaces Group Beamline I05 Figure 1: ARPES measurements for different surface terminations of (Bi 2 Te 3 )n(MnBi 2 Te 4 ) with n = 1 and 2; (a),(b) ARPES data sets for MnBi 4 Te 7 a MnBi 2 Te 4 - and a Bi 2 Te 3 - terminated surface, respectively; (c) Schematic of the possible terminations of MnBi 4 Te 7 and MnBi 6 Te 10 surfaces, labelled with the panel showing the corresponding ARPES data; (d)–(f ) ARPES datasets along for MnBi 6 Te 10 with for different terminations Figure 2: CD-ARPES experiments for the MnBi 2 Te 4 -terminated surface of MnBi 4 Te 7 ;(a) ARPES intensity IR measured with right circularly polarised light; (b) Corresponding CD-ARPES dataset defined as the intensity difference IR – IL; (c) ARPES intensity measured with s-polarised light. The overlaid markers indicate the dispersion extracted from CD-ARPES data in (b) and the red or blue colour refers to the sign of the CD. Figure 3: (a)-(d) Photon-energy-dependent ARPES data for the Bi 2 Te 3 -terminated surface of MnBi 4 Te 7 ; (e) Intensity difference between datasets measured at photon energy hν = 61 and hν = 79 eV; (f ) ARPES intensities traced as a function of hν at specific points in the band structure, indicated by boxes in (a); The colours of the boxes in (a) correspond to the respective data sets in (f ); (g)– (j) Calculations of orbital-projected surface spectral densities for the Bi 2 Te 3 -terminated surface of MnBi 4 Te 7 ; (j) Difference between the surface spectral densities projected on Bi J = 1/2 orbitals in (g) and Te J = 3/2, MJ = 1/2 orbitals in (i).