TOWARDS MAGNETIC TOPOLOGICAL STATES OF MATTER: MnBi4Te7, A COMPOUND FEATURING TOPOLOGICAL ELECTRONIC ORDER AND INTRINSIC NET MAGNETIZATION

Cerdanyola del Vallès, 27th March 2020  Topological insulators – novel materials that conduct electricity on their surfaces, yet behave as insulators in their interiors – derive their peculiar properties from an effect known as a band inversion, where the normal conduction and valence bands swap places. It is thought that magnetization may control this band inversion, offering a potential way to manipulate topological behavior in new and exciting ways. But researchers had yet to find a suitable material that contains a band inversion and an intrinsic net magnetization — until now. In an article published in Physical Review X, an international team lead by scientists from the universities and research institutes in Würzburg and Dresden has reported the first realization of such a material.

Manganese-bismuth telluride MnBi₄Te₇ is a member of the recently discovered structural family of natural heterostructures that exhibit antiferromagnetic and topological behaviors. Applying a variety of experimental techniques to high-quality single crystals of MnBi₄Te₇, researchers have discovered its complex magnetic behavior with competing magnetic states as a function of temperature (Fig. 1). To characterize the electronic and magnetic properties synchrotron radiation approaches have been used besides laboratory techniques, in particular, Angle-Resolved Photoemission spectroscopy (ARPES) at Diamond Light Source, the Advanced Light Source and Petra III-Desy and X-ray Magnetic Circular and Linear dichroism (XMCD&XMLD) at ALBA. Advanced ARPES and theoretical calculations confirmed the presence of an inverted band structure. By cooling the crystals to a few degrees above the absolute zero, a regime was identified, where a net magnetization of the sample coincides with the presence of a topological surface state (Fig. 2). Such phase had not been previously realized in any stoichiometric materials (i.e. compounds which elemental proportions are ratios of natural numbers). The XMLD and XMCD experiments at BOREAS beamline in ALBA were key to directly probe both the antiferromagnetic and ferromagnetic-like states that MnBi₄Te₇ adopts as a function of temperature (Fig. 3).

These results are a major advance in the search for new classes of topological materials. Thanks to its versatile magnetic and electronic properties, MnBi₄Te₇ establishes a unique material platform for the realization of tunable topological quantum phenomena. This provides fascinating perspectives for the realization of quantum effects in bulk materials.

Fig.1 Left: (a) Crystal structure of MnBi₄Te₇ with alternating Bi₂Te₃ and MnBi₂Te₄ layers. Mn atoms are shown in green; Bi in blue; Te in orange. (b) As-grown crystals. Right: schematic topological phase diagram of MnBi₄Te₇. The scheme reflects the experimentally observed trends: a correlated paramagnetic state above T_N is followed by an antiferromagnetic phase that at lower temperatures evolves into a magnetic state with a strong ferromagnetic component. The text in red highlights possible nontrivial topologies.

Fig.2. Electronic structure of the MnBi₄Te₇ (0001) surface as measured by ARPES. (a) Overview data set of the valence band structure obtained at T=8  K showing characteristic surface states SS1 and SS2 and a feature related to Mn 3d states. (b),(c),(f) High-resolution data sets of the electronic structure near E_F obtained at different photon energies and a temperature of T=8  K, showing a topological surface state (TSS) in the gap between conduction and valence-band derived states (BCB and BVB). (d) Photon-energy dependence of the ARPES intensity at E_F (T=8  K). (e) Same as in (f), but for T=80  K. (g) Same as in (f), but for a septuple layer (SL) termination.

Fig3. Spectroscopy of magnetic properties in MnBi₄Te₇. (d) XMCD and (e) XMLD data for MnBi₄Te₇ (0001) obtained at the Mn L2,3 absorption edge with circularly polarized (RCP and LCP) and linearly polarized (LV and LH) light, respectively. Measurements are performed in normal (NI) and grazing (GI) light incidence geometries, as sketched in the inset of (d). XMCD signals are shown for an external field (μ0H=6  T) along the light incidence direction and for remnant conditions (μ0H=0T) at T=2  K. (e) XMLD data without external field are reported for different temperatures.

Reference:  Raphael C. Vidal, Alexander Zeugner, Jorge I. Facio, Rajyavardhan Ray, M. Hossein Haghighi, Anja U. B. Wolter, Laura T. Corredor Bohorquez, Federico Caglieris, Simon Moser, Tim Figgemeier, Thiago R. F. Peixoto, Hari Babu Vasili, Manuel Valvidares, Sungwon Jung, Cephise Cacho, Alexey Alfonsov, Kavita Mehlawat, Vladislav Kataev, Christian Hess, Manuel Richter, Bernd Büchner, Jeroen van den Brink, Michael Ruck, Friedrich Reinert, Hendrik Bentmann, and Anna Isaeva. Topological Electronic Structure and Intrinsic Magnetization in MnBi₄Te₇: A Bi₂Te₃ Derivative with a Periodic Mn Sublattice. Phys. Rev. X9, 041065 (2019).