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Register a new userLarge magnetic gap at the Dirac point in a Mn-induced Bi$_2$Te$_3$ heterostructure
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Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provides quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point but all attempts to observe it directly have been unsuccessful. The gap size is considered crucial to overcoming the present limitations of the QAHE, which so far occurs only at temperatures one to two orders of magnitude below its principle limit set by the ferromagnetic Curie temperature $T_C$. Here, we use low temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi$_2$Te$_3$ films, which is present only below $T_C$. Surprisingly, the gap turns out to be $sim$90 meV wide, which not only exceeds $k_BT$ at room temperature but is also 5 times larger than predicted by density functional theory. By an exhaustive multiscale structure characterization we show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, where Mn is predominantly incorporated in the septuple layers. This self-organized heterostructure substantially enhances the wave-function overlap and the size of the magnetic gap at the Dirac point, as recently predicted. Mn-doped Bi$_2$Se$_3$ forms a similar heterostructure, however, only a large, nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi$_2$Te$_3$ with the most important consequence of a magnetic anisotropy perpendicular to the films, whereas for Bi$_2$Se$_3$ the spin-orbit interaction it is too weak to overcome the dipole-dipole interaction. Our findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room-temperature applications.
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The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$ is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi$_2$Te$_4$ Dirac cone. Here, we study the crystalline and electronic structure of MnBi$_2$Te$_4$(0001) using scanning tunneling microscopy/spectroscopy (STM/S), micro($mu$)-laser angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. Our topographic STM images clearly reveal features corresponding to point defects in the surface Te and subsurface Bi layers that we identify with the aid of STM simulations as Bi$_text{Te}$ antisites (Bi atoms at the Te sites) and Mn$_text{Bi}$ substitutions (Mn atoms at the Bi sites), respectively. X-ray diffraction (XRD) experiments further evidence the presence of cation (Mn-Bi) intermixing. Altogether, this affects the distribution of the Mn atoms, which, inevitably, leads to a deviation of the MnBi$_2$Te$_4$ magnetic structure from that predicted for the ideal crystal structure. Our transport measurements suggest that the degree of this deviation varies from sample to sample. Consistently, the ARPES/STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples/sample cleavages. Our DFT surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, Mn$_text{Bi}$ defects can cause a strong reduction of the MnBi$_2$Te$_4$ Dirac point gap, given the recently proved antiparallel alignment of the Mn$_text{Bi}$ moments with respect to those of the Mn layer. Our results provide a key to puzzle out the MnBi$_2$Te$_4$ Dirac point gap mystery.
Quantum states of matter combining non-trivial topology and magnetism attract a lot of attention nowadays; the special focus is on magnetic topological insulators (MTIs) featuring quantum anomalous Hall and axion insulator phases. Feasibility of many novel phenomena that emph{intrinsic} magnetic TIs may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here, using angle- and spin-resolved photoemission spectroscopy along with emph{ab initio} calculations we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ 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), changes to ferromagnetic (FM) one in MnBi6Te10 (m=2) and disappears with further increase in m. In this way, the AFM and FM TI states are respectively realized in the $m=0,1$ and $m=2$ cases, while for $m ge 3$ a novel and hitherto-unknown topologically-nontrivial phase arises, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MBT, building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.
V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point.
Nuclear magnetic resonance (NMR) was recently shown to measure the bulk band inversion of Bi$_2$Se$_3$ through changes in the $^{209}$Bi nuclear quadrupole interaction, and the corresponding tensor of the local electric field gradient was found to follow, surprisingly, the direction of the external magnetic field if the sample is rotated. This manifests a hidden property of the charge carriers in the bulk of this topological insulator, which is explored here with another material, Bi$_2$Te$_3$. It is found that two electric field gradients appear to be present at $^{209}$Bi, one rests with the lattice, as usual, while a second follows the external field if it is rotated with respect to the crystal axes. These electronic degrees of freedom correspond to an effective rotation of $j$-electrons, and their level life time is believed to be responsible for a new quadrupolar relaxation that should lead to other special properties including the electronic specific heat.
A new type of topological spin-helical surface states was discovered in layered van der Waals bonded (SnTe)$_{n=2,3}$(Bi$_2$Te$_3$)$_{m=1}$ compounds which comprise two covalently bonded band inverted subsystems, SnTe and Bi$_2$Te$_3$, within a building block. This novel topological states demonstrate non-Dirac dispersion within the band gap. The dispersion of the surface state has two linear sections of different slope with shoulder feature between them. Such a dispersion of the topological surface state enables effective switch of the velocity of topological carriers by means of applying an external electric field.
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