Variety of magnetic **topological** phases in the
(MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ family

**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.

4/10 relevant

arXiv

Persistent gapless surface states in MnBi2Te4/Bi2Te3 superlattice
antiferromagnetic **topological** **insulator**

**topological**

**insulator**MnBi2Te4 that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Expand abstract.

**topological**quantum materials (TQMs) provide a fertile ground for the emergence of fascinating

**topological**magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM)

**topological**

**insulator**MnBi2Te4 that could realize quantized anomalous Hall effect and axion

**insulator**phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi4Te7 where Bi2Te3 and MnBi2Te4 layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent)

**topological**electronic structures from both Bi2Te3 and MnBi2Te4 terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the

**topological**surface states show little temperature dependence and remain gapless across the AFM transition. The detailed electronic structure of MnBi4Te7 and its temperature evolution, together with the results of its sister compound MnBi2Te4, will not only help understand the exotic properties of this family of magnetic TQMs, but also guide the design for possible applications.

7/10 relevant

arXiv

Terahertz tuning of Dirac plasmons in Bi$_2$Se$_3$ **Topological** **Insulator**

**Topological**Insulators, where massless fermions show a naturally non-linear optical behavior in the terahertz range. The strong interaction scenario has been considered so far from the point of view of light. In this paper, we investigate instead the effect of strong interaction on the plasmon itself. In particular, we will show that Dirac plasmons in Bi$_2$Se$_3$

**Topological**

**Insulator**are strongly renormalized when excited by high-intensity terahertz radiation by displaying a huge red-shift down to 60% of its characteristic frequency. This opens the road towards tunable terahertz non-linear optical devices based on

**Topological**

**Insulators**.

10/10 relevant

arXiv

Disorder driven phase transitions in weak AIII **topological** **insulators**

**topological**indices can be generalized for arbitrary disorder, and the physical signatures of these indices is not known. Expand abstract.

**topological**phases enumerates all strong

**topological**phases for both clean and disordered systems. These strong

**topological**phases are connected to the existence of robust edge states. However, in addition to the strong

**topological**phases in the tenfold classification, there exist weak

**topological**phases whose properties under disorder are less well understood. It is unknown if the weak

**topological**indices can be generalized for arbitrary disorder, and the physical signatures of these indices is not known. In this paper, we study disordered models of the two dimensional weak AIII

**insulator**. We demonstrate that the weak invariants can be defined at arbitrary disorder, and that these invariants are connected to the presence or absence of bound charge at dislocation sites.

8/10 relevant

arXiv

Novel self-epitaxy for inducing superconductivity in a **topological**
**insulator** with a record-high transparency

**topological**superconducting (TSC) state in a

**topological**

**insulator**(TI) has been extended to various materials platforms and pursued by many researchers, with the hope to realize

**topological**quantum computing. While epitaxially-grown Al on InAs realizing a highly-transparent interface was a major breakthrough in the pursuit of engineered TSC state, precise control of the chemical potential required for

**topological**quantum computing has been an issue in InAs. In this respect, TIs in the original Fu-Kane proposal are more promising. However, epitaxial growth of a superconductor (SC) on the TI surface to realize a highly-transparent interface has never been achieved so far. Here we report our discovery that simply depositing Pd on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$

**topological**

**insulator**films leads to an epitaxial self-formation of PdTe$_2$ superconductor, which proximitizes the TI with a record-high interface transparency documented by Josephson-junction and SQUID devices. This will allow for easy fabrication of high-performance devices to address proximity-induced superconductivity in TIs and eventually the Majorana fermions that are expected to emerge there.

7/10 relevant

arXiv

Pulsed laser deposition of highly c-axis oriented thin films of BSTS
**topological** **insulator**

**topological**

**insulator**(TI) BiSbTe1.5Se1.5 (BSTS) thin films by pulsed laser deposition (PLD) technique. Expand abstract.

**topological**

**insulator**(TI) BiSbTe1.5Se1.5 (BSTS) thin films by pulsed laser deposition (PLD) technique. The various growth parameters such as substrate temperature, Argon pressure in the deposition chamber and target to substrate distance are tuned to obtain the optimized conditions essential for stoichiometric and bulk insulating TI thin films. These films are highly c-axis oriented and exhibit all the four Raman modes characteristic to the R-3m space group. The quality of the deposited thin films is investigated using X-ray diffraction for crystallinity, Raman spectroscopy for lattice dynamics, morphological studies using scanning electron microscope and compositional analysis using Energy dispersive X-ray spectroscopy. Resistance vs temperature measurements confirm bulk insulating nature of the prepared thin films and magnetoresistance data exhibits the phenomena of weak antilocalization with a large phase coherence length.

7/10 relevant

arXiv

Transport on a **topological** **insulator** surface with a time-dependent
magnetic barrier

**topological**

**insulator**. We show that such a barrier can be implemented for Dirac electrons on the surface of a three-dimensional

**topological**

**insulator**by a combination of a proximate magnetic material and linearly polarized external radiation. We find that the conductance of the system can be tuned by varying the frequency and amplitude of the radiation and the energy of an electron incident on the barrier providing us optical control on the conductance of such junctions. We first study a $\delta$-function barrier which shows a number of interesting features such as sharp peaks and dips in the transmission at certain angles of incidence. Approximate methods for studying the limits of small and large frequencies are presented. We then study a barrier with a finite width. This gives rise to some new features which are not present for a $\delta$-function barrier, such as resonances in the conductance at certain values of the system parameters. We present a perturbation theory for studying the limit of large driving amplitude and use this to understand the resonances. Finally, we use a semiclassical approach to study transmission across a time-dependent barrier and show how this can qualitatively explain some of the results found in the earlier analysis. We discuss experiments which can test our theory.

7/10 relevant

arXiv

Crystal and magnetic structures of magnetic **topological** **insulators**
MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$

**topological**

**insulators**MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$, where rich

**topological**quantum states have been recently predicted and observed. Structural refinements reveal that considerable Bi atoms occupied on the Mn sites in both materials, distinct from the previously reported antisite disorder. We show unambiguously that MnBi$_{2}$Te$_{4}$ orders antiferromagnetically below 24 K featured by a magnetic symmetry $R_I$-${3c}$ while MnBi$_{4}$Te$_{7}$ is antiferromagnetic below 13 K with a magnetic space group $P_c$-${3c1}$. They both present antiferromagnetically coupled ferromagnetic layers with spins along the $c$-axis. We put forward a stacking rule for the crystal structure of an infinitely adaptive series MnBi$_{2n}$Te$_{3n+1}$ (n$\geq$1) with the building unit of [Bi$_2$Te$_3$]. A comparison of magnetic properties between MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$, together with the recent density-functional theory calculations, enables us to draw that a two-dimensional magnetism limit might be realized in the derivatives. Our work may promote the theoretical studies of

**topological**magnetic states in the series of MnBi$_{2n}$Te$_{3n+1}$.

10/10 relevant

arXiv

Realizing Corner States in Artificial Crystals Based on **Topological** Spin
Textures

**topological**

**insulators**(HOTIs) has significantly extended our understanding of

**topological**phases of matter. Here, we predict that second-order corner states can emerge in the dipolar-coupled dynamics of

**topological**spin textures in two-dimensional artificial crystals. Taking a breathing honeycomb lattice of magnetic vortices as an example, we derive the full phase diagram of collective vortex gyrations and identify three types of corner states that have not been discovered before. We show that the

**topological**"zero-energy" corner modes are protected by a generalized chiral symmetry in the sexpartite lattice, leading to particular robustness against disorder and defects, although the conventional chiral symmetry of bipartite lattices is absent. We propose the use of the quantized $\mathbb{Z}_{6}$ Berry phase to characterize the nontrivial topology. Interestingly, we observe corner states at either obtuse-angled or acute-angled corners, depending on whether the lattice boundary has an armchair or zigzag shape. Full micromagnetic simulations confirm the theoretical predictions with good agreement. Experimentally, we suggest using the recently developed ultrafast Lorentz microscopy technique [M\"{o}ller \emph{et al}.,{arXiv:1907.04608}] to detect the

**topological**corner states by tracking the nanometer-scale vortex orbits in a time-resolved manner. Our findings open up a promising route for realizing higher-order topologically protected corner states in magnetic systems and finally achieving

**topological**spintronic memory and computing.

4/10 relevant

arXiv

Type-II quadrupole **topological** **insulators**

**topological**insulating phase means that new multipole

**topologic**al

**insulators**with distinct properties can exist in broader contexts beyond classical constraints. Expand abstract.

**topological**phases of matter. Such a formulation has recently been extended to higher electric multipole moments, through the discovery of the so-called quadupole

**topological**

**insulator**. It has been established by a classical electromagnetic theory that in a two-dimensional material the quantized properties for the quadupole

**topological**

**insulator**should satisfy a basic relation. Here we discover a new type of quadupole

**topological**

**insulator**(dubbed as type-II) that violates this relation due to the breakdown of a previously established theory that a Wannier band and an edge energy spectrum are topologically equivalent in a closed quantum system. We find that, similar to the previously discovered (referred to as type-I) quadrupole

**topological**insulator, the type-II hosts topologically protected corner states carrying fractional corner charges. However, the edge polarizations only occur at a pair of boundaries in the type-II insulating phase, leading to the violation of the classical constraint. We propose an experimental scheme to realize such a new

**topological**phase of matter. The existence of the new

**topological**insulating phase means that new multipole

**topological**

**insulators**with distinct properties can exist in broader contexts beyond classical constraints.

10/10 relevant

arXiv