Septuple Layers Research Articles

Berry curvature engineering by gating two-dimensional antiferromagnets

Recent advances in tuning electronic, magnetic, and topological properties of two-dimensional (2D) magnets have opened a new frontier in the study of quantum physics and promised exciting possibilities for future quantum technologies. In this study, we find that the dual-gate technology can well tune the electronic and topological properties of antiferromagnetic (AFM) even septuple-layer (SL) MnBi$_2$Te$_4$ thin films. Under an out-of-plane electric field that breaks $\mathcal{PT}$ symmetry, the Berry curvature of the thin film could be engineered efficiently, resulting in a huge change of anomalous Hall (AH) signal. Beyond the critical electric field, the double-SL MnBi$_2$Te$_4$ thin film becomes a Chern insulator with a high Chern number of 3. We further demonstrate that such 2D material can be used as an AFM switch via electric-field control of the AH signal. These discoveries inspire the design of low-power memory prototype for future AFM spintronic applications.

Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures.

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1-8. The edge states are hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, TC (ref. 8). Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1kelvin of up to 90millielectronvolts, which is five times larger than theoretically predicted9. Using multiscale analysis we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications.

In-plane magnetic-field-induced quantum anomalous Hall plateau transition

Axion insulator is a new state of quantum matter with a vanishing Chern number but a quantized topological axion response. We study the critical properties of the magnetic-field-induced quantum phase transition in axion insulators. Take the even septuple layer film ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ as a concrete example, We find the mirror symmetry breaking from in-plane magnetic field could induce an axion insulator to quantum anomalous Hall (QAH) insulator transition, which belongs to the generic integer quantum Hall plateau transition. The microscopic model proposed here explains the out-of-plane magnetic-field-induced QAH plateau transition observed in this system experimentally. The chiral Majorana fermion does not necessarily emerge at the QAH plateau transition in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ due to strong exchange field, but may be quite feasible in its descendent materials ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$ and ${\mathrm{MnBi}}_{6}{\mathrm{Te}}_{10}$.

Unique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi_{2}Te_{4} Films.

Using density functional theory and MonteCarlo calculations, we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering MnBi_{2}Te_{4} as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topological transitions. While a single septuple layer block of MnBi_{2}Te_{4} is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide band gap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, MnBi_{2}Te_{4} is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theoretically shown to host the exotic axion insulator phase.

Superconductivity in SnSb with a natural superlattice structure

We report the results of electrical resistivity, magnetic, and thermodynamic measurements on polycrystalline SnSb, whose structure consists of stacks of Sb bilayers and Sn4Sb3 septuple layers along the c-axis. The material is found to be a weakly coupled, fully gapped, type-II superconductor with a bulk Tc of 1.50 K, while showing a zero resistivity transition at a significantly higher temperature of 2.48 K. The Sommerfeld coefficient and upper critical field, obtained from specific heat measurements, are 2.29 mJ mol−1 K−2 and 520 Oe, respectively. Compositional inhomogeneity and strain effect at the grain boundaries are proposed as possible origins for the difference in resistive and bulk superconducting transitions. In addition, a comparison with the rock-salt structure SnAs superconductor is presented. Our results provide the first clear evidence of bulk superconductivity in a natural superlattice derived from a topological semimetal.

Probing band parity inversion in the topological insulator GeBi2Te4 by linear dichroism in ARPES

Here we report on experiments performed by linear-dichroism angle resolved photoelectron spectroscopy (LD-ARPES) on septuple layers (SL) topological insulator GeBi2Te4. The linear dichroic signals, both for the valence band states and in the topologically protected surface state are interpreted as a probe of the symmetry of the states. The observation of a change in the LD-ARPES spectrum at the top of VB is attributed to the band parity inversion which is predicted by theory for all materials entering the topologically protected phase. Finally, the opening of a gap in the surface state dispersion along the non-high symmetry direction is reported. This is proposed to originated from the interaction between the surface state and the VB, leading to the formation of surface resonances.

Layer structured bismuth selenides Bi2Se3 and Bi3Se4 for high energy and flexible all-solid-state micro-supercapacitors

In this work, bismuth selenides (Bi2Se3 and Bi3Se4), both of which have a layered rhombohedral crystal structure, have been found to be useful as electrode materials for supercapacitor applications. In a liquid electrolyte system (6M KOH), Bi2Se3 nanoplates exhibit much better performance as an electrode material than Bi3Se4 nanoparticles do, delivering a higher specific capacitance (272.9 F g−1) than that of Bi3Se4 (193.6 F g−1) at 5 mV s−1. This result may be attributed to the fact that Bi2Se3 nanoplates possess more active electrochemical surfaces for the reversible surface redox reactions owing to their planar quintuple stacked layers (septuple layers for Bi3Se4). To meet the demands of electronic skin, we used a novel flexible annular interdigital structure electrode to support the all-solid-state micro-supercapacitors (AMSCs). The Bi2Se3 AMSC device delivers a much better supercapacitor performance, exhibits a large stack capacitance of 89.5 F cm−3 at 20 mV s−1 (Bi3Se4: 79.1 F cm−3), a high energy density of 17.9 mWh cm−3 and a high power density of 18.9 W cm−3. The bismuth selenides also exhibit good cycle stability, with 95.5% retention after 1000 c for Bi2Se3 (Bi3Se4:90.3%). Clearly, Bi2Se3 nanoplates can be promising electrode materials for flexible annular interdigital AMSCs.

Impact of stoichiometry and disorder on the electronic structure of the PbBi2Te4−xSex topological insulator

Detailed comparative theoretical and experimental study of electronic properties and spin structure was carried out for a series of Pb-based quaternary compounds ${\mathrm{PbBi}}_{2}{\mathrm{Te}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$. For all values of $x$, these compounds are theoretically predicted to be topological insulators, possessing at high Se content a remarkably large band gap and a Dirac point isolated from bulk states. Using spin- and angle-resolved photoemission spectroscopy, it was shown that the ${\mathrm{PbBi}}_{2}{\mathrm{Te}}_{2}{\mathrm{Se}}_{2}$ and ${\mathrm{PbBi}}_{2}{\mathrm{Te}}_{1.4}{\mathrm{Se}}_{2.6}$ compounds are characterized by well-defined spin-polarized topological surface state in the bulk gap. To define the probable distribution of atoms over the atomic sites for these samples, we performed ab initio calculations in ordered and disordered configurations of the unit cell. We found that theoretical calculations better reproduce photoemission data when Te atoms are placed in the outermost layers of the septuple layer block.

Perfect quintuple layer Bi2Te3 nanowires: Growth and thermoelectric properties

Bi2Te3 nanowires are promising candidates for thermoelectric applications. Vapor-liquid-solid growth of these nanowires is straightforward, but the traditional Au-catalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications.

Observation of hidden atomic order at the interface between Fe and topological insulator Bi2Te3.

To realize spintronic devices based on topological insulators (TIs), well-defined interfaces between magnetic metals and TIs are required. Here, we characterize atomically precisely the interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. For both sites, our core-level photoemission spectra and density-functional theory calculations demonstrate simultaneous chemical bonding of Fe to both Te and Bi atoms. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm interface layer containing FeTe. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of ∼25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work will be of critical importance when optimizing the efficiency of spin injection in TI-based devices.

Electronic and spin structure of a family of Sn-based ternary topological insulators

We report the bulk and surface electronic properties and spin polarization of a rich family of Sn-based ternary topological insulators studied by means of first-principles calculations within the framework of density functional theory. These compounds exist with the following stoichiometries: $\mathrm{Sn}{X}_{2}{\mathrm{Te}}_{4},\phantom{\rule{0.16em}{0ex}}\mathrm{Sn}{X}_{4}{\mathrm{Te}}_{7}$, and ${\mathrm{SnBi}}_{6}{\mathrm{Te}}_{10}$ ($X$ = Sb and Bi). Where a septuple layer or a quintuple layer and septuple layer blocks alternate along the hexagonal axis. We reveal that the bulk band gap in these compounds is about 100 meV and recognize a strong dependence of the spin polarization on the cleavage surface. The calculated spin polarization reaches 85% in some cases, that is one of the highest predicted values hitherto. Since the electron spin polarization is a relevant parameter for spintronics technology, this new family is suitable for applications within this field.

Modelling near-surface bound electron states in a 3D topological insulator: analytical and numerical approaches

We apply both analytical and ab-initio methods to explore heterostructures composed of a 3D topological insulator (3D TI) and an ultrathin normal insulator (NI) overlayer as a proving ground for the principles of topological phase engineering. Using the continual model of a semi-infinite 3D TI we study the surface potential (SP) effect caused by an attached ultrathin layer of 3D NI on the formation of topological bound states at the interface. The results reveal that the spatial profile and spectrum of these near-surface states strongly depend on both the sign and the strength of the SP. Using ab-initio band structure calculations to take the specificity of the materials into account, we investigate the NI/TI heterostructures formed by a single tetradymite-type quintuple or septuple layer block and the 3D TI substrate. The analytical continuum theory results relate the near-surface state evolution with the SP variation and are in good qualitative agreement with those obtained from density-functional theory (DFT) calculations. We also predict the appearance of the quasi-topological bound state on the 3D NI surface caused by a local band gap inversion induced by an overlayer.

Electronic structure of SnSb2Te4 and PbSb2Te4 topological insulators

The bulk and surface electronic structure of SnSb2Te4 and PbSb2Te4 ternary compounds consisting of three septuple layer blocks separated by van der Waals gaps has been theoretically studied. It has been shown these systems are three dimensional topological insulators. The calculations of the MSb2Te4 (M=Pb, Sn) surface electronic structure show that these compound are well promising for practical application compared to parent Sb2Te3 compound.