Electronic Topology Research Articles

Temperature-dependent electronic structure in a higher-order topological insulator candidate EuIn2As2

The higher order topological insulator (HOTI) has enticed enormous research interests owing to its novelty in supporting gapless states along the hinges of the crystal. Despite several theoretical predictions, enough experimental confirmation of HOTI state in crystalline solids is still lacking. It has been well known that interplay between topology and magnetism can give rise to various magnetic topological states including HOTI and Axion insulator states. Here using the high-resolution angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles calculations, we report a systematic study on the electronic band topology across the magnetic phase transition in EuIn2As2 which possesses an antiferromagnetic ground state below 16 K. Antiferromagnetic EuIn2As2 has been predicted to host both the Axion insulator and HOTI phase. Our experimental results show the clear signature of the evolution of the topological state across the magnetic transition. Our study thus especially suited to understand the interaction of higher order topology with magnetism in materials.

Electronic Band Topology of Monoclinic MoS2 Monolayer: Study Based on Elementary Band Representations for Layer Groups

A monoclinic monolayer is demonstrated to be a topological semimetal (spinless), or zero‐gap semiconductor (spinful, without spin–orbit coupling), or topologically trivial insulator (finite spin–orbit coupling). The latter contradicts a previous prediction that exhibits quantum‐spin Hall (QSH) effect, and the reported value of ‐invariant, calculated from inversion‐parity values at four Brillouin‐zone high‐symmetry points. Namely, as the electronic states of two of these points are double degenerate and transform according to irreducible representations carrying no parity, the band topology of is investigated using elementary band representations (EBRs) for layer groups (LGs). Novel subroutines, which output EBRs and Wilson loop operators for low‐dimensional systems, are incorporated into POLSym code. Based on the calculated band structure decomposition onto EBRs of the relevant symmetry group and Wilson loop eigen‐spectra, it is revealed that valence‐band Wannier functions do not break the symmetry, and that Wannier centers are localized within a unit cell. Moreover, the QSH state is proved to be not realizable within ‐phase group‐VI transition‐metal dichalcogenides, because EBRs for the relevant symmetry groups are topologically trivial. However, it is predicted that among compounds that form other monoclinic structure, with symmorphic LG symmetry, there may exist topologically nontrivial phases, including the QSH state.

Electronic structures and topological properties in nickelates Ln n+1Ni n O2n+2.

After the significant discovery of the hole-doped nickelate compound Nd0.8Sr0.2NiO2, analyses of the electronic structure, orbital components, Fermi surfaces and band topology could be helpful to understand the mechanism of its superconductivity. Based on first-principle calculations, we find that Ni n}{}3d_{x^2-y^2} states contribute the largest Fermi surface. The n}{}Ln 5d_{3z^2-r^2} states form an electron pocket at Γ, while 5dxy states form a relatively bigger electron pocket at A. These Fermi surfaces and symmetry characteristics can be reproduced by our two-band model, which consists of two elementary band representations: B1g@1a ⊕ A1g@1b. We find that there is a band inversion near A, giving rise to a pair of Dirac points along M-A below the Fermi level upon including spin-orbit coupling. Furthermore, we perform density functional theory based Gutzwiller (DFT+Gutzwiller) calculations to treat the strong correlation effect of Ni 3d orbitals. In particular, the bandwidth of n}{}3d_{x^2-y^2} has been renormalized largely. After the renormalization of the correlated bands, the Ni 3dxy states and the Dirac points become very close to the Fermi level. Thus, a hole pocket at A could be introduced by hole doping, which may be related to the observed sign change of the Hall coefficient. By introducing an additional Ni 3dxy orbital, the hole-pocket band and the band inversion can be captured in our modified model. Besides, the nontrivial band topology in the ferromagnetic two-layer compound La3Ni2O6 is discussed and the band inversion is associated with Ni n}{}3d_{x^2-y^2} and La 5dxy orbitals.

DC Current Control for a Single-Stage Current Source Inverter in Motor Drive Application

The current source inverter (CSI) is a power electronics topology that allows for the realization of variable speed drives (VSD). Compared to the most common voltage source inverter (VSI), which can be directly connected to a voltage source, the CSI needs a prestage to generate a constant current bus. This article therefore seeks to challenge this “accepted” consideration that a CSI always needs this precircuit and seeks to remove this circuit by proposing an innovative i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> current control scheme. The proposed scheme is applied to a single stage motor drive driven by a CSI converter. It is shown how implementing this control scheme removes the need for the front-end stage, thus removing an unnecessary converter and optimizing the efficiency at the same time. The CSI state-space equations are presented and the developed models are verified using simulations. Stability analysis of small-signal model is considered through Nyquist criterion with the robustness in presence of variations of the most important system parameters. Experimental results driving a permanent magnet synchronous machine (PMSM) are shown confirming the validity of the proposed control, potentially paving the way to a larger adoption of the CSI topologies for motor drive applications.

Development of high-current power supplies for the TCABR tokamak

Abstract An upgrade is being conducted on the Tokamak a Chauffage Alfven Bresilien (TCABR), which is a small tokamak (R0 = 0.62 m and a = 0.2 m) operated at the University of Sao Paulo, Brazil. An important part of this upgrade is the installation of additional poloidal field coils to allow for the generation of various divertor configurations, such as single-null, double-null, and snowflake divertor. The control of these various magnetic configurations requires the development of 17 robust and high-performance power supplies. To identify the most appropriate solution, thyristor-based and IGBT full-bridge converter are being considered as possible power electronic topologies. In this work, a comparison between these two topologies in terms of controllability and complexity is presented.

Topological band structure transitions and goniopolar transport in honeycomb antimonene as a function of buckling

The electronic band topology of monolayer $\beta$-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the $p_z$-orbitals, and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure becomes a trivial semiconductor. Spin-orbit coupling and edge states are discussed.

A three-phase integrated power electronic transformer with capability of DC fault clearance

This paper presents a three-phase integrated PET topology with capability of DC fault clearance based on the investigation of power electronic transformer topology (PET). The proposed PET has three ports of high-voltage AC, high-voltage DC and low-voltage DC, which can achieve the flexible access of distributed energy resources. The power unit of the PET adopts a three-phase integrated topology, and the single-phase fluctuating power in the DC-link capacitor is reduced by a 4-port DC/DC converter, thereby reducing the capacitance of the DC-link capacitor; Meanwhile, the power unit uses the clamp double sub-module(CDSM) to allow the self-clearing of the short-circuit fault on the high-voltage DC side. Finally, the paper verifies the proposed PET through EMT simulation and experimental prototype.

Real spin and pseudospin topologies in the noncentrosymmetric topological nodal-line semimetal CaAgAs

We present the topology of spin-split Fermi surface of CaAgAs as determined by de Haas-van Alphen (dHvA) effect measurements combined with ab initio calculations. We have determined the torus-shaped nodal-line Fermi surface from the dHvA oscillations of $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ orbits. The former orbit encircles the nodal line, while the latter does not. Nevertheless, a nontrivial Berry phase is found for both orbits. The nontrivial phase of $\ensuremath{\beta}$ arises from the orbital characters, which can be expressed as a pseudospin rotating around the nodal line. On the other hand, the phase of $\ensuremath{\gamma}$ is attributed to the vortex of real spin texture induced by an antisymmetric spin-orbit interaction. Our result demonstrates that both the real spin and pseudospin textures are indispensable in interpreting the electronic topology in noncentrosymmetric nodal-line semimetals.

Mechanical and electronic properties of van der Waals layered hcp PdH2

Mechanical and electronic properties of palladium dihydrides (PdH2) as a function of pressure were studied by ab initio calculations based on density functional theory (DFT). The ab initio random structure searching technique was employed for screening potential PdH2 crystal structures under high pressure. A hexagonal close packed (hcp) phase of PdH2 with space group P63mc was reported. The structure geometry and elastic constants were calculated as a function of pressure. It was found that H atoms are in the interstitial position of Pd atoms layer at 0 GPa. There is an electronic topology transition of hcp PdH2 at 15 GPa. When pressure exceeds above 15 GPa, one hydrogen atom occupies the tetrahedral site and another hydrogen atom locates in the interstitial position. When the c/a ratio is between 1.765 to 1.875, the hcp PdH2 is mechanically stable, and the Pd-H2b bond is the major factor that limits the mechanical stability. The elastic constant C44 is the first one that cannot satisfy the mechanical stability criteria under pressure. The anisotropy parameters are far from 1(one) shows that the hcp PdH2 is a highly anisotropic structure. The electronic structure study indicates that the bonding force between Pd and H atoms along the z-axis direction increases with the increasing pressure. Also, the phonon dispersion study shows that PdH2 is dynamic stability under pressure. The results suggest that hcp PdH2 can be metastable in van der Waals layered structure.

Hall effect evidence for an interplay between electronic correlations and Na order induced electronic bands topology in NaxCoO2

The incidence of topology on the band structure and physical properties of layered compounds has been extensively studied in semimetals. How those evolve in presence of electronic correlations has been less investigated so far. In the sodium cobaltates NaxCoO2 considered here, unexpected magnetic properties associated with correlations on the Co sites have been disclosed about 15 years ago. The distinct Na orders of the stable phases of these cobaltates have been shown to induce specific Co charge disproportionation with large size unit cells in the CoO2 planes, linked with the diverse magnetic behaviors. This provides an original playground in the studies of interplays between topology and correlations in these layered materials. We present here transport measurements on a series of single crystals and demonstrate that we do synthetize pure phases with quite reproducible transport properties. We show that above room T those display a similar behavior whatever the Na content. On the contrary we provide evidence for a great diversity in Hall effect low temperature dependences which underlines the specificities of the Fermi Surface reconstructions induced by the Na order. We study in some detail the difference between two metallic phases, one (x = 0.77) antiferromagnetic below TN = 22 K and the second (x = 2/3) paramagnetic down to T = 0. Both show a sign change in the Hall effect with decreasing T. We demonstrate that this can be attributed to quite distinct physical effects. For the x = 0.77 phase the negative Hall effect occurs in similar conditions to the anomalous Hall effect found in various magnetic metals. In the x = 2/3 phase have to be assigned to specificities of its Fermi surface. In both cases the anomalies detected in the Hall effect are certainly associated with the topology of their Co electronic bands.

A computational study of phase stability, electronic structure, vibrational and semiconducting properties of ScAu-based half-Heusler alloys

The electronic structure, mechanical stability, phonon dispersion and vibrational characteristics of half-Heusler alloys, ScAuX (X = Si, Ge, Sn, Pb) have been analyzed in framework of Density Functional theory(DFT). The alloys are crystallize in face centered cubic having F4‾3m(#216) space group. First-principles simulation reveals that all the alloys are narrow band gap semiconductors. The electronic structure is investigated using generalized gradient approximation under Perdew-Burke-Ernzerhof parameterization(GGA + PBE). To predict the accurate band gap of the compounds, comparatively more refined hybrid functional such as Heyd-Scuseria-Ernzerhof (HSE06) has been taken into account. The calculated elastic constants verify the mechanical stability of all the systems by tolerating Born-Huang criteria. In addition, the phonon dispersion and shear constant also favor in mechanical stability. A detailed investigation of vibrational characteristics have been performed. On the basis of electronic band structure topology and calculated theoretical limit of minimum lattice thermal conductivity, our simulated results predict that all the alloys are promising candidates for thermoelectric applications.

Nodal Semimetals: A Survey on Optical Conductivity

Among different topological and related phases of condensed matter, nodal semimetals occupy a special place—the electronic band topology in these materials is related to 3D bulk, rather than to surface, states. A great variety of different realizations of electronic band crossings (the nodes) leads to a plethora of different electronic properties, ranging from chiral anomaly to solid‐state realizations of a black‐hole horizon. The different nodal phases have similar low‐energy band structure and quasiparticle dynamics, which both can be accessed experimentally by a number of methods. Optical measurements with their large penetration depth and high energy resolution are ideally suited as such a bulk probe, especially at low energies where other spectroscopic methods often lack the required resolution. Herein, recent optical conductivity studies of different nodal semimetals are reviewed, possible limitations of such measurements are discussed, and a comparison between the experimental results, simple theoretical models, and band‐structure‐based calculations is provided.

Circuit Topologies for MOS-Type Gas Sensor

Metal Oxide Semiconductor or MOS-type gas sensors are resistive sensors which can detect different reducible or volatile gases in atmospheres with oxygen. These gas sensors have been used in different areas such as food and drink industries or healthcare, among others. In this type of sensor, the resistance value changes when it detects certain types of gases. Due to the electrical characteristics, the sensors need a conditioning circuit to transform and acquire the data. Four different electronic topologies, two different MOS-type gas sensors, and different concentrations of a gas substance are presented and compared in this paper. The study and experimental analysis of the properties of each of the designed topology allows designers to make a choice of the best circuit for a specific application depending on the situation, considering the required power, noise, linearity, and number of sensors to be used. This study will give more freedom of choice, the more adequate electronic conditioning topology for different applications where MOS-type sensors are used, obtaining the best accuracy.

On the Stability of Interactions between Pairs of Anions - Complexes of MCl3- (M=Be, Mg, Ca, Sr, Ba) with Pyridine and CN.

The ability of the central M atom of the MCl3- anion, with M=Be, Mg, Ca, Sr, Ba, to engage in a noncovalent bond with an approaching nucleophile is gauged by ab initio methods. The N atom of pyridine forms a M⋅⋅⋅N bond with an interaction energy between 12 and 21 kcal mol-1 , even though the π-hole above the M atom is not necessarily positive in sign. Despite a strong Coulombic repulsion between two anions, CN- is also able to approach the M atom so as to engage in a metastable complex that is higher in energy than the individual anions. The energy barrier separating this complex from its constituent anion pair is roughly 20 kcal mol-1 . Despite the endothermic formation reaction energy of the CN- ⋅⋅⋅MCl3- complex, the electron topology signals a strong interaction, more so than in pyridine⋅⋅⋅MCl3- with its exothermic binding energy. The dianionic complex is held together largely on the strength of interorbital interactions, thereby overcoming a repulsive electrostatic component. The latter is partially alleviated by the pyramidalization of the MCl3 unit which makes its π-hole more positive. The complex sinks below the separate monomers in energy when the system is immersed in an aqueous medium, with a binding energy that varies from as much as 20 kcal mol-1 for Be down to 1.2 kcal mol-1 for Ba.

Surface-state Coulomb repulsion accelerates a metal-insulator transition in topological semimetal nanofilms.

The emergence of quantization at the nanoscale, the quantum size effect (QSE), allows flexible control of matter and is a rich source of advanced functionalities. A QSE-induced transition into an insulating phase in semimetallic nanofilms was predicted for bismuth a half-century ago and has regained new interest with regard to its surface states exhibiting nontrivial electronic topology. Here, we reveal an unexpected mechanism of the transition by high-resolution angle-resolved photoelectron spectroscopy combined with theoretical calculations. Anomalous evolution and degeneracy of quantized energy levels indicate that increased Coulomb repulsion from the surface states deforms a quantum confinement potential with decreasing thickness. The potential deformation strongly modulates spatial distributions of quantized wave functions, which leads to acceleration of the transition beyond the original QSE picture. This discovery establishes a complete picture of the long-discussed transition and highlights a new class of size effects dominating nanoscale transport in systems with metallic surface states.

Fractional Quantum Hall Effect in Weyl Semimetals.

Weyl semimetal may be thought of as a gapless topological phase protected by the chiral anomaly, where the symmetries involved in the anomaly are the U(1) charge conservation and the crystal translational symmetry. The absence of a band gap in a weakly interacting Weyl semimetal is mandated by the electronic structure topology and is guaranteed as long as the symmetries and the anomaly are intact. The nontrivial topology also manifests in the Fermi arc surface states and topological response, in particular taking the form of an anomalous Hall effect in magnetic Weyl semimetals, whose magnitude is only determined by the location of the Weyl nodes in the Brillouin zone. Here we consider the situation when the interactions are not weak and ask whether it is possible to open a gap in a magnetic Weyl semimetal while preserving its nontrivial electronic structure topology along with the translational and the charge conservation symmetries. Surprisingly, the answer turns out to be yes. The resulting topologically ordered state provides a nontrivial realization of the fractional quantum Hall effect in three spatial dimensions in the absence of an external magnetic field, which cannot be viewed as a stack of two dimensional states. Our state contains loop excitations with nontrivial braiding statistics when linked with lattice dislocations.

Weyl-Kondo semimetals in nonsymmorphic systems

There is considerable current interest to explore electronic topology in strongly correlated metals, with heavy fermion systems providing a promising setting. Recently, a Weyl-Kondo semimetal phase has been concurrently discovered in theoretical and experimental studies. The theoretical work was carried out in a Kondo lattice model that is time-reversal invariant but inversion-symmetry breaking. In this paper, we show in some detail how nonsymmorphic space-group symmetry and strong correlations cooperate to form Weyl nodal excitations with highly reduced velocity and pin the resulting Weyl nodes to the Fermi energy. A tilted variation of the Weyl-Kondo solution is further analyzed here, following the recent consideration of such effect in the context of understanding a large spontaneous Hall effect in Ce$_3$Bi$_4$Pd$_3$ (Dzsaber et al., arXiv:1811.02819). We discuss the implications of our results for the enrichment of the global phase diagram of heavy fermion metals, and for the space-group symmetry enforcement of topological semimetals in other strongly correlated settings.

Controller-Embeddable Probabilistic Real-Time Digital Twins for Power Electronic Converter Diagnostics

In this article, an approach is proposed for the online diagnostic analysis of power electronic converters utilizing real-time, probabilistic digital twinning. Under this approach, a digital twin (DT) of a power converter is defined as a real-time, probabilistic simulation model with stochastic (random) variables, developed using generalized polynomial chaos expansion. The DT models are partitioned in perspective of control layers for power converter subsystems in the approach, with emphasis on the application and converter control layers. Real-time executed solvers of these divided probabilistic DT models are embedded into power converter controllers running on field programmable gate array (FPGA) computing devices. Using a monitoring system, the real-time probabilistic DTs at each control layer and the corresponding physical twins of the power converter are compared by the controllers to determine if the power converter is operating within probable behavior. Knowing the large computational cost of probabilistic modeling, the resource usage and timing of real-time DT solvers on modern FPGAs is reported for common power electronic converter topologies, showing the approach is feasible and able to perform probabilistic real-time simulation of smaller power converters in perspective of application and converter control layers using $\leq 2\; \mu s$ time steps, with as low as $\text{70}\;{\rm ns}$ steps; while staying embeddable within FPGA-based controllers. To highlight the capabilities of the proposed approach, a case study is presented using a probabilistic DT in the application layer controller of a pair of converters to comparatively monitor their behavior and corresponding controller action under hardware-in-the-loop testing.

Aperiodic topological crystalline insulators

Topological crystalline insulators (TCIs) are usually described with topological protection imposed by the crystalline symmetry. In general, however, the existence of TCI states does not necessitate the periodicity of crystals as long as an essential lattice symmetry can be identified. Here we demonstrate the compatibility of TCIs with aperiodic systems, as exemplified by an octagonal quasicrystal. The aperiodic TCIs we proposed are attributed to a band inversion mechanism, which inverts states with the same parity but opposite eigenvalues of a specific symmetry (such as mirror reflection). The nontrivial topology is characterized by a nonzero integer ``mirror Bott index.'' Moreover, we demonstrate that the topological edge states and quantized conductance of the aperiodic TCI, which are robust against disorder, can be effectively manipulated by external electric fields. Our findings not only provide a better understanding of electronic topology in relation to symmetry but also extend the experimental realization of topological states to much broader material categories beyond crystals.

Theoretical study on prismatic (N6)n (n=16–35) molecules

As a continuation of our study on the stability of pure nitrogen molecules, twenty members of the (N6)n (n = 16–35) molecular sequence with D3h or D3d symmetry alternatively are studied in this work. The structures and energies are examined at the B3LYP/cc‒pvDZ computational level. “Natural Bond Orbital (NBO)” and “Atom In Molecule (AIM)” analyses are performed to investigate the bonding properties and the electronic topologies of the prismatic molecules. The van der Waals forces or the dispersion interactions are identified to be the dominant stabilizing factor for large prismatic nitrogen cages. The van der Waals forces interweave with covalent bonds to form a dense network which tightly bind nitrogen atoms at grid points, and consequently, to make the prismatic (N6)n molecules stable. The mechanism of generating van der Waals forces in the prismatic molecules is the focus of this paper. Geometrically, all the polygons on the prismatic surface are boat hexagons except the two caps. The van der Waals forces or dispersion interactions occur only between the nitrogen atoms at positions 1′,4′ of the boat hexagon on the surface. The atom‒atom proximity effect of nitrogen atoms at the positions 1′,4′ leads to the van der Waals interaction. Topologically, van der Waals forces are generated by partial overlap of sp3 hybrid orbitals of lone pairs of nitrogen atoms at positions 1′,4’. These large prism‒shaped nitrogen cages may also be novel beeline nanotubes which is environmentally friendly. This work is expected to open up a bright prospect of the study and applications of nitrogen nanofibers.