Electronic Topology Research Articles

First-principles theoretical analysis of magnetically tunable topological semimetallic states in antiferromagnetic DyPdBi

A number of compounds in the family of rare-earth half-Heusler alloys have been predicted to be topologically nontrivial semimetals, classified as Weyl, triple-point, and Dirac semimetals based on the multiplicity of degeneracy of the nodal points or crossing of linearly dispersed electronic bands. Here, we present a first-principles theoretical characterization of the electronic topology of the antiferromagnetic half-Heusler alloy DyPdBi. In the antiferromagnetic state preserving ${C}_{3v}$ symmetry of the crystal, DyPdBi is a triple-point semimetal hosting four triply degenerate nodes along the threefold symmetry axis of the Brillouin zone. In contrast, the antiferromagnetic state of DyPdBi with local magnetic moments on Dy rotated to a direction perpendicular to the ${C}_{3}$ axis breaks the threefold rotational symmetry, and hosts four Weyl nodes in its Brillouin zone. Our calculations of the Berry curvature of their electronic states clearly show that the triple-point fermions of DyPdBi exhibit a signature peak in the anomalous Hall conductivity, while the Weyl fermions do not contribute to anomalous Hall conductivity. As these two topologically distinct magnetic states are separated by a small energy difference of $\ensuremath{\approx}$15 meV, we expect them to be switchable with a magnetic field or spin torque and distinguishable experimentally from anomalous Hall conductance.

Visualizing Higher-Fold Topology in Chiral Crystals.

Novel topological phases of matter are fruitful platforms for the discovery of unconventional electromagnetic phenomena. Higher-fold topology is one example, where the low-energy description goes beyond standard model analogs. Despite intensive experimental studies, conclusive evidence remains elusive for the multigap topological nature of higher-fold chiral fermions. In this Letter, we leverage a combination of fine-tuned chemical engineering and photoemission spectroscopy with photon energy contrast to discover the higher-fold topology of a chiral crystal. We identify all bulk branches of a higher-fold chiral fermion for the first time, critically important for allowing us to explore unique Fermi arc surface states in multiple interband gaps, which exhibit an emergent ladder structure. Through designer chemical gating of the samples in combination with our measurements, we uncover an unprecedented multigap bulk boundary correspondence. Our demonstration of multigap electronic topology will propel future research on unconventional topological responses.

Experimental and theoretical revelation of a unique band topology in Sb[formula omitted]Te[formula omitted] topological insulator by substitution of Cu—A high pressure study

TI Sb1.9Cu0.1Te3 is an ideal platform for investigating such semiconducting to metal transition under extreme conditions. The unusual softening of two newly M1, M2 phonon modes combined with the sudden collapse in bond length, angle of succession, intralayer distance and the direct observation of band gap closure from theoretical density functional theory calculation validates a unique metal transition. To the best of our knowledge, the metallic transition pressure in the present observation is the lowest amongst the A2B3 TI family of compounds. This distinctive transition is stimulated by the p-d hybridization which is the concomitant effect of Cu and applied pressure. We also establish structural and vibrational anomalies from pressure induced angle dispersive synchrotron x-ray diffraction and Raman spectra of the bulk that signify changes in electronic topology of the Fermi surface. Our experimental observations on bulk corroborates an ETT (∼2.5 GPa) and 3 structural transitions in the present system.

DC–DC Converters for Bipolar Microgrid Voltage Balancing: A Comprehensive Review of Architectures and Topologies

DC microgrids initiated the change of a paradigm regarding the concept about electrical distribution networks, especially in the context of the distributed generation associated with renewable energies. However, this new reality opens a new area of research, in which several aspects must be carefully studied. Indeed, the bipolar design is one of the principal dc microgrid configurations considering its characteristic wiring. Although holding many promising advantages, the bipolar dc microgrid has a tendency toward voltage and current imbalances due to the unequal distribution of the loads and generators between the two poles. Thus, specific power electronic-based solutions are required to ensure the balance of these dc microgrids. Within this frame, this article gives a comprehensive review of the multiple architectures and power electronic topologies proposed to mitigate/eliminate this undesired condition. The following provides an insightful classification and discussion with the pros and cons of these solutions. This work can serve as a timely review for researcher/engineers who want to enter the voltage balancing field in the bipolar dc grids and promote the innovation of their power electronics-enabled solutions.

Understanding the electronic pi-system of 2D covalent organic frameworks with Wannier functions

We investigate a family of hexagonal 2D covalent organic frameworks (COFs) with phenyl and biphenyl spacer units and different chemical linker species. Chemical trends are elucidated and attributed to microscopic properties of the pi-electron-system spanned by atomic p_z-orbitals. We systematically investigate the electronic structure, delocalization of electronic states, effects of disorder, bond torsion, and doping, and correlate these with variable pi-conjugation and nucleus-independent chemical shift (NICS) aromaticity. Molecular orbitals are obtained from maximally localized Wannier functions that have sigma- and pi-character, forming distinct sigma- and pi-bands for all valence states. The Wannier-orbital description goes beyond simple tight-binding models and enables a detailed understanding of the electronic topology, effective electronic coupling and delocalization. It is shown that a meaningful comparison between COFs with different chemical elements can only be made by examining the entire pi-electron system, while a comparison of individual bands (e.g., bands near the Fermi energy) can be a insufficient to derive general design rules for linker and spacer monomer selection. We further identify delocalized states that are spread across tens or hundreds of pores of the 2D COFs and analyze their robustness against structural and energetic disorders like out-of-plane rotations of molecular fragments, different strength of energetic disorder and energetic shifts due to chemical doping.

Overview of Integration of Power Electronic Topologies and Advanced Control Techniques of Ultra-Fast EV Charging Stations in Standalone Microgrids

For longer journeys, when drivers of electric vehicles need a charge on the road, the best solution is off-board ultra-fast chargers, which offer a short charging time for electric vehicle batteries. Consequently, the ultra-fast charging of batteries is a major issue in electric mobility development globally. Current research in the area of power electronics for electric vehicle charging applications is focused on new high-power chargers. These chargers will significantly increase the charging power of electric vehicles, which will reduce the charging time. Furthermore, electric vehicles can be deployed to achieve improved efficiency and high-quality power if vehicle to microgrid (V2µG) is applied. In this paper, standards for ultra-fast charging stations and types of fast charging methods are reviewed. Various power electronic topologies, the modular design approach used in ultra-fast charging, and integration of the latter into standalone microgrids are also discussed in this paper. Finally, advanced control techniques for ultra-fast chargers are addressed.

Symmetries in Power Electronics and Lattice Converters

The past 50 years witnessed the inventions of more than one thousand power electronic topologies. However, we still lack a common standard to compare and classify various topologies. Moreover, topologies are often created through intuition rather than systematic approaches. The intuition of voltage and current sharing, along with modularity and scalability, gives rise to cascaded-bridge and multi-parallleled converters, respectively. Nevertheless, power converters with both high-voltage and large-current capabilities are still obscure yet important. This article first investigates the symmetries of circuit topologies through group theory. Notably, symmetries lie in twofold—individual modules and connections. Symmetries not only help to classify topologies but also play an important role in the creation of new topologies. Based on wallpaper groups, this article proposes the lattice converters, which readily scale to manage high voltages and large currents. The proposed lattice converters benefit from 1) modularity and scalability, 2) boundless current and voltage ratings, 3) multiple input/output ports, 4) sensorless voltage balancing, 5) high power quality, and 6) flexible control. Simulation and experimental results demonstrate the effectiveness and superiority of the proposed theory and lattice converters.

Interstitial‐Electron‐Induced Topological Molecular Crystals

AbstractTopological phases are usually unreachable in molecular solids, which are characterized by weakly dispersed energy bands with a large gap, in contrast to topological materials. In this work, however, it is proposed that nontrivial electronic topology may ubiquitously emerge in a class of molecular crystals that contain interstitial electronic states, the bands of which are prone to be inverted with those of molecular orbitals. Guidelines are provided to hunt for such interstitial‐electron‐induced topological molecular crystals, especially in the topological insulating state. They exhibit a variety of exceptional qualities, as brought about by the intrinsic interplay of molecular crystals, interstitial electrons, and topological nature: 1) They may host cleavable surfaces along multiple orientations, with pronounced topological boundary states free from dangling bonds. 2) Strong response to moderate mechanical perturbations, whereby topological phase transition would occur under relatively low pressure. 3) Inherent high‐efficiency thermoelectricity as jointly contributed by the non‐parabolic band structure (therewith high thermopower), highly mobile interstitial electrons (high electrical conductivity), and soft phonons (small lattice thermal conductivity). 4) Ultralow work function owing to the active interstitial electrons. First‐principles calculations are utilized to demonstrate these properties with the representative candidate K4Ba2[SnBi4]. This work suggests a pathway of realizing topological phases in bulk molecular systems, which may advance the interdisciplinary research between topological and molecular materials.

Highly Efficient BBFIC for Grid-Connected Photovoltaic-Battery Energy Storage System Using Hybrid Optimization Assisted Framework

The conventional grid has been combined with a number of renewable energy solutions. As a result, the modern power system is constantly evolving with the help of various power electronics topologies. Due to the growing complexity and new system configuration, the modern power grid is subjected to many challenges and the risk factors are increasing exponentially. In order to evaluate the performance of grid-connected solar photovoltaic (PV) energy systems with battery energy storage system (BESS), highly efficient buck-boost-flyback integrated converter (BBFIC) experiments have been conducted. The whole cybernetics methodology utilized for real-time, closed-loop simulation, validation with other researchers, and numerous optimization techniques are applied in the research work. This paper is developing a new framework that focuses on increasing the voltage gain of the BBFIC for grid-tied PV-BESS. For enhancing the voltage gain of BBFIC, it is planned to optimize the duty cycle of the converter. For optimization purposes, this work deploys a hybrid algorithm termed as “alpha score based on salp-grey wolf optimization (AS-GWO)” that combines the concepts of both “grey wolf optimization (GWO) as well as salp swarm algorithm (SSA).” Finally, it has been demonstrated that the results are superior when compared to other approaches in terms of numerous aspects of measurements.

Electronic and topological properties of extended two-dimensional Su-Schrieffer-Heeger models and realization of flat edge bands

One-dimensional Su-Schrieffer-Heeger (SSH) chains are one of the simplest topological models and have been studied extensively. The two-dimensional $(2\mathrm{D})$ version has been confirmed to have a nontrivial topology. In this work, we further extend the $2\mathrm{D}$ SSH model by constructing different configurations, including all possible configurations with 4-site unit cells and another two complicated, but typical, configurations with 8- and 36-site unit cells, respectively. We calculate and analyze the electronic structures and topologies of these SSH models in detail and identify several rich and novel properties, such as topologically protected edge states, metallic chains with different shapes, and a bulk--edge separation in a metal system. In particular, a flat-band feature of topological edge states is obtained. By analyzing the spatial distribution of these edge states, we explain the origin of the flat edge bands and show that bonding squares play a crucial role in the formation of flat bands. Our study can be generalized to more configurations and higher dimensions, providing a basis for further theoretical and experimental explorations.

The CNDOL Fockian with the configuration interaction of single excited wave functions to model the exciton properties of large molecular systems for photovoltaic devices

The approximate CNDOL Fockian, together with the configuration interaction of single excitations ([CIS|CNDOL/21]), is applied to investigate the electronic properties of three different kinds of donors (D) considered suitable for photovoltaic devices, either as isolated systems or complexed with fullerene (C60) as potential acceptors (A). Calculated charge maps, together with the Coulomb-exchange term of CIS lowest transitions, are used to understand the electronic topology and possible behaviour of D-A excitons. The effects of varying donor structures are compared. The lowest excitation energies of the donor-C60 complexes arise from electron densities localised at the central ring units in the donors. Nevertheless, C60 does not always participate in electron sharing upon excitation. It sometimes acts only as a structural companion for enhancing or not the exciton capability to provide adequate photoelectric power conversion efficiencies. The [CIS|CNDOL/21] method is validated as a valuable tool for modelling structure-related properties of molecular excitons and for predicting the performance of D–A materials in relatively big supramolecular systems. It is shown that multi-electron CIS wavefunctions are at least as useful as one-electron molecular orbitals to describe charge-transfer properties of excitons and provide a physical picture of the whole electron density.

A Review on Power Electronic Topologies and Control for Wave Energy Converters

Ocean energy systems (OESs) convert the kinetic, potential, and thermal energy from oceans and seas to electricity. These systems are broadly classified into tidal, wave, thermal, and current marine systems. If fully utilized, the OESs can supply the planet with the required electricity demand as they are capable of generating approximately 2 TW of energy. The wave energy converter (WEC) systems capture the kinetic and potential energy in the waves using suitable mechanical energy capturers such as turbines and paddles. The energy density in the ocean waves is in the range of tens of kilowatts per square meter, which makes them a very attractive energy source due to the high predictability and low variability when compared with other renewable sources. Because the final objective of any renewable energy source (RES), including the WECs, is to produce electricity, the energy capturer of the WEC systems is coupled with an electrical generator, which is controlled then by power electronic converters to generate the electrical power and inject the output current into the utility AC grid. The power electronic converters used in other RESs such as photovoltaics and wind systems have been progressing significantly in the last decade, which improved the energy harvesting process, which can benefit the WECs. In this context, this paper reviews the main power converter architectures used in the present WEC systems to aid in the development of these systems and provide a useful background for researchers in this area.

Extreme Optical Anisotropy in the Type-II Dirac Semimetal NiTe2 for Applications to Nanophotonics

Compared to artificial metamaterials, where nano-fabrication complexities and finite-size inclusions can hamper the desired electromagnetic response, several natural materials like van der Waals crystals hold great promise for designing efficient nanophotonic devices in the optical range. Here, we investigate the unusual optical response of NiTe$_2$, a van der Waals crystal and a type-II Dirac semimetal hosting Lorentz-violating Dirac fermions. By {\it ab~initio~} density functional theory modeling, we show that NiTe$_2$ harbors multiple topological photonic regimes for evanescent waves (such as surface plasmons) across the near-infrared and optical range. By electron energy-loss experiments, we identify surface plasmon resonances near the photonic topological transition points at the epsilon-near-zero (ENZ) frequencies $\approx 0.79$, $1.64$, and $2.22$ eV. Driven by the extreme crystal anisotropy and the presence of Lorentz-violating Dirac fermions, the experimental evidence of ENZ surface plasmon resonances confirm the non-trivial photonic and electronic topology of NiTe$_2$. Our study paves the way for realizing devices for light manipulation at the deep-subwavelength scales based on electronic and photonic topological physics for nanophotonics, optoelectronics, imaging, and biosensing applications.

Giant phonon anomaly in topological nodal-line semimetals

The band topology has been revealed to bring a wide array of novel electronic transport phenomena, but its interaction with phonon properties remains largely unexplored. Here we propose that continuous topological singularities with variable Fermi wave vectors can lead to giant effects on phonon transport in nodal-line semimetals. Using first-principles calculations, we show that such an exotic feature is present in a well-known topological semimetal ZrSiSe, in which the unique nodal-line can guarantee considerable momentum-continuous phonons to satisfy the stringent condition of Kohn anomaly and thereby dramatically strengthen the electron-phonon coupling. Moreover, the momentum-continuous anomaly phonons with eight arms of vertical Fermi surface drive intense phonon-electron scattering that is even comparable with intrinsic phonon-phonon scattering, giving rise to a reduction of ∼45% on room temperature lattice thermal conductivity. Our findings not only uncover giant phonon anomaly in topological nodal-line semimetals but also provide guidance for exploring the vital influence of electronic band topology on thermal transport bridged by the electron-phonon coupling.

An overview of Uninterruptible Power Supply Systems

In the modern world, when there is a power outage or a power failure, telecommunication systems, computer systems, and many other critical equipment, such as medical equipment, require uninterrupted power to support their operation. Uninterruptible power supply (UPS) systems are used for this purpose. Over the years, research on UPS systems and related publications have increased. Also, new opportunities for UPS systems have emerged with the development of novel storage technologies, power electronic topologies, rapid electronic devices, high-performance digital apps, and other technological advances. Servers and storage systems, personal computers, medical equipment, telecommunication systems, and industrial equipment all require clean, stable, and uninterrupted power supply from UPS systems. Several recent studies have focused on the design of UPS systems to provide continuous power under normal or abnormal power conditions, including power outages. Such UPS systems use energy storage technologies such as batteries or flywheels to provide power to loads in the absence of applied power. Typically, static power electronics such as fast-switching high-current insulated gate bipolar transistors (IGBTs) are used to convert power. This article discusses the most typical power line issues and how they relate to the various types of UPS systems available today.

Amorphous Kane-Mele model in disordered hyperuniform two-dimensional networks

As a prototype system of the quantum spin Hall effect, the Kane-Mele model which was proposed initially in graphene promotes the search for two-dimensional topological materials of hexagonal lattices. Here we generalize the Kane-Mele model to exotic amorphous systems which possess a remarkable structural property called hyperuniformity. We show that, in general, the Quantum spin Hall state still survives in disordered hyperuniform lattices that are constructed by structural transformation involving Stone-Wales defects. However, compared to that in the honeycomb lattice, the gapped topological region in the phase diagram shrinks and the size of the corresponding topological gap decreases remarkably in disordered hyperuniform lattices. By introducing random vacancies in either perfectly ordered or disordered hyperuniform lattices, we further show that the degradation or destruction of hyperuniformity is detrimental to the existence of gapped topological states. We therefore propose that the hyperuniform metric of an amorphous lattice, which quantifies the extent of disordered hyperuniformity, reflects its ability to preserve topological states. Our finding not only establishes the possible underlying link between electronic topology and disordered hyperuniformity but also provides useful guidance for the seeking of topological materials in amorphous states of matter.

Different Topologies of Electrical Machines, Storage Systems, and Power Electronic Converters and Their Control for Battery Electric Vehicles—A Technical Review

Electric vehicles (EVs) are emerging as an alternative transportation system owing to a reduction in depleting lubricates usage and greenhouse gas emissions. This paper presents a technical review of each and every sub-system and its feasible control of battery EV (BEV) propulsion units. The study includes the possible combination of electrical machines (EMs), storage system, and power electronic converters and their associated control strategies. The primary unit, i.e., EM, is the heart of the EV, which is used to drive the vehicle at the desired speed as well as to restore the regenerative braking (RB) energy that is generated to enhance the overall system reliability. To electrify the transportation sector, it is necessary to include new options of power electronic converter topologies and their associated control strategies for numerous reasons, which include extracting maximum power from sources in case the EV is powered from renewable energy resources, boosting the energy storage capability for longer electric range, managing power flow from the source to battery or battery to vehicle or vehicle to battery, and regulating the speed of the vehicle and braking control. Based on the survey, the suitable combination of sub-systems and their control for three and four-wheeler EVs are summarized in this paper.

Crystal growth and electronic properties of LaSbSe.

The ZrSiS-type materials have gained intensive attentions. The magnetic version of the ZrSiS-type materials, LnSbTe (Ln = Lanthanide), offers great opportunities to explore new quantum states owing to the interplay between magnetism and electronic band topology. Here, we report the growth and characterization of the non-magnetic LaSbSe of this material family. We found the metallic transport, low magnetoresistance and non-compensated charge carriers with relatively low carrier density in LaSbSe. The specific heat measurement has revealed distinct Sommerfeld coefficient and Debye temperature in comparison to LaSbTe. Such addition of a new LnSbSe selenide compound could provide the alternative material choices in addition to LnSbTe telluride materials.

Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands, and electronic nematicity, our work establishes Fe3Sn2 as an interesting platform that harbors an extraordinarily wide array of topological and correlated electron phenomena.

Structure-Imposed Electronic Topology in Cove-Edged Graphene Nanoribbons.

In cove-edged zigzag graphene nanoribbons (ZGNR-Cs), one terminal CH group per length unit is removed on each zigzag edge, forming a regular pattern of coves that controls their electronic structure. Based on three structural parameters that unambiguously characterize the atomistic structure of ZGNR-Cs, we present a scheme that classifies their electronic state (i.e., if they are metallic, topological insulators, or trivial semiconductors) for all possible widths N, unit lengths a, and cove position offsets at both edges b, thus showing the direct structure-electronic structure relation. We further present an empirical formula to estimate the band gap of the semiconducting ribbons from N, a, and b. Finally, we identify all geometrically possible ribbon terminations and provide rules to construct ZGNR-Cs with a well-defined electronic structure.