Class Of Insulators Research Articles

Control of superexchange interactions with DC electric fields

We discuss DC electric-field controls of superexchange interactions. We first present generic results about antiferromagnetic and ferromagnetic superexchange interactions valid in a broad class of Mott insulators, where we also estimate typical field strength to observe DC electric-field effects: $\sim 1~\mathrm{MV/cm}$ for inorganic Mott insulators such as transition-metal oxides and $\sim 0.1~\mathrm{MV/cm}$ for organic ones. Next, we apply these results to geometrically frustrated quantum spin systems. Our theory widely applies to (quasi-)two-dimensional and thin-film systems and one-dimensional quantum spin systems on various lattices such as square, honeycomb, triangular, and kagome ones. In this paper, we give our attention to those on the square lattice and on the chain. For the square lattice, we show that DC electric fields can control a ratio of the nearest-neighbor and next-nearest-neighbor exchange interactions. In some realistic cases, DC electric fields make the two next-nearest-neighbor interactions nonequivalent and eventually turns the square-lattice quantum spin system into a deformed triangular-lattice one. For the chain, DC electric fields can induce singlet-dimer and Haldane-dimer orders. We show that the DC electric-field-induced spin gap $\propto |\boldsymbol E|^{2/3}$ in the Heisenberg antiferromagnetic chain will reach $\sim 10~\%$ of the dominant superexchange interaction in the case of a spin-chain compound $\mathrm{KCuMoO_4(OH)}$ when the DC electric field of $\sim 1~\mathrm{MV/cm}$ is applied.

Electron-Phonon and Spin-Lattice Coupling in Atomically Thin Layers of MnBi2Te4.

MnBi2Te4 represents a new class of magnetic topological insulators in which novel quantum phases emerge at temperatures higher than those found in magnetically doped thin films. Here, we investigate how couplings between electron, spin, and lattice are manifested in the phonon spectra of few-septuple-layer thick MnBi2Te4. After categorizing phonon modes by their symmetries, we study the systematic changes in frequency, line width, and line shape of a spectrally isolated A1g mode. The electron-phonon coupling increases in thinner flakes as manifested in a broader phonon line width, which is likely due to changes of the electron density of states. In 4- and 5-septuple thick samples, the onset of magnetic order below the Néel temperature is concurrent with a transition to an insulating state. We observe signatures of a reduced electron-phonon scattering across this transition as reflected in the reduced Fano parameter. Finally, spin-lattice coupling is measured and modeled from temperature-dependent phonon frequency.

One-dimensional 2n -root topological insulators and superconductors

Square-root topology is a recently emerged subfield describing a class of insulators and superconductors whose topological nature is only revealed upon squaring their Hamiltonians, i.e., the finite energy edge states of the starting square-root model inherit their topological features from the zero-energy edge states of a known topological insulator/superconductor present in the squared model. Focusing on one-dimensional models, we show how this concept can be generalized to $2^n$-root topological insulators and superconductors, with $n$ any positive integer, whose rules of construction are systematized here. Borrowing from graph theory, we introduce the concept of arborescence of $2^n$-root topological insulators/superconductors which connects the Hamiltonian of the starting model for any $n$, through a series of squaring operations followed by constant energy shifts, to the Hamiltonian of the known topological insulator/superconductor, identified as the source of its topological features. Our work paves the way for an extension of $2^n$-root topology to higher-dimensional systems.

Multipolar topological field theories: Bridging higher order topological insulators and fractons

Two new recently proposed classes of topological phases, namely fractons and higher order topological insulators (HOTIs), share at least superficial similarities. The wide variety of proposals for these phases calls for a universal field theory description that captures their key characteristic physical phenomena. In this work, we construct topological multipolar response theories that capture the essential features of some classes of fractons and higher order topological insulators. Remarkably, we find that despite their distinct symmetry structure, some classes of fractons and HOTIs can be connected through their essentially identical topological response theories. More precisely, we propose a topological quadrupole response theory that describes both a 2D symmetry enriched fracton phase and a related bosonic quadrupolar HOTI with strong interactions. Such a topological quadrupole term encapsulates the protected corner charge modes and, for the HOTI, predicts an anomalous edge with fractional dipole moment. In 3D we propose a dipolar Chern-Simons theory with a quantized coefficient as a description of the response of both second order HOTIs harboring chiral hinge currents, and of a related fracton phase. This theory correctly predicts chiral currents on the hinges and anomalous dipole currents on the surfaces. We generalize these results to higher dimensions to reveal a family of multipolar Chern-Simons terms and related $\theta$-term actions that can be reached via dimensional reduction or extension from the Chern-Simons theories.

Realization of quasicrystalline quadrupole topological insulators in electrical circuits

Quadrupole topological insulators are a new class of topological insulators with quantized quadrupole moments, which support protected gapless corner states. The experimental demonstrations of quadrupole-topological insulators were reported in a series of artificial materials, such as photonic crystals, acoustic crystals, and electrical circuits. In all these cases, the underlying structures have discrete translational symmetry and thus are periodic. Here we experimentally realize two-dimensional aperiodic-quasicrystalline quadrupole-topological insulators by constructing them in electrical circuits, and observe the spectrally and spatially localized corner modes. In measurement, the modes appear as topological boundary resonances in the corner impedance spectra. Additionally, we demonstrate the robustness of corner modes on the circuit. Our circuit design may be extended to study topological phases in higher-dimensional aperiodic structures.

Bulk-boundary-defect correspondence at disclinations in rotation-symmetric topological insulators and superconductors

We study a link between the ground-state topology and the topology of the lattice via the presence of anomalous states at disclinations -- topological lattice defects that violate a rotation symmetry only locally. We first show the existence of anomalous disclination states, such as Majorana zero-modes or helical electronic states, in second-order topological phases by means of Volterra processes. Using the framework of topological crystals to construct d-dimensional crystalline topological phases with rotation and translation symmetry, we then identify all contributions to (d-2)-dimensional anomalous disclination states from weak and first-order topological phases. We perform this procedure for all Cartan symmetry classes of topological insulators and superconductors in two and three dimensions and determine whether the correspondence between bulk topology, boundary signatures, and disclination anomaly is unique.

INVESTIGATION OF THE TEMPERATURE FACTORS INFLUENCE ON OPERATING SAFETY THE SUBMERSIBLE ELECTROMECHANICAL CONVERTER

The article presents a mathematical model of the thermal field for determining the values of temperatures during the submersible electromechanical converter operation. The influence of obtained temperature values on the safety and reliability of the submersible electromechanical converter is analyzed. In a submersible electromechanical converter the windings temperature has great importance. On the one hand, the windings temperature must be such as to transfer a sufficient amount of heat to the viscous loading environment for its processing (movement, transportation, etc.) to begin. On the other hand, the windings temperature must not exceed the limit values for the corresponding insulation class, since this can cause an emergency (fire, short circuit, etc.) The obtained results shows, that temperature on the surface of rotor's cylinders reaches 135 ° C, which provides rapid heating of a viscous substance, and therefore high performance of the pumping process. In this work, the bitumen BND 200/300 was used as the loading and cooling environment. The flash point of this brand of bitumen is 220 ° C and characterizes the degree of flammability of bitumen when it heated. Bitumen does not reach the maximum allowable temperature as a result of heating by means of submersible electromechanical converter that creates a safe temperature corridor in a technological chain at its processing. The temperature of the outer surface of the submersible electromechanical converter, with which the service technical staff may have a contact, does not exceed 20 ° C (Fig. 3), which is completely safe and does not endanger the workers health or life. Also, the obtained temperatures values are within the permissible limits of the heat resistance class of a winding electrical insulating material (H - 180 ° C), which also contributes to the safe submersible electromechanical converter operation.

Lightweight concrete based on foam glass gravel: composition, structure and properties

The article discusses the research results, the purpose of which is to provide the possibility of increasing the physical and mechanical as well as heat-shielding properties of lightweight cement concrete based on foam-glass gravel. The authors also investigated the possibility of reducing the alkali-silica reaction occurring in such concretes. To evaluate the course of the alkali-silica reaction, a method is used based on the study of the microstructure of lightweight concrete samples. The article also assesses the physical and mechanical as well as thermal engineering properties of lightweight concrete based on foam-glass gravel. The determination of properties is carried out according to standard methods. Analysis of the microstructure lightweight concrete samples allowed to fix flowing alkali-silica reaction. To reduce the alkali-silicate interaction, the authors propose introducing into the concrete composition a complex additive consisting of a siliceous modifier and a plasticizing component. The developed compositions of lightweight concrete, belonging to the class of structural and thermal insulation, have a compressive strength of 6.84-9.05 MPa, a density mark grade of D500-D1500 and a thermal conductivity of 0.218-0.282 W/(m°C).

Effect of surface treatment on mechanical, physical and morphological properties of oil palm/bagasse fiber reinforced phenolic hybrid composites for wall thermal insulation application

The effect of 2%v/v silane and 4%v/v hydrogen peroxide treatment on mechanical, physical and morphological characterization of oil palm empty fruit bunch (OPEFB) and sugarcane bagasse (SCB) fiber reinforced bio-phenolic hybrid composites has been evaluated in this work. The treated OPEFB and SCB fibers has been prepared with different ratio while maintaining 50%wt fiber loading, and then incorporated with the bio-phenolic resin by hand layup technique to produce pure and hybrid composites. Universal testing machine INSTRON has been used for tensile, flexural, and compressive strength analysis. Water absorption and thickness swelling are determined after 24 h. Fracture behaviour, void and fiber pull out of the specimen was observed by using scanning electron microscope in morphological analysis. The hybridization of silane treated 7OPEFB:3SCB fiber indicates better highest performance on tensile strength and modulus with 11.67 MPa and 1348.43 MPa. The silane treated 5OPEFB:5SCB hybrid composites show highest flexural and compressive strength, 16.82 MPa and 6.53 MPa, respectively. Obtained result showed silane treated 3OPEFB:7SCB hybrid composites displays lowest water absorption and thickness swelling after 24 h analysis and show less void content. This study indicated that 2%v/v silane coupling agent gives better enhancement of mechanical properties compared to 4%v/v hydrogen peroxide treatment. Silane treated 5OPEFB:5SCB fiber reinforced bio-phenolic hybrid composites fulfil requirement of the mechanical and physical properties needed for insulation board as per standard. It can be concluded from this study that silane treatment improve the performance of agriculture residue and the hybridization of bio-composites have potential to develop new class of eco-friendly thermal insulation and sustainable wall building materials.

ТЕПЛОВА МОДЕЛЬ СИСТЕМИ «АСИНХРОННИЙ ГЕНЕРАТОР–АСИНХРОННИЙ ДВИГУН» ПРИ НЕСИМЕТРІЇ В ОБМОТКАХ СТАТОРА

Purpose. Development of the IG model for estimation of influence of variations of parameters of the generator on quality of process of self–excitation at definition of the basic and boundary operating modes and system of initial excitation at invariable parameters of the generator. Result. The article presents studies of the system "asynchronous generator-asynchronous motor" with parametric asymmetry to determine the quality of generated electricity in load modes of operation on a mathematical model. The assessment of the thermal state in steady-state conditions was carried out using an equivalent thermal equivalent circuit. Thermal transients were investigated when starting an asynchronous electric motor from an autonomous power source based on an asynchronous generator. On a thermal mathematical model, a study of the influence of the asymmetry of the output voltage and its deviation from the nominal value on the heating of the connected asynchronous motor was carried out. A regression model has been developed for studying the operating conditions of electricity consumers when powered by an asynchronous generator with an asymmetry of the stator windings. Practical value. The use of the obtained equations will make it possible to determine the most rational combination of factors affecting the heating of the stator windings of asynchronous machines, at which they will not overheat in excess of the maximum permissible temperature values of the corresponding insulation classes. Figures 9, tables 2, references 23.

Higher-order topological insulator in a dodecagonal quasicrystal

Higher-order topological insulators (HOTIs) are a newly discovered class of topological insulators which exhibit unconventional bulk-boundary correspondence. Very recently, the concept of HOTIs has been extended to aperiodic quasicrystalline systems, where the topological band theory fails to describe topological phases. More importantly, a novel HOTI phase protected by an eightfold rotational symmetry, not found in crystalline materials, was identified in the Ammann-Beenker tiling quasicrystals. Here we report the discovery of a quasicrystalline HOTI in a dodecagonal quasicrystal. The quasicrystalline HOTI supports twelve in-gap zero-energy modes symmetrically distributed at the corners of the quasicrystal dodecagon. These zero-energy corner modes are protected by a combination of a twelvefold rotational symmetry and mirror symmetry, as well as particle-hole symmetry, which has no crystalline counterpart.

Double Majorana vortex zero modes in superconducting topological crystalline insulators with surface rotation anomaly

The interplay of time-reversal and $n$-fold rotation symmetries ($n=2,4,6$) is known to bring a new class of topological crystalline insulators (TCIs) having $n$ surface Dirac cones due to surface rotation anomaly. We show that the proximity-induced $s$-wave superconductivity on the surface of these TCIs yields a topological superconducting phase in which two Majorana zero modes are bound to a vortex, and that $n$-fold rotation symmetry ($n=2,4,6$) enriches the topological classification of a superconducting vortex from $\mathbb{Z}_2$ to $\mathbb{Z}_2\times\mathbb{Z}_2$. Using a model of a three-dimensional high-spin topological insulator with $s$-wave superconductivity and two-fold rotation symmetry, we show that, with increasing chemical potential, the number of Majorana zero modes at one end of a vortex changes as $2\to1\to0$ through two topological vortex phase transitions. In addition, we show that additional magnetic-mirror symmetry further enhances the topological classification to $\mathbb{Z} \times \mathbb{Z}$

From magnetoelectric response to optical activity

We apply a microscopic theory of polarization and magnetization to crystalline insulators at zero temperature and consider the orbital electronic contribution of the linear response to spatially varying, time-dependent electromagnetic fields. The charge and current density expectation values generally depend on both the microscopic polarization and magnetization fields, and on the microscopic free charge and current densities. But contributions from the latter vanish in linear response for the class of insulators we consider. Thus we need only consider the former, which can be decomposed into "site" polarization and magnetization fields, from which "site multipole moments" can be constructed. Macroscopic polarization and magnetization fields follow, and we identify the relevant contributions to them; for electromagnetic fields varying little over a lattice constant these are the electric and magnetic dipole moments per unit volume, and the electric quadrupole moment per unit volume. A description of optical activity and related magneto-optical phenomena follows from the response of these macroscopic quantities to the electromagnetic field and, while in this paper we work within the independent particle and frozen-ion approximations, both optical rotary dispersion and circular dichroism can be described with this strategy. Earlier expressions describing the magnetoelectric effect are recovered as the zero frequency limit of our more general equations. Since our site quantities are introduced with the use of Wannier functions, the site multipole moments and their macroscopic analogs are generally gauge dependent. However, the resulting macroscopic charge and current densities, together with the optical effects to which they lead, are gauge invariant, as would be physically expected.

Comparison of electro-thermal performance of advanced cooling techniques for electric vehicle motors

Permanent magnet synchronous motors are commonly used in the powertrain of electric vehicles because of their high power and torque densities. Attempts to increase power and torque densities beyond the state-of-the-art often suffer from thermal limitations of the adopted winding-wire insulation class. In this paper, overall effectiveness of four different cooling technologies, namely conventional stator jacket cooling, embedded circular and rectangular cooling channel within stator core, and direct winding heat exchanger have been studied numerically. By realizing temperature dependent magnetic and material properties, winding and core losses, a two-way coupling algorithm has been utilized to study the detailed electro-magnetic and thermal performance of BMW i3 motor with aforementioned cooling techniques. The numerical results illustrate that the direct winding heat exchanger approach provides better electro-thermal performance in comparison to the other cooling techniques analyzed. On the contrary, embedded rectangular and circular channel in stator core adversely affect the electro-magnetic performance and overall efficiency of the motor, despite their superior thermal performance compared to the jacket cooling.

Modified Becke-Johnson calculations using norm-conserving pseudopotential plane-wave approach: Systematic analysis

The modified Becke-Johnson potential for exchange and local density approximation for correlation (mBJLDA) approach is proved to provide a highly improved description of the bandgap. This work presents the results of a systematic investigation of the mBJLDA electronic band structure calculations using the norm-conserving pseudopotential (PP) approach, as implemented in the ABINIT code. The considered systems represent different classes of semiconductors and insulators. Moreover, all-electron (AE) mBJLDA potential and screened hybrid functional YS-PBE0 calculations are also performed for comparison purposes. We show that the mBJLDA@PP bandgaps obtained using typical PP's (i.e., valence and semicore electrons are allowed to relax) are generally speaking smaller than that of the mBJLDA@AE approach. This artifact cannot be attributed to a small “c” parameter of the mBJLDA, since we found that this parameter is quite insensitive to the adopted computational method. Interestingly, the number of electrons treated as valence, especially for the cations, can be used as an extra adjustable parameter to improve the mBJLDA@PP bandgaps. In particular, the best agreement with the mBJLDA@AE bandgaps is obtained by including the outer core p electrons as valence. The optimal PP's for mBJLDA@PP calculations are transferable. A similar agreement is also achieved for the binding energies of the semicore d electrons (in the IIB-VI and IIIB-V compounds), upper valence bandwidths and electron effective masses. Sound explanations of the mBJLDA@PP results for the considered electronic structure properties are provided.

Intrinsic Bulk Quantum Oscillations in a Bulk Unconventional Insulator SmB6.

SummaryThe finding of bulk quantum oscillations in the Kondo insulator SmB6 proved a considerable surprise. Subsequent measurements of bulk quantum oscillations in other correlated insulators including YbB12 lent support to our discovery of a class of bulk unconventional insulators that host bulk quantum oscillations. Here we perform a series of experiments to examine evidence for the intrinsic character of bulk quantum oscillations in floating zone-grown single crystals of SmB6 that have been the subject of our quantum oscillation studies. We present results of thermodynamic, transport, and composition analysis experiments on pristine floating zone-grown single crystals of SmB6 and compare quantum oscillations with metallic LaB6 and elemental aluminum. These results establish the intrinsic origin of quantum oscillations from the insulating bulk of floating zone-grown SmB6. The similarity of the Fermi surface in insulating SmB6 with the conduction-electron Fermi surface in metallic hexaborides is at the heart of a theoretical mystery.

Applications for new high-frequency impact ratings

In January 2020, a new ASTM classification standard that defines high-frequency impact ratings was published as ASTM E3222. These ratings are named High-frequency Impact Rating, or HIR, for field testing, and High-frequency Impact Insulation Class, or HIIC, for laboratory testing. These ratings describe the high-frequency region of a proposed two-rating method of evaluating impact noise isolation [LoVerde and Dong, J. Acoust. Soc. Am. 141 (2017)], in which low- and high-frequency components are evaluated independently. This paper addresses practical use of HIR and HIIC, focusing on applications where exclusive use of HIR or HIIC can be beneficial.

Wannier band transitions in disordered π -flux ladders

Boundary obstructed topological insulators are an unusual class of higher-order topological insulators with topological characteristics determined by the so-called Wannier bands. Boundary obstructed phases can harbor hinge/corner modes, but these modes can often be destabilized by a phase transition on the boundary instead of the bulk. While there has been much work on the stability of topological insulators in the presence disorder, the topology of a disordered Wannier band, and disorder-induced Wannier transitions have not been extensively studied. In this work, we focus on the simplest example of a Wannier topological insulator: a mirror-symmetric $\pi$-flux ladder in 1D. We find that the Wannier topology is robust to disorder, and derive a real-space renormalization group procedure to understand a new type of strong disorder-induced transition between non-trivial and trivial Wannier topological phases. We also establish a connection between the Wannier topology of the ladder and the energy band topology of a related system with a physical boundary cut, something which has generally been conjectured for clean models, but has not been studied in the presence of disorder.

Three-dimensional higher-order topological acoustic system with multidimensional topological states

Topologically protected gapless edge/surface states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with gapped edge states and in-gap corner states have been theoretically predicted in electric systems 1,2, and experimentally realized in two-dimensional (2D) mechanical and electromagnetic systems 3,4, electrical circuits 5, optical and sonic crystals 6-11, and elastic phononic plates 12. Here, we elaborately design a strong three-dimensional (3D) topological acoustic system, by arranging acoustic meta-atoms in a simple cubic lattice. Under the direct field measurements, besides of the 2D surface propagations on all of the six surfaces, the 1D hinge propagations behaving as robust acoustic fibers along the twelve hinges and the 0D corner modes working as robust localized resonances at the eight corners are experimentally confirmed. As these multidimensional topological states are activated in different frequencies and independent spaces, our works pave feasible ways for applications in the topological acoustic cavities, communications and signal-processing.

Valleylike Edge States in Chiral Phononic Crystals with Dirac Degeneracies beyond High-Symmetry Points and Boundaries of Brillouin Zones

Recently, the field of topological acoustics has rapidly developed, where the topological phase transition in phononic crystals is unveiled to occur through band inversion via the Dirac degeneracy in momentum space. Here, we focus on a class of low-symmetry acoustic valleylike topological insulators (VTIs), with honeycomb-lattice scatterers being chiral with ${C}_{1}$ symmetry. The valley Hall transition can be generated by simply rotating the chiral scatterers, where the topological phase transition is triggered by reopening the Dirac degeneracies beyond high-symmetry points and boundaries in the Brillouin zones. We numerically and experimentally demonstrate robust transport against different defects in chiral VTIs. For applications, we construct a topological beam splitter to verify the valley-selective one-way transport in chiral VTIs. Based on valleylike edge states, we design a topological acoustic switch with one input and two outputs, where the switching functionality at outputs with the on-off, off-off, and off-on states is confirmed by simulations and experimental tests. We also provide a more complex four-port acoustic switch based on chiral VTIs. The chiral VTI-based splitter and switches may have potential applications in constructing intelligent acoustic devices.