Inverse Band Research Articles

Three-dimensional photonic band gap cavity with finite support: Enhanced energy density and optical absorption

We study numerically the transport and storage of light in a 3D photonic band gap crystal doped by a single embedded resonant cavity. The crystal has finite support since it is surrounded by vacuum, as in experiments. Therefore, we employ the finite element method to model the diamond-like inverse woodpile crystal that consists of two orthogonal arrays of pores in a high-index dielectric such as silicon and that has experimentally been realized by CMOS-compatible methods. A point defect that functions as the resonant cavity is formed in the proximal region of two selected orthogonal pores with a radius smaller than the ones in the bulk of the crystal. We present a field-field cross-correlation method to identify resonances in the finite-support crystal with defect states that appear in the 3D photonic band gap of the infinite crystal. Out of 5 observed angle-independent cavity resonances, one is s-polarized and 4 are p-polarized for light incident in the X or Z directions. It is remarkable that quality factors up to Q = 1000 appear in such thin structures (3 unit cells). We find that the optical energy density is remarkably enhanced at the cavity resonances by up to 2400x the incident energy density in vacuum or up to 1200x the energy density of the equivalent effective medium. We find that an inverse woodpile photonic band gap cavity with a suitably adapted lattice parameter reveals substantial absorption in the visible range. Below the 3D photonic band gap, Fano resonances arise due to interference between the discrete fundamental cavity mode and the continuum light scattered by the photonic crystal. We argue that the five eigenstates of our 3D photonic band gap cavity have quadrupolar symmetry, in analogy to d-like orbitals of transition metals. We conclude that inverse woodpile cavities offer interesting perspectives for applications in optical sensing and photovoltaics.

Titania photonic crystal photocatalysts functionalized by graphene oxide nanocolloids

Photonic crystal-assisted semiconductor photocatalysis has been attracting significant attention as an advanced photon management approach that combines light harvesting with the macro/mesoporous structured materials properties permitting enhanced mass transport and high adsorption. In this work, surface functionalization of well-ordered photonic band gap engineered TiO2 inverse opal films fabricated by the convective evaporation-induced co-assembly method was performed by graphene oxide nanocolloids (nanoGO). The loading of GO nanosheets was determined by the films’ macropore size, with minimal effects on their long range periodicity and photonic properties. While nanoGO deposition reduced mesoporosity of the nanocrystalline titania walls, their surface functionality was greatly improved by the abundant oxygen groups of the GO nanosheets leading to increased pollutant adsorption. Slow photon amplification in the aqueous phase methylene blue photodegradation was identified for the unmodified TiO2 photonic films under both UV–vis and Vis illumination upon spectral overlap of the low energy edge of the inverse opal stop band (in water) with the dye electronic absorption, due to (red) slow photons localized in the titania skeleton that distinctly accelerated dye photodegradation kinetics. The photocatalytic efficiency was further improved for the nanoGO functionalized TiO2 inverse opal films via the synergetic action of interfacial electron transfer from TiO2 to the GO nanosheets. Under UV–vis light, the functionalized photonic films outperformed benchmark mesoporous Aeroxide® P25 TiO2 films where nanoGO modification, despite the enhanced dye adsorption, resulted in adverse effects in photocatalytic degradation due to pore clogging. Combination of the exceptional structural and photonic properties of TiO2 inverse opals with the high adsorption capacity and charge separation afforded by GO nanocolloids is proposed as a promising modification route for the development of efficient photocatalytic films.

An inverse chiral EBG manufactured by a 3D printing method

In this work, a novel inverse chiral electromagnetic band gap (EBG) structure with dual band gaps is studied. The EBG, which is made of alumina ceramic, was fabricated utilizing a 3D printing technology combining with a gel casting method. The band diagram of this EBG is simulated using a high frequency structure simulator (HFSS). Dual full band gaps of 9.2 GHz – 9.9 GHz and 10.7 GHz – 11.5 GHz that may stop the propagation of microwaves despite the directions or polarizations are found in the band diagram. As a contrast, the transmission property of the manufactured sample is measured by a transmission/reflection method. Two band gaps in accordance with the simulation are confirmed.

Fractional order inverse filters using operational amplifier

Two new op-amp-based multifunction filter structures are proposed which can realize fractional order inverse low pass, high pass and band pass filters. In the first configuration, op-amp is used in an inverting mode while in the second structure op-amp is employed in non-inverting mode. To the best knowledge of the authors, any fractional order inverse filter structure employing any active element/device has not been reported in the open-literature so far. The proposed inverse filters have been simulated in PSPICE using µA741-type op-amp as well as in MATLAB to validate the theoretical propositions.

Quantum anomalous Hall effect and topological phase transition in two-dimensional antiferromagnetic Chern insulator NiOsCl6

By doing calculations based on density functional theory, we predict that the two-dimensional anti-ferromagnetic (AFM) NiOsCl6 as a Chern insulator can realize the quantum anomalous Hall (QAH) effect. We investigate the magnetocrystalline anisotropy energies in different magnetic configurations and the Néel AFM configuration is proved to be ground state. When considering spin–orbit coupling (SOC), this layered material with spins perpendicular to the plane shows properties as a Chern insulator characterized by an inversion band structure and a nonzero Chern number. The nontrivial band gap is 37 meV and the Chern number C = −1, which are induced by a strong SOC and AFM order. With strong SOC, the NiOsCl6 system performs a continuous topological phase transition from the Chern insulator to the trivial insulator upon the increasing Coulomb repulsion U. The critical Uc is indicated as 0.23 eV, at which the system is in a metallic phase with . Upon increasing U, the Eg reduces linearly with C = −1 for 0 < U < Uc and increases linearly with C = 0 for U > Uc. At last we analysis the QAH properties and this continuous topological phase transition theoretically in a two-band model. This AFM Chern insulator NiOsCl6 proposes not only a promising way to realize the QAH effect, but also a new material to study the continuous topological phase transition.

Topological Dirac Nodal‐Line Structure in Orthorhombic‐Ti3N2 (Adv. Theory Simul. 3/2018)

As a new type of titanium nitrides phase, o-Ti3N2 possesses simultaneous time-reversal symmetry, spatial inversion symmetry, band inversion and negligible spin-orbit-coupling, therefore it is expected to be a topological nodal-line material. In article number 1700018, Bin Wen and co-workers systematically investigate the electronic and topological properties of o-Ti3N2 with a combination of first-principles calculations and Wannier tight-binding analysis. The analysis suggests that o-Ti3N2 may possess interesting surface transport properties making it a new member in topological materials, with promising novel applications in electronic devices.

Inverse Band Structure Design via Materials Database Screening: Application to Square Planar Thermoelectrics

Electronic band structure contains a wealth of information on the electronic properties of a solid and is routinely computed. However, the more difficult problem of designing a solid with a desired band structure is an outstanding challenge. In order to address this inverse band structure design problem, we devise an approach using materials database screening with materials attributes based on the constituent elements, nominal electron count, crystal structure, and thermodynamics. Our strategy is tested in the context of thermoelectric materials, for which a targeted band structure containing both flat and dispersive components with respect to crystal momentum is highly desirable. We screen for thermodynamically stable or metastable compounds containing d8 transition metals coordinated by anions in a square planar geometry in order to mimic the properties of recently identified oxide thermoelectrics with such a band structure. In doing so, we identify 157 compounds out of a total of more than half a mill...

Topological Dirac Nodal‐Line Structure in Orthorhombic‐Ti3N2

AbstractAs a new type of titanium nitrides phase, orthorhombic‐Ti3N2 (o‐Ti3N2) possesses coexistence of time‐reversal symmetry, spatial inversion symmetry, band inversion and negligible spin‐orbit‐coupling (SOC), hence it is expected to have special physical and chemical properties. In this work, its electronic and topological properties were systematically investigated by combing first‐principles calculations and Wannier tight‐binding analysis. It is found that o‐Ti3N2 is a 3D topological nodal‐line material characterized by hornlike nodal lines in 3D momentum space. The “drumhead‐like” surface states exist on the (001) surface, while surface states possess the exotic single–double conversion, implying that o‐Ti3N2 may process interesting surface transport properties. These features make o‐Ti3N2 a new member in topological materials, with promising novel applications in electronic devices.

Jump-and-return sandwiches: A new family of binomial-like selective inversion sequences with improved performance

A new family of binomial-like inversion sequences, named jump-and-return sandwiches (JRS), has been developed by inserting a binomial-like sequence into a standard jump-and-return sequence, discovered through use of a stochastic Genetic Algorithm optimisation. Compared to currently used binomial-like inversion sequences (e.g., 3–9–19 and W5), the new sequences afford wider inversion bands and narrower non-inversion bands with an equal number of pulses. As an example, two jump-and-return sandwich 10-pulse sequences achieved 95% inversion at offsets corresponding to 9.4% and 10.3% of the non-inversion band spacing, compared to 14.7% for the binomial-like W5 inversion sequence, i.e., they afforded non-inversion bands about two thirds the width of the W5 non-inversion band.

Sputtering pressure effects on microstructure and grain orientation distribution in FePt thin films

We use the magnetron sputtering method to fabricate FePt thin films on amorphous oxidized Si substrates by controlling the sputtering pressure and investigate the evolution of microstructure and grain orientation using X-ray diffraction and electron back-scatter diffraction, and discuss the effect of microstructure on magnetic properties of L10-FePt thin film. The experimental results show that the L10-FePt phase is formed when the sputtering pressure is 0.25 Pa, and the L10-FePt phase is not found when the sputtering pressure is higher. The higher sputtering pressure leads to poor crystallinity and smaller grain size in FePt thin films, which inhibit disorder–order phase transformation. Meanwhile, higher sputtering pressure results in a larger residual strain, because it increases the degree of local gain misorientation in FePt thin films. The inverse pole figure map and band contrast map reveal the grain growth orientation and microstructure of L10-FePt thin film from cross-sectional, indicating that the grain grows along different direction, and exists in the form of nanocolumnar crystal. The experimental results demonstrate that when there is no texture in L10-FePt thin film, the out-of-plane magneto anisotropy of L10-FePt thin film should be mainly contributed by the shape anisotropy.

Electronic Phase Separation and Dramatic Inverse Band Renormalization in the Mixed-Valence Cuprate LiCu_{2}O_{2}.

We measured, by angle-resolved photoemission spectroscopy, the electronic structure of LiCu_{2}O_{2}, a mixed-valence cuprate where planes of Cu(I) (3d^{10}) ions are sandwiched between layers containing one-dimensional edge-sharing Cu(II) (3d^{9}) chains. We find that the Cu(I)- and Cu(II)-derived electronic states form separate electronic subsystems, in spite of being coupled by bridging O ions. The valence band, of the Cu(I) character, disperses within the charge-transfer gap of the strongly correlated Cu(II) states, displaying an unprecedented 250% broadening of the bandwidth with respect to the predictions of density functional theory. Our observation is at odds with the widely accepted tenet of many-body theory that correlation effects generally yield narrower bands and larger electron masses and suggests that present-day electronic structure techniques provide an intrinsically inappropriate description of ligand-to-d hybridizations in late transition metal oxides.

The electronic band structure of Ge1−xSnx in the full composition range: indirect, direct, and inverted gaps regimes, band offsets, and the Burstein–Moss effect

A comprehensive and detailed study of the composition dependence of lattice constants, band gaps and band offsets has been performed for bulk Ge1−xSnx alloy in the full composition range using state-of-the-art density functional theory methods. A spectral weight approach to band unfolding has been applied as a means of distinguishing the indirect and direct band gaps from folded supercell band structures. In this way, four characteristic regions of the band gap character have been identified for Ge1−xSnx alloy: an indirect band gap (x < 6.5%), a direct band gap (6.5% < x < 25%) and an inverse band gap (x > 25%) with inverse spin–orbit split-off for 45% < x < 85%. In general, it has been observed that the bowing parameters of band edges (Γ and L-point in conduction band (CBΓ and CBL), valence band (VB), and spin–orbit (SO) band) are rather large ( = 2.43 ± 0.06 eV, = 0.64 ± 0.04 eV, = −0.59 ± 0.04 eV, and = −0.49 ± 0.05 eV). This indicates that Ge1−xSnx behaves like a highly mismatched group IV alloy. The composition dependence of lattice constant shows negligible bowing (ba = −0.083 Å). Obtained results have been compared with available experimental data. The origin of band gap reduction and large bowing has been analyzed and conclusions have been drawn regarding the relationship between experimental and theoretical results. It is shown that due to the low DOS at the Γ-point, a significant filling of CB by electrons in the direct gap regime may easily take place. Therefore, the Burstein–Moss effect should be considered when comparing experimental data with theoretical predictions as has already been shown for other intrinsic n-type narrow gap semiconductors (e.g. InN).

Torsional Vibration Reduction with Augmented Inverse Model-based Controller in Wind Turbine Drivetrain

Wind energy has shown promising advantages in reducing the greenhouse effect by minimizing carbon dioxide emissions to improve earth climate. Wind turbine which falls under the umbrella of renewable energy family promises cleaner environment while generating electricity from wind energy with no burnt fossil fuel. However, it portrays challenges in terms of high operating cost due to component failure. Thus this paper discusses on mitigating one of the problems related to wind turbine failure, the torsional vibration reduction in drive train. A generator torque control is investigated together with the particle swarm optimization technique in search for accurate parameters of the controller. This control strategy is a solution to low wind speed areas especially around South East Asian region. An augmented inverse model-based controller and band pass filter is proposed to obtain vibration attenuation at the dominant mode. The modelling endeavor is firstly obtained via particle swarm optimization search capability to obtain an accurate transfer function of the inverse model. A band pass filter (BPF) is then augmented with the inverse model as controller for torsional vibration suppression. Results have shown favorable comparison between the proposed and conventional methods in terms of vibration attenuation level.

Hybrid crystalline sp2[sbnd]sp3 carbon as a high-efficiency solar cell absorber

Carbon is a versatile element that has allotropes with both sp2 (graphene) and sp3 (diamond) bonding. However, none of the allotropes can be used as light-absorber materials in solar cells due to either too large or too small band gap. Here, we propose a novel concept that enables a tunable band gap of carbon phases with sp2 carbon atoms within a sp3 carbon structure. The tunability is due to the quantum confinement effect. By embedding the sp2 atoms within the sp3 structure, we can design new carbon allotropes with ideal optical properties for optoelectronic applications. Five carbon allotropes incorporated this structural feature were identified by combining this new concept with our freshly developed multi-objective inverse band structure design approach. They all have proper band gaps for optical absorption, and the simulated photovoltaic efficiency of C10-C is even higher than conventional absorber materials such as GaAs, which indicates that C10-C with mixed sp2sp3 hybridization may have potential application as light-absorber material in electronic and optoelectronic devices.

In Russian

The surface photoconductivity in macroporous silicon with an oxide coating with thickness of 3 to 30 nm with a strong surface light absorption, the absorption coefficient is varied from 330 to 105 cm –1 (wavelength is from 0.93 to 0.4 micron) has been investigated.The starting material consisted of n-type silicon with [100] orientation and 4.5 Ω∙cm resistivity. Macropores with diameter Dp = 4 μ m and depths hp = 100 μ m were formed by electrochemical etching. At the wavelengths of 0.4 and 0.57 microns, and the thickness of the oxide layer of 3 to 15 nm the surface monopolar negative photoconductivity was manifested, due to the inversion band bending in the surface layers of the semiconductor and due to capture of majority carriers on the so-called «slow» surface levels. When the thickness of the oxide layer is 30 nm, a positive photoconductivity with anomalous spectral dependence takes peace with an increase in the absorption coefficient from 330 to 105cm -1 (decreasing wavelength from 0.93 to 0.4 microns) photoconductivity is not decreasing, but increases – localization effect of the surface photoconductivity in the space charge region was manifested. The experimental results are explained as follows. With the increasing of oxide coating thickness its positive charge also increases and compensates the negative charge of the slow surface levels. The inversion band bending at the same time becomes depletion band bending. In general, the semiconductor photoconductivity, taking into account surface effects (space charge region) consists of three components: bipolar photoconductivity of the space charge region, monopolar photoconductivity of the space charge region and bipolar photoconductivity of the quasi-neutral volume. Effect of localization photoconductivity occurs in all cases where the bipolar photoconductivity of the space charge region is more than bipolar photoconductivity quasi-neutral volume. This occurs in the case of the depleting band bending on the semiconductor surface.

Structural, mechanical, electronic and optical properties of BBi, BP and their ternary alloys BBi1−xPx

The ground state properties of BBi, BP and their ternary alloys BBi1−xPx are reported using first-principles calculations based on density functional theory (DFT). The modified Becke–Johnson (mBJ) potential together with the generalized gradient approximation (GGA) for the correlation potential has been used here as it is a superior method for estimating band inversion strength and band order. The zincblende phase is found to be more stable than the other phases for all studied materials. The calculated lattice constants exhibit a small deviation from the linear Vegard’s law with a downward bowing value of 0.11 Å. The calculated ground state parameters for the studied binary compounds agree with available theoretical and experimental results. The bandgap value of the studied materials calculated with the mBJ potential is considerably enhanced with respect to values from the GGA functional. Optical properties have been calculated and analysed with photon incident energy up to 21.0 eV. The spin–orbit interaction (SOI) has also been considered for structural and electronic calculations and the results are compared with those of non-SOI calculations. The real and imaginary parts of the dielectric function have also been calculated and discussed.

Influence of the Starting Solution on the Growth and Morphology of Rare Earth-Doped Yttrium Oxide Spherical Particles By the Urea Precipitation Method

Since the start of this millennium, the need for amorphous submicron spherical rare earth element (REE) precursors that after subsequent heat treatment (~1000°C) forms crystalline REE-doped yttrium or gadolinium oxide (Y2O3:REE or Gd2O3:REE) phosphors has generated an active area of research.1 The original impetus was for a variety of display applications such as cathode ray tubes for high definition television and field emission devices.2 Later uses have included X-ray imaging and X–ray computed tomography (after converting these spherical REE phosphor particles to oxysulfide phosphor particles using sulfur and further heat treatment whereby they retained their former size and morphology).3 These spherical REE phosphor particles have the ability to close pack to form a thin emissive layer.2 These REE phosphor particles are produced via a urea homogeneous precipitation method and its modifications. In this work REE nitrate stock solutions which yielded spherical REE-doped hydroxycarbonate precursor particles.1 It was assumed due to the ‘boring’ similarity of the chemical properties of REEs that all the spherical phosphor particles precipitated using different dopants would have identical particle sizes and spherical morphologies. Our observations have found this not to be the case, and that their chemistry is much more interesting. For example precipitated Y2O3:Eu spherical particles are larger than Y2O3:Tb precursor particles, this is also the case after annealing at 980°C where the original morphology is retained (see Figure 1). Also, it has been found that highly viscous Y2O3:REE and Gd2O3:REE nitrate precursor solutions used for the preparation of three-dimensional photonic structures such as inverse photonic band gap crystals4 and bio-templates5vary in stability (non-precipitation) for each dopant. The reason behind these findings will be discussed in terms of the pH of the precursor solutions, electron configuration, oxidation state (preferred) and stoichiometry. Figure 1. Annealed spherical phosphor particles of (a) Y2O3:Eu and (b) Y2O3:Tb. References X. Jing, T. Ireland, C. Gibbons, D. Barber, J. Silver, A. Vecht, G. Fern, P. Trogwa, and D. Morton, J. Electrochem. Soc., 146, 4654 (1999).M. I. Martinez-Rubio, T. G. Ireland, G. R. Fern, J. Silver, and M. J. Snowden, Langmuir, 17, 7145 (2001).G.R. Fern, T.G. Ireland, J. Silver, R. Withnall, A. Michette, C. McFaul and S. Pfauntsch, Nucl. Instr. Methods Phys Res., A600, 434 (2009).J. Silver, T.G. Ireland and R. Withnall, J. Mater. Res., 19, 1656 (2004).J. Silver, R. Withnall, T.G. Ireland, G.R. Fern and S. Zhang, Nanotechnology, 19, 095302 (2008). Figure 1

Comparative analysis of karyotypes of Chironomussolitus Linevich & Erbaeva, 1971 and Chironomusanthracinus Zetterstedt, 1860 (Diptera, Chironomidae) from East Siberia.

A comparative chromosome banding analysis of Chironomus solitus Linevich & Erbaeva, 1971 and Chironomus anthracinus Zetterstedt, 1860 from East Siberia (Lakes Baikal, Gusinoe, Arakhley and Irkutsk Reservoir) showed close similarity of banding sequences. Chironomus solitus differs from Chironomus anthracinus in one species-specific sequence of arm B. Arms C (43%) and D (30%) had inversion banding sequences previously reported in Chironomus anthracinus The similarity of karyotypic features of Chironomus solitus and Chironomus anthracinus in combination with morphological features of larvae provide evidence in favour of including Chironomus solitus in the Chironomus anthracinus group of sibling species long with Chironomus reservatus Shobanov, 1997.

Structural, Magnetic, and Electronic Properties and the Topological Phase of CePdBi Bulk and Nanolayers

We have investigated the structural, electronic, and topological band structures and the linear coefficient γ of the electronic-specific heat and magnetic properties for ferromagnetic phase of CePdBi in bulk and nanolayers using density functional theory. The total energies as a function of volume are calculated and the bulk modulus and their pressure derivatives are determined. Also, the total energy calculation within generalized gradient approximation (GGA), local density approximation (LDA), and (LDA + U) indicate that at zero pressure the ferromagnetic phase of CePdBi compound is the most stable phase. This result is in agreement with the experimental results. The GGA and Engel–Vosko generalized gradient approximation have been used to obtain the band inversion strength and band order. The effect of hydrostatic pressure on the band inversion strength, band order, and total and local magnetic moments of CePdBi in bulk has been investigated. The magnetic moment decreases with the increasing pressure. The calculations of the structural, electronic properties, topological band structures, magnetic moment, and the linear coefficient γ of the electronic-specific heat have been performed under the presence of spin–orbit coupling. As far as we know, this is the first report on the pressure dependence of the magnetic moment of this compound and the electronic and magnetic properties and the linear coefficient γ of the electronic-specific heat of CePdBi nanolayers.

Influence of lattice expansion on the topological band order of InAsxSb1−x (x = 0, 0.25, 0.5, 0.75, 1) alloys

The topological band structures of InAsxSb1−x (x=0, 0.25, 0.5, 0.75, 1) alloys have been investigated using density functional theory by utilizing the Wien2k package. These alloys are in a topologically trivial phase in their unstrained states and exhibit a small band gap. Since in small band-gap cubic semiconductors the nontrivial topological phase can be achieved by lattice expansion, we investigate the effect of hydrostatic and biaxial lattice expansion on band inversion strength and band order of these alloys. It is found that under reasonable hydrostatic lattice expansion, InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys with cubic symmetry and InAs0.5Sb0.5 alloy with tetragonal symmetry, are converted to nontrivial topological semiconductors with zero band gap and non-zero band gap, respectively. In order to convert InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys into topological semiconductors with non-zero band gap, we let the systems of InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys undergo a biaxial lattice expansion. Thus by breaking the cubic symmetry in these alloys, not only they are converted to topologically nontrivial phase but also a small band gap is opened at Γ point.