Localized States Research Articles

Planned stitching, practical suturing: assembling community voices and mobilisation across difference in Johannesburg’s corridors of freedom

The City of Johannesburg’s Corridors of Freedom (CoF), launched in 2013, were intended to cut across the economically and racially divided city using infrastructure and interventions in the built environment around new transport nodes. Undertaken in haste for political reasons and projected to be delivered as swiftly as possible, those driving this mega project oversaw substantial consultation exercises, but provided relatively few spaces for direct engagement to shape the project. This paper presents the experiences of a team of engaged-researchers, a long-standing NGO in partnership with University-based scholars jointly investigating the CoF development process. Interested in the ways in which the CoF initiative sought to ‘stitch’ the city together, our contribution to the project was to engage with different communities, clarify their different experiences with participation in the Corridors development and explore the possibility of collaboration across these different communities. Using the conceptual framework of stitching and suturing, the paper, in two parts, interrogates the roles that engaged partners can have in complex and diverse communities and our ability to support engagement. We reveal the limitations of engaged research when faced with political and institutional cycles that do not synchronise with the research projects, and point to the cleavages and disruptions that result when the local state does not systematically incorporate the needs and lived realities of its residents.

Disordered Heisenberg XYZ magnetic chains with exponential correlations: Localization and quantum state transfer

Our study investigates the Quantum State Transfer Protocol within a one-dimensional quantum Heisenberg spin chain model that features exponentially correlated disorder in the spin coupling distribution. We conducted extensive numerical simulations to explore how varying degrees of correlation impact the transfer of quantum states through this model of a quantum channel. Our results demonstrate that the extent of correlation significantly influences quantum communication along the channel. Specifically, we observed that in the strongly correlated regime, quantum communication remains largely unaffected even in large systems. These findings are interpreted in the context of the localization properties inherent to the disordered channel.

Protracted Kondo coherence with dilute carrier density in cerium based nickel pnictides

Nozières' exhaustion theory argues that the temperature for coherently screening of all local moments in Kondo lattice could be much lower than the temperature of single moment screening in case of insufficient conduction electrons. Experiments indicate that cerium based nickel pnictides CeNi2−δAs2(δ≈0.28) is an ideal material to exam such protracted Kondo screening. Using density functional theory plus dynamical mean-field theory, we found CeNi2As2 presents a transition from the local moment states to the coherent Kondo screening states under compression, accompanied by topological changes of the Fermi surface. Similar phase transition could be driven by chemical pressure via the isovalence As→P substitution. The derived coherent Kondo temperature Tcoh is much lower than the single-impurity Kondo temperature TK due to the diluted Ni-3d conduction electrons, indicating the protraction of Kondo screening in crystal. In contrast to structurally analogous layered iron pnictides, the electronic structures of the present systems show strong three-dimensionality.

Recombination efficiency in c-plane (In,Ga)N/GaN quantum wells: Saturation of localisation sites versus Auger-Meitner recombination

Abstract Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells can have >90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN quantum wells is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the quantum well, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger-Meitner recombination at the carrier densities required for saturation.

Numerical investigations of heat transfer and pressure drop in packed bed duct exposed to forced convection boundaries under local thermal non-equilibrium conditions

The present study is a 2-D numerical study which discusses the thermohydraulic performance of packed bed duct under local thermal non-equilibrium and steady state conditions exposed to forced convection boundaries (BC-6). The numerical model is well validated with the experimental work reported in the literature and found to be accurate enough to perform further parametric investigations. The analysis is done to see the effect of different ball diameter (5 mm, 9 mm and 11 mm), bed porosity, heat transfer fluid (air, water and engine oil - Pr = 0.70–645) and ball material (EPS, steel and bronze) on heat transfer and fluid flow characteristics in turbulent flow region of Reynolds number ranging from 900 to 14,320. The numerical results obtained using commercial CFD software COMSOL shows that, the heat transfer coefficient and pressure drop in packed bed increases with increase in ball diameter, thermal conductivity of ball and bed porosity which is exactly reverse as reported in literature for BC-1 to BC-5. The maximum thermal performance factor with bronze particles is 2.45 and 24.5 times more than stainless steel and EPS particles respectively for larger particle size and bed porosity. Engine oil exhibits significantly higher heat transfer coefficient (9.7 × 105 W/m2K) and pressure drop (3.3 × 106 Pa) compared to water (40,126 W/m2K and 2028 Pa) and air (600 W/m2K and 250 Pa) respectively. Overall, the combination of water as heat transfer fluid along with bronze particles of larger diameter and larger bed porosity emerges as the optimal choice for enhancing heat transfer in the packed duct exposed to forced convection boundary condition BC-6.

Imaging Quantum Interference in a Monolayer Kitaev Quantum Spin Liquid Candidate

Single atomic defects are prominent windows to look into host quantum states because collective responses from the host states emerge as localized states around the defects. Friedel oscillations and Kondo clouds in Fermi liquids are quintessential examples. However, the situation is quite different for quantum spin liquid (QSL), an exotic state of matter with fractionalized quasiparticles and topological order arising from a profound impact of quantum entanglement. Elucidating the underlying local electronic property has been challenging due to the charge neutrality of fractionalized quasiparticles and the insulating nature of QSLs. Here, using spectroscopic-imaging scanning tunneling microscopy, we report atomically resolved images of monolayer α−RuCl3, the most promising Kitaev QSL candidate, on metallic substrates. We find quantum interference in the insulator manifesting as incommensurate and decaying spatial oscillations of the local density of states around defects with a characteristic bias dependence. The oscillation differs from any known spatial structures in its nature and does not exist in other Mott insulators, implying it is an exotic oscillation involved with excitations unique to α−RuCl3. Numerical simulations suggest that the observed oscillation can be reproduced by assuming that itinerant Majorana fermions of Kitaev QSL are scattered across the Majorana Fermi surface. The oscillation provides a new approach to exploring Kitaev QSLs through the local response against defects like Friedel oscillations in metals. Published by the American Physical Society 2024

Topological Band Engineering of One-Dimensional π-d Conjugated Metal-Organic Frameworks.

One-dimensional (1D) π-d conjugated metal-organic frameworks (c-MOFs) have garnered widespread research interest in chemical energy storage and conversion. In this work, we introduce a universal principle to engineer the topological bands of 1D c-MOFs. Connected by d orbitals of transition metals, two equivalent hidden molecular π orbitals in 1D c-MOFs can generate a staggered hopping within and between the organic ligands, forming Su-Schrieffer-Heeger-shaped 1D topological bands. Guided by this discovery, we investigate the electronic structures of the typical 1D c-MOF assembled from Ni atoms and 2HQDI (QDI = 2,5-diamino-1,4-benzoquinonediimine) precursors (NiQDI) by first-principles calculations, revealing 1D topological bands around the Fermi level. Due to local bonding variations at the QDI terminations, these two hidden molecular π orbitals become atomically bonded but electronically separated at the edge QDI, creating spatially localized in-gap topological edge states at the end of the NiQDI chain. This definitive signature for 1D topological bands is identified through differential conductance spectra in scanning tunneling microscopy measurements. Our results provide conclusive experimental evidence for topological bands in 1D c-MOFs, paving the way for exploring the topological physics in organic materials through frontier molecular orbitals.

Nonlinear Anderson Localized States at Arbitrary Disorder

Nonlinear Anderson Localized States at Arbitrary Disorder

Entropy Engineering of 2D Materials.

Entropy, a measure of disorder or uncertainty in the thermodynamics system, has been widely used to confer desirable functions to alloys and ceramics. The incorporation of three or more principal elements into a single sublattice increases the entropy to medium and high levels, imparting these materials a mélange of advanced mechanical and catalytic properties. In particular, when scaling down the dimensionality of crystals from bulk to the 2D space, the interplay between entropy stabilization and quantum confinement offers enticing opportunities for exploring new fundamental science and applications, since the structural ordering, phase stability, and local electronic states of these distorted 2D materials get significantly reshaped. During the last few years, the large family of high-entropy 2D materials is rapidly expanding to host MXenes, hydrotalcites, chalcogenides, metal-organic frameworks (MOFs), and many other uncharted members. Here, the recent advances in this dynamic field are reviewed, elucidating the influence of entropy on the fundamental properties and underlying elementary mechanisms of 2D materials. In particular, their structure-property relationships resulting from theoretical predictions and experimental findings are discussed. Furthermore, an outlook on the key challenges and opportunities of such an emerging field of 2D materials is also provided.

Scale-tailored localization and its observation in non-Hermitian electrical circuits

Anderson localization and non-Hermitian skin effect are two paradigmatic wave localization phenomena, resulting from wave interference and the intrinsic non-Hermitian point gap, respectively. In this study, we unveil a novel localization phenomenon associated with long-range asymmetric coupling, termed scale-tailored localization, where the number of induced localized modes and their localization lengths scale exclusively with the coupling range. We show that the long-range coupling fundamentally reshapes the energy spectra and eigenstates by creating multiple connected paths on the lattice. Furthermore, we present experimental observations of scale-tailored localization in non-Hermitian electrical circuits utilizing adjustable voltage followers and switches. The circuit admittance spectra possess separate point-shaped and loop-shaped components in the complex energy plane, corresponding respectively to skin modes and scale-tailored localized states. Our findings not only expand and deepen the understanding of peculiar effects induced by non-Hermiticity but also offer a feasible experimental platform for exploring and controlling wave localizations.

Improving the photoswitching performance of a transistor with amorphous metal oxide semiconductor thin film by a gradient annealing approach

Metal oxides are attracting attention as electronic mate rials in research and industry. Thin films of amorphous indium gallium zinc oxide (a-IGZO) exhibit low absorbance in the visible spectrum, making them ideal components in transparent electronics. To widen the scope of use for thin film transistor (TFT) devices based on a-IGZO in on-chip sensing applications, photoresponsive behavior has been achieved by proper engineering of the active layers by either introducing a photosensitive top layer or using a method to generate localized states inside the band gap. In this paper, we propose a bilayer structured with the use of thermal annealing of a-IGZO film at different temperatures. Thermal annealing has been shown to improve the electrical performance of the TFT devices because of the improved film quality but negatively affects the photoresponsivity by removing tarp sites that play an important role in both charge generation and photomultiplication via the photogating effect. Therefore, here we propose an a-IGZO film with a high temperature-annealed bottom layer and pristine top layer. The bottom layer plays a vital role in the charge transport behavior, resulting in a low threshold voltage and subthreshold swing similar to the device with a fully annealed film, while the photoresponse of the device is driven by the higher density of gap states in the pristine top layer. This proposed method is advantageous to previously published procedures due to the simplicity of using no additional materials and complex steps to introduce trap sites into the photoactive layer, but only differential annealing temperature.

Valley-Hybridized Gate-Tunable 1D Exciton Confinement in MoSe2.

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the quantum photonics and optoelectronics fields. Monolayers of transition metal dichalcogenides (TMDs) offer inherent 2D confinement and possess significant exciton binding energies, making them promising candidates for achieving electric-field-based confinement of excitons without dissociation. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum-confined states in MoSe2. Building on previous reports of tunable trapping potentials and linearly polarized emission, we extend this understanding by demonstrating how nonuniform in-plane electric fields enable in situ control of these effects and highlight the role of gate-tunable valley hybridization in these localized states. Their polarization is entirely engineered through either the 1D confinement potential's geometry or an out-of-plane magnetic field. Controlling nonuniform in-plane electric fields in TMDs enables control of the energy (up to five times its line width), polarization state (from circular to linear), and position of 1D confined excitonic states (5 nm V-1).

Interaction-Induced Crystalline Topology of Excitons

We apply the topological theory of symmetry indicators to interaction-induced exciton band structures in centrosymmetric semiconductors. Crucially, we distinguish between the topological invariants inherited from the underlying electron and hole bands and those that are intrinsic to the exciton wave function itself. Focusing on the latter, we show that there exists a class of exciton bands for which the maximally localized exciton Wannier states are shifted with respect to the electronic Wannier states by a quantized amount; we call these excitons shift excitons. Our analysis explains how the exciton spectrum can be topologically nontrivial and sustain exciton edge states in open boundary conditions even when the underlying noninteracting bands have a trivial atomic limit. We demonstrate the presence of shift excitons as the lowest energy neutral excitations of the Su-Schrieffer-Heeger model in its trivial phase when supplemented by local two-body interactions. Published by the American Physical Society 2024

Evolution of topological extended state in multidimensional non-Hermitian topolectrical circuits

The extended state pertains to the dispersion of the system's eigenfunctions across the whole lattice. Recent studies have shown that the non-Hermitian skin effect (NHSE) can reshape the wavefunction of topological modes. The localized states of topological modes within the bandgap gradually delocalized into extended states through the manipulation of NHSE. Here, we clarify the NHSE direction using the Bloch spectral winding numbers and reestablish the bulk-boundary correspondence through the non-Bloch winding numbers in the generalized Brillouin zone. We elucidate the formation of extended state by employing the localized decay length. Then, we have designed non-Hermitian topological circuits for experimental verification based on the voltage follower. The corner states, edge states, and extended states in 1D, 2D, and 3D circuits were observed through the measurement of node voltage. Our work can achieve the sustainable extended mode and provides significant cases for the analysis of topolectrical circuits.

The comprehensive large-scale redevelopment of Pazhou Village in Guangzhou, China: The interaction of state, market and local community

Urban villages are an important element of the urban environment in China. Often characterized by high densities, illegal construction and poor quality, the key issue of urban villages for city planners and officials is whether to try to comprehensively redevelop them or opt for gradual upgrading. This paper provides an evaluation of the comprehensive redevelopment of Pazhou Village in Guangzhou, China, after a decade and shows that under certain important conditions including good co-operation between governments, developers and village collectives, land use intensification and participatory planning, it is possible for comprehensive redevelopment to benefit the original villagers. The redevelopment was initiated by the local state and the case study examines the dynamic interactions between the key actors and their respective roles in ensuring the success of the development. Using the theory of state entrepreneurialism, the findings highlight the leading yet subtle role of the state in Chinese cities.

Modified Centroid of Root Projection Method for Determining the Center of Resistance of a Tooth

Abstract Center of resistance (CR) has been widely accepted in dentistry as a reference point for controlling tooth moment, which depends on the direction of loading and the morphology of the periodontal ligament (PDL). In clinical practice, dentists estimate the location of CR based on the morphology of the root of teeth, which may lead to a misestimation of orthodontic treatment. A quick method was proposed to efficiently determine the CR by identifying the centroid of the root projection (CRP), according to the orthodontic force. However, the original CRP method was limited to single-rooted teeth and it did not provide a strategy for handling the overlapping roots projection of multi-rooted teeth. To address this issue, we expanded the CRP method to accommodate multi-rooted teeth by applying the weighted average method. We further validated the modified CRP method using finite element simulation (FEA) for both single-rooted and multi-rooted teeth considering mesial-distal and buccal-lingual force directions. The evaluation of displacement distribution along the projection direction allowed us to assess translation and rotation movements, which confirmed that the centroid of root projection can accurately serve as the CR for the multi-rooted teeth. Additionally, we observed heterogeneous stress distributions in the multi-rooted teeth. Considering the well-acknowledged bone remodeling effect in response to local stress states, this indicated that the comprehensive indexes beyond the CR are desired for evaluating or controlling the tooth movement.

Ru nanoparticles on mesostructured carbon for glucose hydrogenation; catalysts synthesis and characterization

Ru-containing hydrogenation catalysts based on functionalized carbon material CMK-3 (Carbon Mesostructured by KAIST) were developed. Mesostructured silicate SBA-15 with enlarged wall channels was used as a template for the carbon replica synthesis. The effect of carbon material functionalization via moist air oxidation and sulfonation on the morphology, physicochemical properties and activity of the catalyst was studied. The dispersion, localization, and electronic state of supported ruthenium were determined depending on the support functionalization method. The initial support structure preservation after Ru deposition was confirmed by a set of physicochemical methods. Metal particles are finely distributed with no agglomerated present, providing a high active site accessibility and ensures a superb catalyst activity. The catalysts were tested in glucose to sorbitol hydrogenation. The results showed that pore morphology and carbon support initial structure preservation account for the catalytic activity of Ru nanoparticles.

Electronic Structure Of Cobalt Phosphates Co1-xmxpo4 Doped With Iron And Nickel Atoms

In this research, the electronic states, band structures, and bond properties of the framework compounds of CoPO4, Co1-xFexPO4, and Co1–xNixPO4 were investigated by the density functional theory calculations. The potential capabilities of these systems in the photocatalytic water splitting to produce hydrogen were analyzed. The spin-up electron densities of states for the CoPO4, Co1–xFexPO4, and Co1–xNixPO4 systems have band gaps of 2.7, 3.4, and 3.45 eV, respectively. The band of spin-down electron states has several energy gaps above the Fermi level. The density of states of electron with spin up near the Fermi level is obviously greater than that of electrons with spin down. In this case, localized states of electrons appear in the band gap of doped semiconductors due to impurity atoms. The calculated value of the energy at the lower edge of the conduction band for CoPO4 was –0.7 eV, which is more negative than the energy required for water splitting. Meanwhile, the calculated value of the energy at the upper edge of the valence band was 2.01 eV, which is more positive than the oxygen evolution energy of 1.23 eV.

Topological State in Heterostructured Gap Waveguides and Their Transitions to Classical Transmission Line

AbstractGap waveguides play a crucial role in millimeter‐wave information transmission and processing for next‐generation wireless communication and radar sensing systems. However, they suffer from challenges, such as reflection and scattering losses at sharp bends and defects. While topological photonic crystals offer innovative solutions for robust signal transmission, their localized interface states and unique electromagnetic modes pose compatibility challenges with practical circuits and systems. In this paper, a novel heterostructured gap waveguide support valley‐locked topological guided wave transport is introduced. This design ensures robust propagation against defects and bends while maintaining compatibility with classical transmission lines. Its topological characteristic reduce sensitivity to fabrication and assembly errors. More importantly, an efficient transition structure is developed to seamlessly convert quasi‐transverse electromagnetic (quasi‐TEM) modes in planar transmission lines to topological guided wave states, thereby effectively exciting topological waves. All theoretical predictions are quantitatively verified by the simulations and experiments at millimeter wave frequency. This heterostructured gap waveguide can find unique applications, such as power divider, as verified numerically. By leveraging topological principles, the proposed gap waveguide offers a significant performance boost for antennas and passive/active components in millimeter‐wave applications, including automotive radar, 5G communication, and synthetic‐aperture radar for imaging.

Corner and edge states in topological Sierpinski Carpet systems

Fractal lattices, with their self-similar and intricate structures, offer potential platforms for engineering physical properties on the nanoscale and also for realizing and manipulating high order topological insulator states in novel ways. Here we present a theoretical study on localized corner and edge states, emerging from topological phases in Sierpinski Carpet within a $\pi$-flux regime. A topological phase diagram is presented correlating the quadrupole moment with different hopping parameters. Particular localized states are identified following spatial signatures in distinct fractal generations. The specific geometry and scaling properties of the fractal systems can guide the supported topological states types and their associated functionalities. A conductive device is proposed by coupling identical Sierpinski Carpet units providing transport response through projected edge states which carry on the details of the system's topology. Our findings suggest that fractal lattices may also work as alternative routes to tune energy channels in different devices.