Edge Spectra Research Articles

Percolation-Induced PT Symmetry Breaking.

We propose a new avenue in which percolation, which has been much associated with critical phase transitions, can also dictate the asymptotic dynamics of non-Hermitian systems by breaking PT symmetry. Central to it is our newly designed mechanism of topologically guided gain, where chiral edge wave packets in a topological system experience non-Hermitian gain or loss based on how they are topologically steered. For sufficiently wide topological islands, this leads to irreversible growth due to positive feedback from interlayer tunneling. As such, a percolation transition that merges small topological islands into larger ones also drives the edge spectrum across a real to complex transition. Our discovery showcases intriguing dynamical consequences from the triple interplay of chiral topology, directed gain, and interlayer tunneling, and suggests new routes for the topology to be harnessed in the control of feedback systems.

Quantum chaos on edge

Recently, the physics of many-body quantum chaotic systems close to their ground states has come under intensified scrutiny. Such studies are motivated by the emergence of model systems exhibiting chaotic fluctuations throughout the entire spectrum [the Sachdev-Ye-Kitaev (SYK) model being a renowned representative] as well as by the physics of holographic principles, which likewise unfold close to ground states. Interpreting the edge of the spectrum as a quantum critical point, here we combine a wide range of analytical and numerical methods to the identification and comprehensive description of two different universality classes: the near edge physics of “sparse” and the near edge of “dense” chaotic systems. The distinction lies in the ratio between the number of a system's random parameters and its Hilbert space dimension, which is exponentially small or algebraically small in the sparse and dense case, respectively. Notable representatives of the two classes are generic chaotic many-body models (sparse) and invariant random matrix ensembles or chaotic gravitational systems (dense). While the two families share identical spectral correlations at energy scales comparable to the level spacing, the density of states and its fluctuations near the edge are different. Considering the SYK model as a representative of the sparse class, we apply a combination of field theory and exact diagonalization to a detailed discussion of its edge spectrum. Conversely, Jackiw-Teitelboim gravity is our reference model for the dense class, where an analysis of the gravitational path integral and random matrix theory reveal universal differences to the sparse class, whose implications for the construction of holographic principles we discuss. Published by the American Physical Society 2024

DFT study of electron energy loss spectra of sulfur in Janus MoSSe, MoSTe, WSSe and WSTe monolayers

In this paper, electronic properties of Janus MSX monolayers (M = Mo, W; X = Se or Te), including electron energy loss near edge structures (ELNES), as K and [Formula: see text] edge spectra of sulfur atoms, have been calculated using a full potential scheme and the augmented plane wave plus local orbitals (APW+lo) method in the framework of density functional theory (DFT). Due to the lack of experimental results, the obtained spectra are compared with the corresponding spectra of MoS2 monolayer. The band structure analysis shows that the Janus MoSSe and WSSe monolayers are direct bandgap semiconductors, while the Janus MoSTe and WSTe monolayers are indirect bandgap semiconductors. In comparison to MoS2, the main structures of the K edge ELNES spectra of sulfur atoms in Janus monolayers occur at lower energies, due to the structural change and increased bond length. The relative reduction of intensities in the main structures predicts the possibility of reducing the number of partial densities of states (PDOS), that is, the reduced p PDOS for the K edge in the corresponding energy range. In the K ([Formula: see text] edge ELNES spectra of the sulfur atom in Janus monolayers and MoS2 monolayers, the main structures are due to the electron transfer to the unoccupied [Formula: see text] and [Formula: see text] states (3s of sulfur and 4d states of the transition metal).

Bulk-entanglement spectrum correspondence in PT - and PC -symmetric topological insulators and superconductors

In this study, we discuss a type of bulk-boundary correspondence, which holds for topological insulators and superconductors when the parity-time (PT) and/or parity-particle-hole (PC) symmetry are present. In these systems, even when the bulk topology is nontrivial, the edge spectrum is generally gapped, and thus the conventional bulk-boundary correspondence does not hold. We find that, instead of the edge spectrum, the single-particle entanglement spectrum becomes gapless when the bulk topology is nontrivial: i.e., the holds in PT- and/or PC-symmetric topological insulators and superconductors. After showing the correspondence using K-theoretic approach, we provide concrete models for each symmetry class up to three dimensions where nontrivial topology because of PT and/or PC is expected. An implication of our results is that, when the bulk topology under PT and/or PC symmetry is nontrivial, the noninteracting many-body entanglement spectrum is multiply degenerate in one dimension and is gapless in two or higher dimensions. Published by the American Physical Society 2024

Absolutely continuous edge spectrum of topological insulators with an odd time-reversal symmetry

We show that non-trivial two-dimensional topological insulators protected by an odd time-reversal symmetry have absolutely continuous edge spectrum. To accomplish this, we establish a time-reversal symmetric version of the Wold decomposition that singles out extended edge modes of the topological insulator.

Intermediate valence behavior of the ternary cerium-nickel-phosphide Ce2Ni12P5

The crystal structure of the ternary phosphide Ce2Ni12P5 (La2Ni12P5-type structure) has been determined from X-ray powder diffraction data: full profile refinement, monoclinic symmetry, space group P21/m, a = 10.7809(2) Å, b = 3.6869(1) Å, c = 13.1490(3) Å, β = 107.776(4)º, RI = 0.068, RP = 0.044, RwP = 0.060. Ce2Ni12P5 is a ferromagnet with a Curie temperature of 5.8(5) K. A significant deviation from the linearity of the temperature dependence of the electrical resistance for Ce2Ni12P5 has been found. The low-temperature part of the electrical resistance indicates the presence of magnetic interactions in Ce2Ni12P5. The influence of a magnetic field on the electrical resistance of Ce2Ni12P5 has been studied. The X-ray absorption spectrum at the Ce LIII edge and X-ray emission spectra of Ni and P at the K and LIII edges have been studied experimentally and theoretically using DFT+U calculations. The calculations show good agreement with the experimental measurements. The effective valence of Ce in Ce2Ni12P5 determined based on the Ce LIII absorption spectrum is ϑeff ∼ 3.05.

Topology classification in bi-anisotropic topological photonic crystals via the Wilson loop approach [Invited

Topological photonic crystals have attracted tremendous attention due to their promise of robust optical properties and great potential for applications in on-chip devices. Numerous successful experimental demonstrations have shown or proved their topological properties, however, many of them turn out to have a nature of fragile topological phases. Here, using theoretical methods of fragile topology, we analyze two cases of topological photonic crystals with preserved time reversal symmetry, which utilize (1), the intrinsic duality and bi-anisotropy, and (2), accidental duality and structural bi-anisotropy respectively to induce their topological order. Our results show that the former case belongs to a Wannier-obstructed type of topological phase, indicating strong topological protection in their edge states. However, the latter meta-waveguide designs with structural bi-anisotropy widely implemented in experiments are Wannierizable, implying the fragile properties of their topology and gapped edge spectra. Our results provide new insights into the topological properties of photonic crystals as well as other bosonic systems with time-reversal symmetry.

Atomic-Layer IrOx Enabling Ligand Effect Boosts Water Oxidation Electrocatalysis.

An in situ formed IrOx (x ≤ 2) layer driven by anodic bias serves as the essential active site of Ir-based materials for oxygen evolution reaction (OER) electrocatalysis. Once being confined to atomic thickness, such an IrOx layer possesses both a favorable ligand effect and maximized active Ir sites with a lower O-coordination number. However, limited by a poor understanding of surface reconstruction dynamics, obtaining atomic layers of IrOx remains experimentally challenging. Herein, we report an idea of material design using intermetallic IrVMn nanoparticles to induce in situ formation of an ultrathin IrOx layer (O-IrVMn/IrOx) to enable the ligand effect for achieving superior OER electrocatalysis. Theoretical calculations predict that a strong electronic interaction originating from an orderly atomic arrangement can effectively hamper the excessive leaching of transition metals, minimizing vacancies for oxygen coordination. Linear X-ray absorption near edge spectra analysis, extended X-ray absorption fine structure fitting outcomes, and X-ray photoelectron spectroscopy collectively confirm that Ir is present in lower oxidation states in O-IrVMn/IrOx due to the presence of unsaturated O-coordination. Consequently, the O-IrVMn/IrOx delivers excellent acidic OER performances with an overpotential of only 279 mV at 10 mA cm-2 and a high mass activity of 2.3 A mg-1 at 1.53 V (vs RHE), exceeding most Ir-based catalysts reported. Moreover, O-IrVMn/IrOx also showed excellent catalytic stability with only 0.05 at. % Ir dissolution under electrochemical oxidation, much lower than that of disordered D-IrVMn/IrOx (0.20 at. %). Density functional theory calculations unravel that the intensified ligand effect optimizes the adsorption energies of multiple intermediates involved in the OER and stabilizes the as-formed catalytic IrOx layer.

The role of the coupling matrix elements in time-dependent density functional theory on the simulation of core-level spectra of transition metal complexes

Time-dependent density functional theory (TD-DFT) stands out as an efficient tool for computing core-level spectra in large molecules, particularly transition metal complexes. However, despite their relatively moderate computational demands, TD-DFT methods can still pose challenges for typical computations involving transition metal complexes with over a thousand basis functions. In this study, we investigate the role of the Coulomb, Hartree–Fock exchange, and exchange-correlation kernel contributions to the TD-DFT coupling matrix elements when simulating core-level spectra in transition metal complexes. Our observations reveal that the exchange-correlation kernel contribution, responsible for more than 50% of the computational time in a hybrid TD-DFT calculation, surprisingly has no discernible impact on the qualitative aspects of the calculated spectra. While the Coulomb term plays a crucial role in describing L2,3 -edge spectra, its significance becomes negligible when considering K, L 1, and M4,5 edges. In contrast, the scaled Hartree–Fock exchange is demonstrated to be the most influential term, underscoring the necessity for hybrid density functional approximations in accurately simulating core-level spectra. These trends hold irrespective of the chosen basis set and exchange-correlation functional, providing valuable insights for the development of approximate methods for incorporating two-electron interactions within the realm of core-level spectroscopies.

Robust Machine Learning Inference from X-ray Absorption Near Edge Spectra through Featurization

Robust Machine Learning Inference from X-ray Absorption Near Edge Spectra through Featurization

Spin-textured Volkov–Pankratov states and their tunnel magnetoresistance response

Volkov–Pankratov (VP) states are a family of sub-gap states that appear at the smooth interface/domain wall between topologically distinct gaped states. We carry out quantum transport simulations on one- and two-dimensional lattice models to demonstrate the emergence of such states in the edge spectrum of a quantum spin Hall system subjected to a smoothly varying exchange field that switches its sign at a given spatial point. We show the VP states possess non-trivial spin textures that can be characterized by a winding number in real space. It is further demonstrated that the application of an electric field along the edge provides control of this spin texture without altering the winding number. Finally, we illuminate how these spin textures can be read off via the local tunnel magnetoresistance (TMR) response of spin-polarized tunnel probes attached to the edge and the TMR can be controlled by purely electrical means akin to a Datta–Das type spin transistor.

Application of Exciton Coupling for Characterization of Nanographene Edge.

The structural characterization of nonstoichiometric nanographene (NG)-organic hybrid materials is usually difficult. The number of substituents on the edge and their arrangements are frequently questioned but are difficult to answer. Since the number of functional groups is closely related to the distance between the nearest neighbors (dISD ), the extraction of dISD from spectroscopic data could provide important information on their structural characterization. We show that exciton coupling, which is a theoretical prediction of the absolute structures of discrete molecules, is a possible candidate to address this issue. The comparison of the calculated CD spectra of the chiral chromophores extracted from the model NG edge with the observed edge spectra indicated a dISD of ca. 8 Å; this corresponded to substitution on every other armchair edge. Furthermore, an up-up-down-down alternate orientation was found to be a possible edge structure. Although the procedure was limited to NGs carrying chiral substituents, our method could facilitate the detailed structural characterization of NG-organic hybrid materials.

Layer-coupled corner states in two-dimensional topological multiferroics.

The structural diversity and controllability in two-dimensional (2D) materials offers an intriguing platform for exploring a wide range of topological phenomena. The layer degree of freedom, as a novel technique for material manipulation, requires further investigation regarding its association with topological states. Here, using first-principles calculations and a tight-binding model, we propose a novel mechanism that couples the second-order topological corner states with the layer degree of freedom. By analyzing the edge states, topological indices, and spectra of nanoflakes, we identify ferromagnetic H'-Co2XF2 (X = C, N) as 2D second-order topological insulators with intrinsic ferroelectricity. Moreover, the topological corner states strongly couple with the layer degree of freedom, and, remarkably, ferroelectricity provides a nonvolatile handle to manipulate the layer-polarized corner states. These findings open an avenue for the manipulation of second-order topological states and establish a bridge between ferroelectricity and nontrivial topology.

Estimation of Apple Leaf Nitrogen Concentration Using Hyperspectral Imaging-Based Wavelength Selection and Machine Learning

In apple cultivation, the total nitrogen content is an important indicator of plant growth, fruit quality, and yield. Timely monitoring of growth becomes imperative, since an imbalance, either in deficiency or excess nitrogen, can result in physiological disorders, adversely impacting both the quantity and quality of fruit. Leaf nitrogen content can be determined using simple chlorophyll meters or destructive testing; however, these methods are time-consuming. However, by employing spectral imaging technology, it is possible to swiftly predict leaf nitrogen content. This study estimated the total nitrogen content in apple trees via hyperspectral imaging and machine learning-based regression analysis (partial least-squares regression (PLSR), support vector regression (SVR), and eXtreme gradient boosting regression (XGBoost). Additionally, to reduce computational costs and improve reproducibility, spectral binning was divided into three stages (4, 8, and 16 bins), and models were compared with a 2-binning estimation model. The analysis focused on green, red, red edge, and near-infrared (NIR) spectra, with 5–10 selected wavelengths, and the SVR-based prediction model showed a similar or greater performance to that of the full spectrum. At 4- and 8-binning, the selected wavelengths were similar to those at 2-binning, maintaining similar prediction model performance. However, at 16 bp, the performance of the prediction model decreased owing to spectral data loss, leading to a significant reduction in wavelengths for nitrogen content estimation. These results can support informed nitrogen fertilization decisions, enabling precise, real-time monitoring of nitrogen content for enhanced plant growth, fruit quality, and yield in apple trees. Additionally, the selected wavelengths can be considered in the development of new types of multispectral sensors.

Manipulating Spin Exchange Interactions and Spin‐Selected Electron Transfers of 2D Metal Phosphorus Trisulfide Crystals for Efficient Oxygen Evolution Reaction

AbstractBecause oxygen molecules in the ground state favor a triplet spin configuration, spin‐polarized electrons at electrocatalysts may promote the generation of parallel spin‐aligned oxygen atoms, enhancing oxygen evolution reaction (OER) kinetics. In this study, a significant enhancement of OER performance is demonstrated by controlling the spin‐exchange interaction and spin‐selected electron transfer of 2D CoxFe1−xPS3 (x = 0–0.45) van der Waals (vdW) single crystals through Co doping. The pristine FePS3 exhibits antiferromagnetic orbital ordering, while the Co‐doped FePS3 exhibits the emergence of interatomic ferromagnetism due to doping‐mediated magnetic exchange interactions. The coupling between Fe and Co ions in the Co‐doped FePS3 crystal allows the formation of efficient spin‐selective electron transfer channels compared to the pristine FePS3. The correlation of spin‐exchange interactions and spin‐selected electron transfers of 2D Co‐doped FePS3 crystals with a superior OER performance is further revealed by superconducting quantum interference device magnetometer, in situ X‐ray absorption near edge spectra and density functional theory simulations. The result suggests that manipulating the spin‐exchange interactions of 2D vdW crystals to enhance the spin‐selected electron transfer efficiencies through doping is an effective strategy to boost their OER catalytic performances.

Dynamics of femtosecond heated warm dense copper with time-resolved L3-edge XANES.

Combining experimental set up and ab initio molecular dynamics simulations, we were able to follow the time evolution of the X-ray absorption near edge spectrum (XANES) of a dense copper plasma. This provides a deep insight into femtosecond laser interaction with a metallic copper target. This paper presents a review of the experimental developments we made to reduce the X-ray probe duration, from approximately 10 ps to fs duration with table-top laser systems. Moreover, we present microscopic scale simulations, performed with Density Functional Theory, as well as macroscopic simulations considering the Two-Temperature Model. These tools allow us to get a complete picture of the evolution of the target at a microscopic level, from the heating process to the melting and expansion stages, with a clear view of the physics involved during these processes. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Tunability of bandgap from UV to visible region by transition metal ion implantation in CeO2 and their electronic structures

The present report examines the tunability of optical band gap by 150 keV Fe ion implantation in cerium oxide (CeO2) thin films and their electronic structures. X-ray diffraction (XRD) confirms the stable FCC structure and these films lose their crystalline nature with increasing ion fluences. Atomic force microscopy (AFM) shows the morphology changes. Raman measurement exhibits the presence of defect states. UV–vis spectroscopy reveals that the optical band gap reduces from 3.14 eV to 2.70 eV. These results are well correlated with the electronic structure studies from the x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS). The XPS indicates the existence of Ce3+ due to the replacement of Fe3+ which reduces the Ce4+ to Ce3+ ions. XAS at the Ce-M4,5 edge reveals the change in Ce4+→Ce3+ with Fe ion fluence and the O-K edge spectra show that the vacancies are introduced after Fe ion implantation. The edge shifting of the O-K edge confirmed the presence of Ce4+-VO-Ce3+ and Ce3+-VO-Fe3+ networks in Fe-implanted thin films. This reversible ability enables the CeO2-based nanomaterial for energy and environmental-related applications.

Carbon K edge spectra of functionalized molybdenum-based MXenes, Mo2CT2 (T = F, OH and O), from first-principles calculations

In this study, the effect of the termination groups (T = F, OH, O) on the energy loss near edge structure (ELNES) of carbon K edge in Mo2C MXene at orientation-independent conditions has been investigated using first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that within the YS-PBE0 functional, the Mo2CF2 is a semiconductor with an indirect band gap of 0.723 eV. For Mo2CO2, the indirect band gap increases to 0.17 eV within the screened hybrid functional. The calculation results of ELNES spectra with the affection of core-hole show that, in comparison to pristine Mo2C, as a fingerprint of termination groups, the spectral structures in Mo2CT2 are reproduced at higher energies. Moreover, the spectral features of Mo2CT2 are sensitive to the chemical nature and the location of the T groups on the pristine Mo2C MXene surface. When going from T = O to T = F and, further, to T = OH, the energy separation between the main peaks increases, which is a sign of decreasing the Mo-C bond length, respectively, from T = O to T = F and to T = OH. The comparison of ELNES spectra and the unoccupied densities of states (DOS) reveal that, the origin of the first structure at the carbon K edge of Mo2CT2 is mostly result from electron transition to pz state, while in pristine Mo2C, mainly due to the transition to px + py state. Other structures at higher energies mainly arise from electron transitions to px + py state and partially to pz state. The spectral decomposition of the ELNES into in-plane (l′ = 1, m′ = ± 1) and out-of-plane (l′ = 1, m′ = 0) components also confirms these results. Generally, in both Mo2C and Mo2CT2, the contribution of in-plane element in most of the structures is more considerable.

On the saturation spectrum of families of cycle subdivisions

Given a family of graphs F, a graph G is F-saturated if G contains no member of F as a subgraph, but G+e contains a member of F as a subgraph for each edge e in the complement of G. The edge spectrum of F is the set of all possible sizes of F-saturated graphs on n vertices. A G-subdivision is a graph derived from G by replacing each edge of G with a path of arbitrary length. Let C≥k denote the family of Ck-subdivisions, where Ck is a cycle of length k with k≥3. Determining the minimum or maximum number of edges in n-vertex F-saturated graphs are two of the most important problems in the study of extremal graph theory. This is also a very important optimization problem in graph theory. The study of this problem is closely related to the development of other branches of mathematics, computer science, network, modern information science and technology. In this paper, we determine the edge spectrum of C≥k for each k∈{3,4,5,6}.

Electron energy loss spectra of sulfur in HgX2S4 (X = Al, Ga, In): A comparative theoretical study

By means of density functional theory based electronic structure calculations and energy loss near edge structure (ELNES) analysis, the present study calculates and describes the sulfur K and L2,3 edge spectra of mercury-based defective chalcopyrites (HgAl2S4 and HgGa2S4) and of its spinel structure (HgIn2S4), and compares with those of cadmium thiogallate (CdGa2S4) and the existing experimental spectra. The substitution of gallium by aluminium changes the sulfur K and L2,3 edge ELNES, decreasing the relative spectral intensities and shifting the main peaks to higher energies. The covalency power and the bond length play the considerable roles in the overall dispersions of the edge spectra.