Electronic Origin Research Articles

Thermoluminescence characteristics of BeO doped with Si, Mg, and Cr: Density Functional Theory calculations and One Trap – One Recombination Center model simulations

The incorporation of first principles computational methods in Thermoluminescence (TL) enables the comprehensive investigation of the luminescent characteristics exhibited by solid materials. These calculations offer valuable insights into the electronic structures and thermal behavior, providing a deeper understanding of the underlying mechanisms. Density Functional Theory (DFT) is a powerful tool for predicting, among others, the electronic structure properties, like the Density of States (DOS) of materials. By theoretically depicting the electronic states arising from electron transitions, the present study focuses on the DFT study of undoped and of BeO with substitutional and doping, and the connection of TL emission caused by recombination of electrons in centers, as outlined in the One Trap – One Recombination (OTOR) center model. This approach offers essential details regarding the origins of electron traps, facilitating a deeper exploration of the intrinsic relationship between TL and DFT in Stimulated Luminescence (SL) analysis.

Monte Carlo modeling of the origin of contaminant electrons on a 0.5T bi-planar Linac-MR.

In the last decade, hybrid linear accelerator magnetic resonance imaging (Linac-MR) devices have evolved into FDA-cleared clinical tools, facilitating magnetic resonance guided radiotherapy (MRgRT). The addition of a magnetic field to radiation therapy has previously demonstrated dosimetric and electron effects regardless of magnetic field orientation. This study uses Monte Carlo simulations to investigate the importance and efficacy of the magnetic field design in mitigating surface dose enhancement in the Aurora-RT, focusing specifically on contaminant electrons, their origin, and energy spectrum. The Aurora-RT 0.5 T Biplanar Linac-MR device was modeled using the BEAMnrc package using the updated EM macros, a magnetic field map generated from Opera 3D. Simulation generated phasespace data at the distal side of the first magnetic pole plate (89cm) and at machine isocenter (120cm) were analyzed with respect to electron energy spectra and electron creation origins, both with and without the static magnetic field. The presence of the main magnetic field was verified to affect the origin and distribution of contaminant electrons, removing them from the air column up to 60cm from the target, and focusing them along the CAX within the region below. Analysis of the remaining electron energy fluence reveals the net removal of electrons with energies>2 MeV and generation of electrons with energies<2 MeV in the presence of the static magnetic field as compared to no magnetic field. Moreover, in the presence of the magnetic field the integral energy contained in the contaminant electrons increases from 89cm to isocenter but is still 15% less overall than the integral energy contained in contaminant electrons without the magnetic field. This study provides an analysis of contaminant electrons in the Aurora-RT 0.5 T Linac-MR, emphasizing the role of magnetic field design in successfully minimizing electron contaminants.

Self-diffusion in Germanium-rich liquid Germanium-Nickel investigated by quasielastic neutron scattering

Abstract We report atomic dynamics in liquid Ge98Ni2 measured using quasielastic neutron scattering. Isotopic substitution enabled to separately determine Ge and Ni self-diffusion with high accuracy. The Ge self-diffusion coefficient in liquid Ge98Ni2 is equal within error bars to that in pure liquid Ge. However, the Ni self-diffusion coefficient lies at least 30% below the Ge self-diffusion coefficient. This behaviour differs from previously reported atomic dynamics in Ge-rich GeAu, GeSi, GeIn and GeCe melts, where no separation of the&amp;#xD;self-diffusion coefficients between the minor component and Ge is observed. The change of the atomic dynamics already at an addition of 2 at% Ni points to electronic and chemical origins in Ge98Ni2. Moreover, the slower self-diffusion of the minor component compared to Ge might be associated with two different local structural environments, as observed in Ge-Ni alloys at higher Ni concentration.

Vibronic Splitting of the Electronic Origin in Two Conformers of the 3-Tolunitrile Dimer.

3-tolunitrile (3-TN) can form three different dimers, which differ in the relative orientation of the methyl groups. We determined the geometry changes of two of these conformers of 3-TN upon electronic excitation via a Franck-Condon fit of the vibronic intensities in the fluorescence emission spectra. Both aromatic rings expand upon electronic excitation, showing a delocalized one-photon excitation of the cluster. The conformer with the smaller center-of-mass distance shows an unusual order of the split components of the electronic origin. We attribute this changed order to the stronger charge transfer contributions to the splitting and a partial breakdown of the point dipole approximation, made in the Frenkel type interaction.

Chirality-Mediated 1D-to-2D Phase Transition in Hybrid Lead Halide Perovskites.

Dimensionality engineering plays a pivotal role in optimizing the performance, ensuring long-term stability, and expanding the versatile applications of lead halide perovskites (LHPs). Currently, the manipulation of LHP dimensions primarily occurs during the synthesis stage, a procedure hampered by constraints, including synthetic complexity and irreversibility. This investigation successfully achieved a transition from one-dimensional (1D) to two-dimensional (2D) structures in chiral LHPs by applying hydrostatic pressure. Remarkably, this pressure-induced transition in dimensionality is absent in the racemic analogue due to the staggered arrangement of inorganic chains and the elevated steric hindrance posed by the organic cations. Notably, the hydrogen bonding between organic cations and the inorganic framework adopts a symmetrical arrangement in the racemic system but a helical configuration along the 1D chain direction in the chiral counterparts. This distinct helical arrangement induces a consequential distortion in the inorganic moiety, resulting in the emergence of a spin-polarized Rashba-Dresselhaus texture that explains the chirality's electronic spin origin. Furthermore, both experimental and density functional theory calculation results demonstrate that the 1D-to-2D phase transition in chiral halide perovskites can induce significant modifications in the electronic structures and associated optical emissions. In summary, the findings unveil novel avenues for manipulating optoelectronic properties in chiral perovskites through dimensionality engineering.

Data-Driven Design of Single-Atom Electrocatalysts with Intrinsic Descriptors for Carbon Dioxide Reduction Reaction

The strategic manipulation of the interaction between a central metal atom and its coordinating environment in single-atom catalysts (SACs) is crucial for catalyzing the CO2 reduction reaction (CO2RR). However, it remains a major challenge. While density-functional theory calculations serve as a powerful tool for catalyst screening, their time-consuming nature poses limitations. This paper presents a machine learning (ML) model based on easily accessible intrinsic descriptors to enable rapid, cost-effective, and high-throughput screening of efficient SACs in complex systems. Our ML model comprehensively captures the influences of interactions between 3 and 5d metal centers and 8 C, N-based coordination environments on CO2RR activity and selectivity. We reveal the electronic origin of the different activity trends observed in early and late transition metals during coordination with N atoms. The extreme gradient boosting regression model shows optimal performance in predicting binding energy and limiting potential for both HCOOH and CO production. We confirm that the product of the electronegativity and the valence electron number of metals, the radius of metals, and the average electronegativity of neighboring coordination atoms are the critical intrinsic factors determining CO2RR activity. Our developed ML models successfully predict several high-performance SACs beyond the existing database, demonstrating their potential applicability to other systems. This work provides insights into the low-cost and rational design of high-performance SACs.

Electronic and magnetic origins of unconventional charge density wave in kagome FeGe

Electronic and magnetic origins of unconventional charge density wave in kagome FeGe

Deciphering Pauling's Third Rule: Uncovering Strong Anharmonicity and Exceptionally Low Thermal Conductivity in TlAgSe for Thermoelectrics.

The elucidation of chemical bonding, coupled with an exploration of the correlated dynamics of constituent atoms, is essential for unravelling the underlying mechanism responsible for low lattice thermal conductivity (κL) exhibited by a crystalline solid, which is essential for thermoelectrics and thermal barrier coatings. In this regard, Pauling's third empirical rule, which deals with the cationic repulsion due to proximity in the face or edge shared polyhedra in a crystal structure, can bring about the lattice instability required to suppress the κL. Here, we demonstrate the presence of such instability in a ternary selenide, TlAgSe, leading to a ultra-low κL of 0.17 W/m.K at 573 K. Our study reveals the instability arising from Ag-Ag repulsion within edge-shared AgSe4 tetrahedra through investigation of the local structure using synchrotron X-ray pair distribution function (PDF) analysis and supported by first-principles density functional theory calculations. We observe correlation between weakening in the Ag and the Tl-sublattice, providing direct experimental evidence of Pauling's third empirical rule. The correlated rattling of Ag and Tl induces a highly anharmonic lattice and low energy optical phonons, resulting in suppressed sound velocity and ultralow κL in TlAgSe. The electronic origin of soft and anharmonic lattice is the presence of filled antibonding states in the valence band near the Fermi level constructed by Ag(4d)-Se(4p) and Tl(6s)-Se(4p) interactions. This work demonstrates that the evidence of dynamic distortion in a crystal lattice is governed by the third empirical rule given by Pauling, which can act as a potential new strategy for diminishing κL in crystalline solids.

Deciphering Pauling's Third Rule: Uncovering Strong Anharmonicity and Exceptionally Low Thermal Conductivity in TlAgSe for Thermoelectrics

AbstractThe elucidation of chemical bonding, coupled with an exploration of the correlated dynamics of constituent atoms, is essential for unravelling the underlying mechanism responsible for low lattice thermal conductivity (κL) exhibited by a crystalline solid, which is essential for thermoelectrics and thermal barrier coatings. In this regard, Pauling's third empirical rule, which deals with the cationic repulsion due to proximity in the face or edge shared polyhedra in a crystal structure, can bring about the lattice instability required to suppress the κL. Here, we demonstrate the presence of such instability in a ternary selenide, TlAgSe, leading to a ultra‐low κL of 0.17 W/m.K at 573 K. Our study reveals the instability arising from Ag−Ag repulsion within edge‐shared AgSe4 tetrahedra through investigation of the local structure using synchrotron X‐ray pair distribution function (PDF) analysis and supported by first‐principles density functional theory calculations. We observe correlation between weakening in the Ag and the Tl‐sublattice, providing direct experimental evidence of Pauling's third empirical rule. The correlated rattling of Ag and Tl induces a highly anharmonic lattice and low energy optical phonons, resulting in suppressed sound velocity and ultralow κL in TlAgSe. The electronic origin of soft and anharmonic lattice is the presence of filled antibonding states in the valence band near the Fermi level constructed by Ag(4d)−Se(4p) and Tl(6s)−Se(4p) interactions. This work demonstrates that the evidence of dynamic distortion in a crystal lattice is governed by the third empirical rule given by Pauling, which can act as a potential new strategy for diminishing κL in crystalline solids.

Stacking-dependent Van Hove singularity shifts in three-dimensional charge density waves of kagome metals AV3Sb5 (A = K, Rb, Cs)

Vanadium-based kagomé systems AV3Sb5 (A = K, Rb, Cs) have emerged as paradigmatic examples exhibiting unconventional charge density waves (CDWs) and superconductivity linked to van Hove singularities (VHSs). Despite extensive studies, the three-dimensional (3D) nature of CDW states in these systems remains elusive. This study employs first-principles density functional theory and a tight-binding model to investigate the stacking-dependent electronic structures of 3D CDWs in AV3Sb5, emphasizing the significant role of interlayer coupling in behaviors of the VHSs associated with diverse 3D CDW orders. We develop a minimal 3D tight-binding model and present a detailed analysis of band structures and density of states for various 3D CDW stacking configurations, including those with and without a π-phase shift stacking of the inverse star of David, as well as alternating stacking of the inverse star of David and the star of David. We find that VHSs exist below the Fermi level even in 3D CDWs without π-phase shift stackings, and that these VHSs shift downward in the π-phase shift stacking CDW structure, stabilizing the 2×2×2π-shifted inverse star of David distortions in alternating vanadium layers as the ground state 3D CDW order of AV3Sb5. Our work provides the electronic origin of 3D CDW orders, paving the way for a deeper understanding of CDWs and superconductivity in AV3Sb5 kagomé metals.

Electronic transport in amorphous Ge2Sb2Te5 phase-change memory line cells and its response to photoexcitation

We electrically characterized melt-quenched amorphized Ge2Sb2Te5 (GST) phase-change memory cells of 20 nm thickness, ∼66–124 nm width, and ∼100–600 nm length with and without photoexcitation in the 80–275 K temperature range. The cells show distinctly different current–voltage characteristics in the low-field (≲19 MV/m), with a clear response to optical excitation by red light, and high-field (≳19 MV/m) regimes, with a very weak response to optical excitation. The reduction in carrier activation energy with photoexcitation in the low-field regime increases from ∼10 meV at 80 K to ∼50 meV at 150 K (highest sensitivity) and decreases again to 5 meV at 275 K. The heterojunctions at the amorphous–crystalline GST interfaces at the two sides of the amorphous region lead to formation of a potential well for holes and a potential barrier for electrons with activation energies in the order of 0.7 eV at room temperature. The alignment of the steady state energy bands suggests the formation of tunnel junctions at the interfaces for electrons and an overall electronic conduction by electrons. When photoexcited, the photo-generated holes are expected to be stored in the amorphous region, leading to positive charging of the amorphous region, reducing the barrier for electrons at the junctions and hence the device resistance in the low-field regime. Holes accumulated in the amorphous region are drained under a high electric field. Hence, the potential barrier cannot be modulated by photogenerated holes, and the photo-response is significantly reduced. These results support the electronic origin of resistance drift in amorphous GST.

Fluorescence Excitation and Dispersed Fluorescence Spectra of the First Electronic Excited (S1) State of peri-Hexabenzocoronene (C42H18) Isolated in Solid para-Hydrogen.

Large polycyclic aromatic hydrocarbons (PAH) and their cationic, hydrogenated, and protonated derivatives have long been considered as promising candidates for the carriers of the diffuse interstellar bands. peri-Hexabenzocoronene (peri-HBC, C42H18) is a large, compact PAH, and, to the best of our knowledge, the largest centrosymmetric all-benzenoid PAH for which electronic spectroscopy data has been published. In this work, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S1) state of peri-HBC isolated in solid para-H2 and provide the first detailed vibronic analysis of observed features. The observed spectra agree with the emission and absorption spectra simulated according to optimized geometries and scaled harmonic vibrational frequencies calculated at the density functional theory (DFT) level using a Franck-Condon Herzberg-Teller approach; the spectral bands are associated solely with vibrational normal modes of approximate e2g symmetry and their combinations with vibrational modes of approximately a1g symmetry. We clearly observed the position of the S1-S0 electronic transition origin of peri-HBC at 22,088 cm-1 (452.7 nm), which was unreported previously. The matrix shift of ∼110 cm-1 to the red relative to the gas-phase value was estimated by comparison of two reported gas-phase bands with our work. Because of the significant deviation from the reported wavelengths of DIB, the weakness of the S1-S0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S4 ← S0 band of peri-HBC is located, peri-HBC is unlikely to contribute to DIB.

Direct laser acceleration: A model for the electron injection from the walls of a cylindrical guiding structure.

We use analytical methods and particle-in-cell simulation to investigate the origin of electrons accelerated by the process of direct laser acceleration driven by high-power laser pulses in preformed narrow cylindrical plasma channels. The simulation shows that the majority of accelerated electrons are originally located along the interface between the channel wall and the channel interior. The analytical model based on the electron hydrodynamics illustrates the underlying physical mechanism of the release of electrons from the channel wall when irradiated by an intense laser, the subsequent electron dynamics, and the corresponding evolution of the channel density profile. The quantitative predictions of the total charge of released electrons and the average electron density inside the channel are validated by comparison with the simulation results.

Unraveling electronic origins for boosting thermoelectric performance of p-type (Bi,Sb)2Te3.

P-type Bi2-xSbxTe3 compounds are crucial for thermoelectric applications at room temperature, with Bi0.5Sb1.5Te3 demonstrating superior performance, attributed to its maximum density-of-states effective mass (m*). However, the underlying electronic origin remains obscure, impeding further performance optimization. Herein, we synthesized high-quality Bi2-xSbxTe3 (00 l) films and performed comprehensive angle-resolved photoemission spectroscopy (ARPES) measurements and band structure calculations to shed light on the electronic structures. ARPES results directly evidenced that the band convergence along the [Formula: see text]-[Formula: see text] direction contributes to the maximum m* of Bi0.5Sb1.5Te3. Moreover, strategic manipulation of intrinsic defects optimized the hole density of Bi0.5Sb1.5Te3, allowing the extra valence band along [Formula: see text]-[Formula: see text] to contribute to the electrical transport. The synergy of the above two aspects documented the electronic origins of the Bi0.5Sb1.5Te3's superior performance that resulted in an extraordinary power factor of ~5.5 milliwatts per meter per square kelvin. The study offers valuable guidance for further performance optimization of p-type Bi2-xSbxTe3.

Orbital-dependent electron correlation in double-layer nickelate La3Ni2O7

The latest discovery of high temperature superconductivity near 80 K in La3Ni2O7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La3Ni2O7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La3Ni2O7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3dz2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{{z}^{2}}$$\\end{document} orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La3Ni2O7. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La3Ni2O7.

Unraveling the Nature of Spin Coupling in a Metal-Free Diradical: Theoretical Distinction of Ferromagnetic and Antiferromagnetic Interactions.

Metal-free diradicals based on polycyclic aromatic hydrocarbons are promising candidates for organic spintronics due to their stable magnetism and tunable spin coupling. However, distinguishing and elucidating the origins of ferromagnetic and antiferromagnetic interactions in these systems remain challenging. Here, we investigate the 2-OS diradical molecule sandwiched between gold electrodes using a combined density functional theory and hierarchical equations of motion approach. We find that the dihedral angle between the radical moieties controls the nature and strength of the intramolecular spin coupling, transitioning smoothly from antiferromagnetic to ferromagnetic as the angle increases. Distinct features in the inelastic electron tunneling spectra are identified that can discern the two coupling regimes, including spin excitation steps whose energies directly reveal the exchange coupling constant. Mechanical stretching of the junction is predicted to modulate the spectral line shapes by adjusting the hybridization of the molecular radicals with the electrodes. Our work elucidates the electronic origin of tunable spin interactions in 2-OS and provides spectroscopic fingerprints for characterizing magnetism in metal-free diradicals.

Computed Gyration Tensors of Knotted Chiral and Achiral Topological Stereoisomers of C60 Cyclocarbons.

The electronic origins of the computed optical rotations of the simplest chiral and achiral chemical knots with comparatively simple compositions and large, anticipated magnetoelectric polarizabilities are provided. Linear response theory (LRT) is used to calculate the gyration at 1064 nm of two knotted polyyne chains, topological stereoisomers of cyclo[60]carbon. One isomer is analogous to the trefoil knot with approximate D3 symmetry and the other to the figure eight knot with approximate S4 symmetry. The response in each case can be attributed largely to the magnetic dipole term that arises in a near degenerate E-like excited state. An oriented achiral figure eight knot is as optically active in some directions as the chiral knot in any direction, and its absolute eigenvalues are larger.

Unveiling the electronic origin of lanthanide based Metal Organic Framework with ideal spin filtering capacity

Recently, a three dimensional metal-organic framework (MOF) based on Dy(III) and the L-tartrate ligand was experimentally shown to exhibit a spin polarization (SP) power of 100% at room temperature. The material’s spin filtering ability was ascribed to the chiral-induced spin selectivity (CISS) effect. In this work, we computationally characterize the electronic structure of this MOF, revealing that the high SP of the material is linked to the asymmetric arrangement, around the Fermi level, of the alpha- and beta-spin electron states arising from the 4f-states of the lanthanide Dy atom, which results in two different conduction channels (band gaps) for each spin state. Based on the understanding gathered in this work, we propose that the substitution of the hydroxyl groups of the ligand by mercaptan groups should boost the electrical conductivity, while retaining the spin filtering power of the material.

Broadband Emission Induced by Band-Edge Carrier Reconfiguration in 2D Hybrid Lead Halide Perovskites.

Broadband emission in hybrid lead halide perovskites (LHPs) has gained significant attention due to its potential applications in optoelectronic devices. The origin of this broadband emission is primarily attributed to the interactions between electrons and phonons. Most investigations have focused on the impact of structural characteristics of LHPs on broadband emission, while neglecting the role of electronic mobility. In this work, the study investigates the electronic origins of broadband emission in a family of 2D LHPs. Through spectroscopic experiments and density functional theory calculations, the study unveils that the electronic states of the organic ligands with conjugate effect in LHPs can extend to the band edges. These band-edge carriers are no longer localized only within the inorganic layers, leading to electronic coupling with molecular states in the barrier and giving rise to additional interactions with phonon modes, thereby resulting in broadband emission. The high-pressure photoluminescence measurements and theoretical calculations reveal that hydrostatic pressure can induce the reconfiguration of band-edge states of charge carriers, leading to different types of band alignment and achieving macroscopic control of carrier dynamics. The findings can provide valuable guidance for targeted synthesis of LHPs with broadband emission and corresponding design of state-of-the-art optoelectronic devices.

Electronic origin of structural degradation in Li-rich transition metal oxides: The case of Li2MnO3 and Li2RuO3

Li2MnO3 and Li2RuO3 represent two prototype Li-rich transition metal (TM) oxides as high-capacity cathodes for Li-ion batteries, which have similar crystal structures but show quite different cycling performances. Here, based on the first-principles calculations, we systematically studied the electronic structures and defect properties of these two Li-rich cathodes, in order to get more understanding on the structural degradation mechanism in Li-rich TM oxides. Our calculations indicated that the structural and cycling stability of Li2MnO3 and Li2RuO3 depend closely on their electronic structures, especially the energy of their highest occupied electronic states (HOS), as it largely determines the defect properties of these cathodes. For Li2MnO3 with low-energy HOS, we found that, due to the defect charge transfer mechanism, various defects can form spontaneously in its host structure as Li ions are extracted upon delithiation, which seriously deteriorates its structural and cycling stability. While for Li2RuO3, on the other hand, we identified that the high-energy HOS prevents it from the defect formation upon delithiation and thus preserve its cycling reversibility. Our studies thus illustrated an electronic origin of the structural degradation in Li-rich TM oxides and implied that it is possible to improve their cycling performances by carefully adjusting their TM components.