Study Of Electronic Structure Research Articles

Nature of quasiparticle interference in three dimensions

Quasiparticle interference (QPI) imaging is a powerful tool for the study of the low-energy electronic structure of quantum materials. However, the measurement of QPI by scanning tunneling microscopy (STM) is restricted to surfaces and is thus inherently constrained to two dimensions. QPI has proved immensely successful for the study of materials that exhibit a quasi-two-dimensional electronic structure, yet it raises questions about how to interpret QPI in materials that have a highly three-dimensional electronic structure. In this paper, we address this question and establish the methodology required to simulate and understand QPI arising from three-dimensional systems as measured by STM. We calculate the continuum surface Green's function in the presence of a defect, which captures the role of the surface and the vacuum decay of the wave functions. We find that defects at different depths from the surface will produce unique sets of scattering vectors for three-dimensional systems, which nevertheless can be related to the three-dimensional electronic structure of the bulk material. We illustrate the consequences that the three-dimensionality of the electronic structure has on the measured QPI for a simple cubic nearest-neighbor tight-binding model, and then demonstrate application to a real material using a realistic model for PbS. Our method unlocks the use of QPI imaging for the study of quantum materials with three-dimensional electronic structures and introduces a framework to generically account for ${k}_{z}$ dispersions within QPI simulations.

First principles calculation of the ZnV2O6(001) surface terminations: the thermodynamic stability and electronic structure study.

We have investigated the surface structure and relative stability of ZnV2O6(001) using a thermodynamic technique based on density functional theory (DFT). We built Zn-V-O surface phase diagrams of various surface terminations using the obtained surface Gibbs free energy. In this study, we selected nine different surface terminations along the (001) crystal plane to elucidate that the E, G, H, and I terminations (as shown in Table 1) are the most stable configurations. We found that although their stability varies widely, the four terminations on the ZnV2O6(001) surface can be stabilized under specific thermodynamic equilibrium circumstances. Furthermore, we calculated the surface electronic structures of the four surface terminations and found that there are surface states conducive to visible light absorption at the G, H, and I terminations. The different termination structures are significant in improving the range and intensity of light absorption of ZnV2O6 in specific regions. The fact that the work functions fluctuate significantly for different surface terminations suggests that the work function of ZnV2O6 can be changed to increase photocatalytic activity by achieving thermodynamically favored surface terminations under appropriate conditions. The obtained surface phase diagram will further lay a foundation for the study of the ZnV2O6 surface. These results may help to explore the inherent properties of the ZnV2O6 surface and provide useful strategies for future experimental research on ZnV2O6-based photocatalysts.

Insight into the electronic structure of the centrosymmetric skyrmion magnet GdRu2Si2.

The discovery of a square magnetic-skyrmion lattice in GdRu2Si2, with the smallest so far found skyrmion size and without a geometrically frustrated lattice, has attracted significant attention. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu2Si2 by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4f magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along kz at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman-Kittel-Kasuya-Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4f moments responsible for the skyrmion physics in GdRu2Si2. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based systems.

Towards hard-magnetic behavior of CoFe2O4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties.

We have used the coprecipitation and mechanical-milling methods to fabricate CoFe2O4 nanoparticles with an average crystallite size (d) varying from 81 to ∼12 nm when changing the milling time (t m) up to 180 min. X-ray diffraction and Raman-scattering studies have proved the samples crystalizing in the spinel structure. Both the lattice constant and residual strain tend to increase when t m(d) increases (decreases). The analysis of magnetization data has revealed a change in the coercivity (H c) towards the hard-magnetic properties. Specifically, the maximum H c is about 2.2 kOe when t m = 10 min corresponding to d ≈ 29 nm; beyond this t m(d) value, H c gradually decreases. Meanwhile, the increase of t m always reduces the saturation magnetization (M s) from ∼69 emu g-1 for t m = 0 to 35 emu g-1 for t m = 180 min. The results collected as analyzing X-ray absorption data have indicated a mixed valence state of Fe2+,3+ and Co2+ ions. We think that the migration and redistribution of these cations between the tetrahedral and octahedral sites together with lattice distortions and defects induced by the milling process have impacted the magnetic properties of the CoFe2O4 nanoparticles.

Study of experimental method for warm dense matter ionization distribution based on x-ray fluorescence spectroscopy

Warm dense matter (WDM), a state of matter which lies at the frontiers between condensed matter and plasma, is one of the main research objects of high energy density physics (HEDP). Compared to the isolated atom, the electron structure of WDM will change because of the influence of density and temperature effect. Both the accurate theoretical represent and the accurate experimental study of WDM electron structure are challenging, as it is strongly coupled and partially degenerated. In this paper, an experimental method for studying the ionization distribution of warm dense matter based on x-ray fluorescence spectroscopy is developed. In the experiment, warm dense titanium with several tens eV and near solid density is created by a simultaneous drive from high energy xray heating and shock compression in a special designed hohlraum. Then, using the characteristic line spectrum emitted by the laser irradiation on pump material (Vanadium) as pump source, the titanium emits fluorescence. The x-ray fluorescence spectroscopy of titanium with different states (cold sample, 1.8-4.5 g/<i>cm</i><sup>3</sup> and 1-25 eV) is diagnosed by changing the experimental strategy. The experimental results indicate that the line profiles of <i>K<sub>α</sub></i> and <i>K<sub>β</sub></i> fluorescence spectrum of the heated sample change obviously relative to that of the cold sample. Associating a theoretical calculation from two-step Hartree-Fock-Slater (TSHFS) method, the reason for the variation of the line profile is the change of ionization distribution mainly caused by temperature rise. The future work will focus on optimizing the experimental method of x-ray fluorescence spectroscopy, such as improving the spectrum resolution, characterizing the temperature and density experimentally, obtaining a set of ionization distribution data, and then study the influence of dense environment on electronic structure.

Band-gap engineering and optoelectronic properties of 2D WSi2N4 nanosheets: A first principle calculations

The successful growth of centimeter-scale monolayer films of MoSi 2 N 4 and WSi 2 N 4 has attracted great interest (Science 369, 670, 2020). Here, we systematically investigate the effects of biaxial and uniaxial strain on the electronic structures of WSi 2 N 4 nanosheets with different layers of monolayer(ML), bilayer(BL) and trilayer(TL), respectively, by using first-principles calculations. In addition, combined with the non-equilibrium Green’s function methods, the photoelectronic response of WSi 2 N 4 ML is also explored. The results indicate that: 1) The biaxial and uniaxial strain can flexibly increase or decrease the band gap of WSi 2 N 4 nanosheets. Particularly, tensile strain realizes the transition of WSi 2 N 4 nanosheets from semiconductor to metal. 2) The tensile strain effectively improves the optical properties of WSi 2 N 4 nanosheets in the visible light region, and the red/blue shift of the absorption peaks is observed in the optical spectrum by tensile/compressive strain. 3) The pin-junction photodiode of WSi 2 N 4 ML generates sizable photocurrents under illumination, showing a strong photoelectronic response to purple light. Furthermore, the biaxial tensile strain significantly enhances the photoelectric performance of the WSi 2 N 4 photodiode and effectively modulates its detection range in the visible region. Hence, our findings suggest that WSi 2 N 4 nanosheets are a promising multifunctional material for nanoelectronic and optoelectronic applications. • Electronic structure and optoelectronic properties studies of the 2D WSi 2 N 4 nanosheets. • Strain engineering can effectively modulate the band gap and optical properties of WSi 2 N 4 nanosheets. • The pristine WSi 2 N 4 monolayer is predicted to be a good photoelectric sensor, showing a strong photoelectronic response to purple light. • The tensile strain significantly enhance the photoelectric performance of the WSi 2 N 4 photodiode and effectively modulates its detection range in the visible region.

Optical Measurement of Photorecombination Time Delays

Recollision physics and attosecond pulse generation meld the precision of optics with collision physics. As a follow-up to our previous work, we reveal a new direction for the study of electronic structure and multielectron dynamics by exploiting the collision-physics nature of recollision. We show experimentally that, by perturbing recollision trajectories with an infrared field, photorecombination time delays can be measured entirely optically using the Cooper minimum in argon as an example. In doing so, we demonstrate the relationship between recollision trajectories and the transition moment coupling the ground and continuum states. In particular, we show that recollision trajectories are influenced by their parent ion, while it is commonly assumed they are not. Our work paves the way for the entirely optical measurement of ultrafast electron dynamics and photorecombination delays due to electronic structure, multielectron interaction, and strong-field-driven dynamics in complex molecular systems and correlated solid-state systems.

Electronic structure study of dual-doped II-VI semiconductor quantum dots towards single-source white light emission.

Single-source white light emitting colloidal semiconductor quantum dots (QDs) is one of the most exciting and promising high-quality solid-state light sources to meet the current global demand for sustainable resources. While most of the previous methods involve dual (green-red) emissive nanostructures coated on blue LEDs to achieve white light, this work describes a single-source white light emitter of robust and superior quality using dual-doping. A modified synthesis method for intense white light emitting Cu, Mn dual-doped ZnSe QDs is engineered such that the extent of doping and concentration of ligands can alter their electronic structures. This is then customized to obtain various types of white light emissions ranging from warm white to cool white. Further, the composition-driven change in the electronic structure of the host QDs is exploited to achieve emission tunability over the entire visible range.

Electronic structural studies of newly synthesized sterically hindered analogues of meta-benziporphodimethene and its metal complexes

meta-BPDM is a new type of discrete conjugated porphyrinoid system and shows tremendous properties related to fluorescence and different behaviors with substituents on the sp3meso carbons.

Electronic structure study of H3BXH3 (X═B, N and P) as hydrogen storage materials using calculated NMR and XPS spectra

Boron-based materials have been used for hydrogen storage applications owing to their high volumetric and gravimetric hydrogen density. The present study quantum mechanically investigates the electronic structures of three compounds: diborane (DB, B2H6), ammonia borane (AB, H3BNH3) and phosphine borane (PB, H3BPH3). The exploration is facilitated using calculated nuclear magnetic resonance (NMR) chemical shifts, together with outer valence ionisation potentials (IP) and core electron binding energy (CEBE). The findings show a distinct electronic structure for diborane, differing notably from AB and PB, which exhibit certain similarities. Noteworthy dissimilarities are observed in the chemical environments of the bridge hydrogens and terminal hydrogens in diborane, resulting in a substantial chemical shift difference of up to 5.31 ppm. Conversely, in AB and PB, two distinct sets of hydrogens emerge: protic hydrogens (Hp–N and Hp–P) and hydridic hydrogens (Hh–B). This leads to chemical shifts as small as 0.42 ppm in AB and as significant as 3.0 ppm in PB. The absolute isotropic NMR shielding constant (σB) of 11B in DB is 85.40 ppm, in contrast to 126.21 ppm in AB and 151.46 ppm in PB. This discrepancy indicates that boron in PB has the most robust chemical environment among the boranes. This assertion finds support in the calculated CEBE for B 1s of 196.53, 194.01 and 193.93 eV for DB, AB and PB respectively. It is clear that boron in PB is the most reactive atom. Ultimately, understanding the chemical environment of the boranes is pivotal in the context of dehydrogenation processes for boron-based hydrogen storage materials.

Hybrid-Density Functional Calculations of Structural, Electronic, Magnetic, and Thermodynamic Properties of α-Cu2P2O7

We present a comparative study (using PBE, PBE0, and HSE functionals) of electronic and atomic structure, magnetism, and phonon dispersion relations of α-Cu2P2O7. Four possible magnetic configurations are considered, FM, AFM-1, AFM-2, and AFM-3. The calculations reveal that α-Cu2P2O7 is mechanically and thermodynamically stable. The elastic moduli indicate a weak resistance of the compound to volume and shear deformations. The electronic structure at the valence band maximum is dominated by O, with a small admixture of Cu-dx2−y2 states. The conduction band results from the hybridization between Cu and O states which, in the case of AFM-2, produces the largest band gap of 3.966 eV and the smallest magnetic moment of ±0.785 μB on Cu. AFM-2 is found to be the lowest-energy structure that may be viewed as consisting of quasi-one-dimensional −Cu1−Cu2−Cu3−Cu4− chains along the b axis; the antiferromagnetism is due to two identical Cu−O−Cu paths with a bond angle of 100.301∘. The phonon spectra exhibit four distinct frequency ranges corresponding to different vibrational modes of ions and ionic groups. Thus, a quantitative description of the structural, electronic, and magnetic properties of α-Cu2P2O7 is possible using the HSE hybrid functional, which enables computational studies of transition metal pyro compounds.

Unraveling the Photoelectron Spectrum of 1-Phospha-2,3,4-triazolate Anion, HCPN3–, A Theoretical Approach

The first five low-lying electronic states of HCPN3 are probed through extensive ab initio electronic structure and quantum dynamics studies to reproduce the 193 nm photoelectron spectrum. Vibronic Hamiltonian is constructed and availed for time-dependent (TD) and time-independent (TI) quantum dynamical studies. The presence of numerous conical intersections (CIs) and crossings among electronic states yielded interesting nonadiabatic effects in the photoelectron bands of the overall spectrum. Moreover, the theoretical bands corresponding to five electronic states have reproduced all three experimental spectral bands. Among these, the first two bands originated due to a combination of four electronic states as predicted by previous studies. The third band corresponds to the fifth electronic state. The results calculated via TD and TI approaches exhibited satisfying agreement with the experimental results.

Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8

Kagome materials have become solid grounds to study the interplay among geometry, topology, correlation, and magnetism. Recently, niobium halide semiconductors Nb3X8 (X = Cl, Br, I) have been predicted to be two-dimensional magnets and these materials are also interesting for their breathing kagome geometry. However, experimental electronic structure studies of these promising materials are still lacking. Here, we report the spectroscopic evidence of flat and weakly dispersing bands in breathing-kagome semiconductor Nb3I8 around 500 meV binding energy, which is well supported by our first-principles calculations. These bands originate from the breathing kagome lattice of niobium atoms and have niobium d-orbital character. They are found to be sensitive to the polarization of the incident photon beam. Our study provides insight into the electronic structure and flat band topology in an exfoliable kagome semiconductor, thereby providing an important platform to understand the interaction of geometry and electron correlations in two-dimensional materials.

Na2Mn(CO3)2: A carbonate based prototype cathode material for Na-ion batteries with high rate capability — An ab-initio study

Considering the mounting demand for energy storage systems and the issues associated with the current cathode materials, it is necessary that the search space for novel cathode materials for rechargeable batteries should be widened. In this regard, we have presented the structural, lattice dynamical, electrochemical, and electronic structure studies of a carbonate-based prototype cathode material for Na-ion batteries, using first-principles calculations. The existence of a carbon-containing Mn compound Na2Mn(CO3)2 is predicted using first-principles calculations and the compound is expected to crystallize in trigonal crystal structure with R3̄ space group. The electrochemical analyses reveal that this cathode material could deliver a theoretical capacity of 243mAh/g and a theoretical average voltage of 3.6 V. The desodiation process is modeled via finding the stable crystal structure of intermediate compositions for different Na/vacancy concentrations. The electronic structure and charge density analysis of these intermediate compositions reveal that Mn, O, and a small contribution from C involve in the desodiation process. Even though the involvement of O in the redox is high, we have found that the oxygen evolution during desodiation is not favored thermodynamically, indicating a possible enhancement in the specific capacity. The chemical bonding analyses indicate that the Mn-O bonding interaction is iono-covalent, whereas the C-O bond exhibits a more covalent character with small ionicity. The activation energy of the diffusion barrier for the Na diffusion through the lower energy path in Na2Mn(CO3)2 is estimated to be 0.22 eV using nudged elastic band method. We postulate high ionic conductivity and thereby high rate capability for Na2Mn(CO3)2 as the Na diffusion coefficient is relatively higher compared with that of Li in cathodes used for commercial Li-ion batteries.

Meta-magnetism and exchange interaction in binary alloy Fe2Ge

First principle studies of electronic structure and magnetic properties of binary alloy Fe2Ge are presented. We have observed the increase of total magnetic moment per unit cell with lattice dilation. The underlying cause for this increased magnetic moments is explained in terms of electronic structure. We found that the lattice dilation has facilitated the band filling especially the majority spin band resulting in increasing magnetic moment. Later, we reasoned out the interaction between magnetic Fe and non-magnetic germanium atoms responsible for the observed magnetic properties of Fe2Ge. From the current electronic structure studies, the insight gained has allowed us to explain the experimentally observed magneto-resistance properties of Fe2Ge thin films.

Effect of warpage on the electronic structure and optical properties of bilayer germanene

The electronic structure and optical properties of bilayer germanene under different warpages are studied by the first-principles method of density functional theory. The effects of warpages on the electronic structure and optical properties of bilayer germanene are analyzed. The results of the electronic structure study show that the bottom of the conduction band of bilayer germanene moves to the lower energy direction with the increase of warpages at the K point, and the top of the valence band stays constant at the K point, and so the band gap decreases with the increase of warpage. When the warpage is 0.075 nm, the top of the valence band of bilayer germanene changes from K point to G point, and the bilayer germanene becomes an indirect band gap semiconductor. This is an effective means to modulate the conversion of bilayer germanene between direct band gap semiconductor and indirect band gap semiconductor by adjusting the band structure of bilayer germanene effectively. The study of optical properties shows that the effect of warpage on the optical properties of bilayer germanene is mainly distributed in the ultraviolet and visible regions, and the warpage can effectively regulate the electronic structure and optical properties of bilayer germanene. When the warpage is 0.069 nm, the first peak of dielectric function and extinction coefficient is the largest, and the energy corresponding to the absorption band edge is the smallest. Therefore, the electron utilization rate is the best when the warpage is 0.069 nm.

Nitrogen reduction reaction mechanism on Fe-doped TiO2 from theoretical perspective: A kinetic and electronic structure study

TiO2 has been recognized as a promising electrocatalyst for N2 reduction reaction (NRR), the strategies to construct oxygen vacancies and doping of iron atoms have been implemented to improve its performance. Nevertheless, the inherent NRR reaction kinetics on the modified TiO2 is rarely reported. In this study, selected key NRR pathways governing its reaction mechanisms on Fe-atom doped-TiO2 (Fe-TiO2) and oxygen vacancy TiO2 (101) surface (OV-TiO2) were systematically studied by density functional theory (DFT) calculation. It demonstrates that both models experience the same NRR reaction path (distal). Fe-TiO2 (101) delivers significantly lower energy barriers in some key reaction steps, resulting in promoted NRR kinetics compared with OV-TiO2 (101). In addition, Fe-TiO2 (101) also shows better NRR selectivity with hydrogen evolution reaction than OV-TiO2 (101). The enhanced NRR reaction on Fe-TiO2 (101) is found contributed to its unique electronic structure. That is, the doped Fe atom acts as effective electron-rich active site for efficient N2 adsorption. Meanwhile, the doped-Fe atom would affect the electron distribution of the adsorbed N2 molecule, and more electrons tend to accumulate around the outward N, which promotes the following protonation process and accelerate the sequential NRR steps. These results provide new insights on NRR mechanism on TiO2-based catalysts.

First-principles calculations to investigate structural stability, half-metallic behavior, thermophysical and thermoelectric properties of Co2YAl (Y = Mo, Tc) full Heusler compounds

The present manuscript reports structural stability, half-metallic behavior, thermophysical and thermoelectric properties of Co2MoAl and Co2TcAl full Heusler compounds using first-principle calculations. The full-potential linearized augmented plane wave method within the framework of density functional theory is implemented and WIEN2k code is incorporated in the stable Fm-3 m phase to attain the desired results. The L21 phase is found as the most stable phase for both compounds. The optimized equilibrium lattice parameters in the stable phase are 5.89 Å, 5.86 Å for Co2MoAl and Co2TcAl respectively. The electronic band structure study proves that both compounds Co2MoAl and Co2TcAl have indirect band gaps of 0.74 eV and 0.85 eV respectively in majority spin alignment. The half-metallic performance of the present set of compounds is explained by the spin-resolved density of states. The Fermi level at the Brillouin zone point approves the metallic as well as semiconducting behavior. To elucidate the thermodynamical stability of these materials against pressure and temperature by implementing the quasi-harmonic approximation of various parameters like Debye temperature, Gruneisen parameters and specific heat are studied successfully. To know the thermoelectric response, Seebeck coefficient and electrical conductivity are computed using the BoltzTrap code. The thermodynamic profile delivers that the present set of compounds possesses no anomalies with respect to the phase transition or instabilities. The computational study insights that the compounds can be synthesized experimentally and find a route towards spintronic and thermoelectric applicability.

Increasing the Distance of Adjacent Palladium Atoms for Configuration Matching in Selective Hydrogenation

AbstractPrecisely tailoring the distance between adjacent metal sites to match adsorption configurations of key species for the targeted reaction pathway is a great challenge in heterogeneous catalysis. Here, we report a proof‐of‐concept study on the atomically sites‐tailored pathway in Pd‐catalyzed acetylene hydrogenation, i.e., increasing the distance of adjacent Pd atoms (dPd‐a‐Pd) for configuration matching in acetylene semi‐hydrogenation against coupling.dPd‐a‐Pdis identified as a structural descriptor for describing the competitiveness for reaction pathways, and the increaseddPd‐a‐Pdprefers the semi‐hydrogenation pathway due to simultaneously promoted C2H4desorption and the destabilized transition state of the C2H3* coupling. Spectroscopic, kinetics and electronic structure studies reveal that increasingdPd‐a‐Pdto 3.31 Å delivers superior selectivity and stability due to energy matching and appropriate hybridization of Pd 4d with In 2s and, especially, 2p orbitals.

Revisiting Co 2p core‐level photoemission in LiCoO2 by in‐lab soft and hard X‐ray photoelectron spectroscopy: A depth‐dependent study of cobalt electronic structure

Addressing the oxidation state of functional materials such as transition metal oxides is a current critical challenge and requires new methodologies to characterize their electronic structure within surface to bulk resolution at the nanometer scale. One approach to this issue is the combination of co‐localized soft and hard X‐ray photoelectron spectroscopies for a non‐destructive depth profiling. In this work, we demonstrate the capability to characterize the oxidation state of cobalt in LiCoO2 thin films, a model electrode material in the Li‐ion battery field, within the first 15 nm. The capability of the methodology to address the surface evolution of the cobalt oxidation state is tested through a proof of concept surface modification introduced by Ar‐ion sputtering. To address the cobalt local electronic structure at different depths from the extreme surface, we exploited Auger‐free Hard X‐ray photoelectron spectroscopy (HAXPES) spectra to fit the Co 2p core‐level features in the X‐ray photoelectron spectroscopy (XPS) spectra. This approach may pave the way for a better understanding of the surface electronic structure changes in transition metal oxides driven by their applications in a broad range of technologies.