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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Topological Electronic Structure Evolution with Symmetry Breaking Spin Reorientation in (Fe 1–x Co x )Sn

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The (Fe 1–x Co x )Sn system not only hosts a kagome lattice but has a tunable symmetry-breaking magnetic moment with temperature and doping. In this study, angle-resolved photoemission spectroscopy and first-principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. Here, evidence for a theoretically predicted gap at the Dirac point is revealed in the low-temperature axial phase, but no gap opening is observed across a temperature-dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature-dependent bulk gap, but this highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electronic structure factors and the importance of adsorbate effects in chemisorption on surface alloys

The chemisorption energy is an integral aspect of surface chemistry, central to numerous fields such as catalysis, corrosion, and nanotechnology. Electronic-structure-based methods such as the Newns-Anderson model are therefore of great importance in guiding the engineering of material surfaces with optimal properties. However, existing methods are inadequate for interpreting complex, multi-metallic systems. Herein, we introduce a physics-based chemisorption model for alloyed transition metal surfaces employing primarily metal d-band properties that accounts for perturbations in both the substrate and adsorbate electronic states upon interaction. Importantly, we show that adsorbate-induced changes in the adsorption site interact with its chemical environment leading to a second-order response in chemisorption energy with the d-filling of the neighboring atoms. We demonstrate the robustness of the model on a wide range of transition metal alloys with O, N, CH, and Li adsorbates yielding a mean absolute error of 0.13 eV versus density functional theory reference chemisorption energies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing the interplay between geometric and electronic-structure features via high-harmonic spectroscopy

We present molecular-frame measurements of the recombination dipole matrix element (RDME) in CO 2 , N 2 O, and carbonyl sulfide (OCS) molecules using high-harmonic spectroscopy. Both the amplitudes and phases of the RDMEs exhibit clear imprints of a two-center interference minimum, which moves in energy with the molecular alignment angle relative to the laser polarization. Here, we find that whereas the angle dependence of this minimum is consistent with the molecular geometry in CO 2 and N 2 O, it behaves very differently in OCS; in particular, the phase shift which accompanies the two-center minimum changes sign for different alignment angles. Our results suggest that two interfering structural features contribute to the OCS RDME, namely, (i) the geometrical two-center minimum and (ii) a Cooper-like, electronic-structure minimum associated with the sulfur end of the molecule. We compare our results to ab initio calculations using time-dependent density functional theory and present an empirical model that captures both the two-center and the Cooper-like interferences. We also show that the yield from unaligned samples of two-center molecules is, in general, reduced at high photon energies compared to aligned samples, due to the destructive interference between molecules with different alignments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Electronic structure and charge-density wave transition in monolayer $\mathrm{VS}_2$

Vanadium disulfide (VS 2 ) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS 2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Here, our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Beyond real: alternative unitary cluster Jastrow models for molecular electronic structure calculations on near-term quantum computers

Near-term quantum devices require wavefunction ansätze that are expressive while also of shallow circuit depth in order to both accurately and efficiently simulate molecular electronic structure. While the unitary coupled cluster ansatz (e.g., UCCSD) has become a standard, the high gate count associated with the implementation of this limits its feasibility on noisy intermediate-scale quantum (NISQ) hardware. k -Fold unitary cluster Jastrow (uCJ) ansätze mitigate this challenge by providing O( kN 2 ) circuit scaling and favorable linear depth circuit implementation. Previous work has focused on the real orbitalrotation (Re-uCJ) variant of uCJ, which allows an exact (Trotter-free) implementation. Here we extend and generalize the k -fold uCJ framework by introducing two new variants, Im-uCJ and g-uCJ, which incorporate imaginary and fully complex orbital rotation operators, respectively. Similar to Re-uCJ, both of the new variants achieve quadratic gate-count scaling. Our results focus on the simplest k = 1 model, and show that the uCJ models frequently maintain energy errors within chemical accuracy (∼1 kcal mol −1 ). Both g-uCJ and Im-uCJ are more expressive in terms of capturing electron correlation and are also more accurate than the earlier Re-uCJ ansatz. We further show that Im-uCJ and g-uCJ circuits can also be implemented exactly, without any Trotter decomposition. Numerical tests using k = 1 on H 2 , H 3 + , Be 2 , C 2 H 4 , C 2 H 6 and C 6 H 6 in various basis sets confirm the practical feasibility of these shallow Jastrow-based ansätze for applications on near-term quantum hardware.

Tkachenko, Nikolay V. [University of California, B↗
DOE OSTI2022

Correlating Crystallography, Magnetism, and Electronic Structure Across Anhysteretic First-Order Phase Transition in Pr 2 In

Here, temperature-dependent powder X-ray diffraction and magnetization measurements of Pr 2 In conclusively prove that the unusual anhysteretic first-order paramagnetic-ferromagnetic phase transition in the compound is related to concurrent changes in both the magnetic and crystallographic lattices. At the same time, the hexagonal Ni 2 In-type structure is stable at least between 6 and 298 K, including at T C = ~57 K. From the density functional theory calculations, the electronic structure of the compound is extraordinarily sensitive to minor changes in lattice parameters that occur across the phase transition, revealing the origin of strong magnetoelastic coupling. In the vicinity of T C , the maximum entropy change, ΔS Max = -16 J Kg -1 K -1 induced by a moderate magnetic field change of 20 kOe (ΔS Max = -20 J Kg -1 K -1 for 50 kOe magnetic field change) is comparable to other known potentially functional materials demonstrating large cryogenic magnetocaloric effect.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Structural, electronic and magnetic properties of the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions: Spin polarized first principles total energy calculations

In this work, using first principles total energy calculations within the periodic spin polarized density functional theory, we have investigated the structural, electronic, and magnetic properties of manganese gallium (MnGa) alloys. Specifically, we explore the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions. The surface formation energies reveal that selected substitutions occur under Mn (Ga) rich growth conditions. Structures with top layers missing all Mn (Ga) atoms and two layers deep substitutions are also investigated. However, the formation energy shows that these structures are less favorable. For the stable structures, the magnetic properties per layer are proportional to the Mn:Ga ratio. Also, the density of states shows that the MnGa surfaces are metallic. The projected density of states shows that the electronic states in the vicinity of the Fermi level are due mainly to the manganese 3d orbitals. However charge density plots reveal that Mn 3d electrons are closer to the nucleus than Ga sp electrons. Consequently, experimental scanning tunneling microscopy images reveal periodically-arranged bright features, corresponding to the Ga atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Size Effects on the Electronic Structure of Hexagonal Gallium Telluride

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quaternary Pavonites A 1+ x S 2– x Bi 5+ x S 10 (A + = Li + , Na + ): Site Occupancy Disorder Defines Electronic Structure

The field of mineralogy represents an area of untapped potential for the synthetic chemist, as there are numerous structure types that can be utilized to form analogues of mineral structures with useful optoelectronic properties. In this work, we describe the synthesis and characterization of two novel quaternary sulfides A 1+x Sn 2–x Bi 5+x S 10 (A = Li + , Na + ). Though not natural minerals themselves, both compounds adopt the pavonite structure, which crystallizes in the C2/m space group and consists of two connected, alternating defect rock-salt slabs of varying thicknesses to create a three-dimensional lattice. Despite their commonalities in structure, their crystallography is noticeably different, as both structures have a heavy degree of site occupancy disorder that affects the actual positions of the atoms. The differences in site occupancy alter their electronic structures, with band gap values of 0.31(2) eV and 0.07(2) eV for the lithium and sodium analogues, respectively. LiSn 2 Bi 5 S 10 exhibits ultralow thermal conductivity of 0.62 W m –1 K –1 at 723 K, and this result is corroborated by phonon dispersion calculations. Here, this structure type is a promising host candidate for future thermoelectric materials investigation, as these materials have narrow band gaps and intrinsically low thermal conductivities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Structural, electronic, and optical-absorption properties of 2D Si thin films

Recent experimental studies highlighted the potential of thin-film crystalline silicon (Si) for high-efficiency solar cells. Using density functional theory, we investigated 2D Si thin films across various orientations, thicknesses, and surface structures to elucidate their structure–property relationships. Through surface-energy calculations and Wulff construction, we determined the crystal habit of Si, which aligns with available experimental observations. Electronic-structure calculations underscored the critical role of valence saturation on surfaces in enabling semiconducting behavior in Si thin films, essential for optical applications. From optical-absorption calculations, we identified the surface index exhibiting the highest absorption coefficients for thin films Si solar cell applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The First-Principle Study of the Electronic Structure, Ferromagnetic and Thermoelectric Properties of Spinel Alloy FeAl{sub 2}O{sub 4} Using mBJ Functional Approach

In this research work, the modified Becke and Johnson potential has been used to realize the actual electronic states and band gap properties of FeAl{sub 2}O{sub 4}. Ferromagnetism is illustrated in terms of John-Teller energy and exchange energies involved in the system due to electron spin. Moreover, the exchange constants also support the ferromagnetism in the studied compound. The reduction of the magnetic moment of Fe from its free space value and the shifting of the magnetic moment to nonmagnetic sites are the consequence of strong magnetization. In the end, the electrical conductivity and Seebeck coefficient incorporated to give a high value of the power factor and thermal efficiency for energy renewable devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electronic structure calculations of twisted multi-layer graphene superlattices

Quantum confinement endows two-dimensional (2D) layered materials with exceptional physics and novel properties compared to their bulk counterparts. Although certain two- and few-layer configurations of graphene have been realized and studied, a systematic investigation of the properties of arbitrarily layered graphene assemblies is still lacking. We introduce theoretical concepts and methods for the processing of materials information, and as a case study, apply them to investigate the electronic structure of multi-layer graphene-based assemblies in a high-throughput fashion. We provide a critical discussion of patterns and trends in tight binding band structures and we identify specific layered assemblies using low-dispersion electronic bands as indicators of potentially interesting physics like strongly correlated behavior. A combination of data-driven models for visualization and prediction is used to intelligently explore the materials space. This work more generally aims to increase confidence in the combined use of physics-based and data-driven modeling for the systematic refinement of knowledge about 2D layered materials, with implications for the development of novel quantum devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Impurity effects on local electronic structure of twisted bilayer cuprates

It has been recently proposed that a fully gapped d-wave superconducting phase emerges in twisted bilayer cuprates when twist angle is in the vicinity of 45°. By considering a twisted bilayer model with a moiré cell, we study the local electronic structure around a single-site impurity and impurity dimer in both topologically trivial and nontrivial superconducting states. Here we find that the topological nontrivial superconducting phase is robust against single scalar-potential impurity scattering but is very sensitive to the interlayer impurity dimer scattering, which avoids and enforces the creation of the subgap quasiparticle state. This behavior is intrinsic to the bilayer structure and topological nature, in contrast to the other types of impurities that inevitably bring subgap quasiparticle states. We expect that our results can contribute to the understanding of electronic properties of twisted bilayer cuprates systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural, Electronic, Magnetic, and Optical Properties of Half-Heusler Alloys RuMnZ (Z = P, As): a First-Principle Study

Half-Heusler alloys RuMnZ (Z = P, As ) are studied in the framework of Density Functional Theory (DFT). Structural, electronic, magnetic, and optical properties are calculated and analyzed using the WIEN2k simulation code. All the calculations are done using the full potential linearized augmented plane wave (FP-LAPW) method. Equilibrium lattice parameters are found to be in the range 5.5–5.6 Å. Band gaps of the compounds and density of states (DoS) analysis reveal that the minority spin-down states are semi-conducting while the majority spin-up states are conducting confirming the half-metallic nature of the compounds. Hence, at Fermi level, states are 100 % polarized. The value of the total magnetic moment is found to be 2, i.e., M{sub Tot} = 2μ{sub B}. Several optical properties, including dielectric function, reflectivity, refractive index, conductivity, and absorption coefficient are calculated as well. It is revealed from the imaginary part of the dielectric function that the compounds are optically metallic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Electronic structures and pseudogap nature of the strongly correlated Kondo semimetals and insulators CeNiSn and CeRhX(X=As, Sb)

Employing temperature (T)-dependent angle-resolved photoemission spectroscopy (ARPES) near the Ce 4d absorption edge, we have investigated the electronic structures of the strongly correlated Kondo insulators (KIs) and/or Kondo semimetals of CeNiSn and CeRhX (X=As, Sb), which also belong to potential topological KIs (TKIs). Good agreement is found between the Fermi surfaces (FSs) measured at the Ce 4d→4f on resonance and those calculated and unfolded into the Ce-only Brillouin zone, supporting the crucial Ce 4f contribution to the Fermi-edge states in them. While the FSs of CeRhSb and CeNiSn are similar to each other, they are quite different from those of CeRhAs, which is understood to originate from the semimetallic Kondo ground states of CeNiSn and CeRhSb in contrast to the insulating Kondo ground state of CeRhAs. T-dependent ARPES demonstrates the existence of the Kondo resonance above the Fermi level (EF) and its T-driven decoherence in CeNiSn and CeRhSb, and the existence of the Ce 4f Kondo-like peak well below EF with a pseudogap in CeRhAs. With increasing T, the tail of the Kondo resonance in CeNiSn and CeRhSb loses its coherence, whereas the Ce 4f Kondo-like peak in CeRhAs persists up to T≥200 K. The Kondo temperatures (TKs) are estimated via the analysis of T-dependent ARPES, yielding TK≈80 K (CeNiSn), TK≈180 K (CeRhSb), and TK≫200 K (CeRhAs). High-resolution, low-T ARPES confirms the Kondo resonance just above EF in CeNiSn and CeRhSb Kondo semimetals and the KI ground state of CeRhAs with a pseudogap of Δ≈30 meV, demonstrating the importance of the coherent Kondo states in determining their potential topological properties. The differences in the T-dependent ARPES of CeNiSn and CeRhSb suggest the weaker f-c hybridization in CeNiSn than in CeRhSb (c: conduction electron), which is likely due to the more localized Ni 3d orbitals than the delocalized Rh 4d orbitals. The differences between the T-dependent ARPES of CeNiSn/CeRhSb Kondo semimetals and those of CeRhAs Kondo insulator are likely due to the smaller volume of CeRhAs than those of CeNiSn/CeRhSb.

Kang, J-S↗
DOE OSTI2019

Catalysis-Hub.org, an open electronic structure database for surface reactions

Abstract We present a new open repository for chemical reactions on catalytic surfaces, available at https://www.catalysis-hub.org . The featured database for surface reactions contains more than 100,000 chemisorption and reaction energies obtained from electronic structure calculations, and is continuously being updated with new datasets. In addition to providing quantum-mechanical results for a broad range of reactions and surfaces from different publications, the database features a systematic, large-scale study of chemical adsorption and hydrogenation on bimetallic alloy surfaces. The database contains reaction specific information, such as the surface composition and reaction energy for each reaction, as well as the surface geometries and calculational parameters, essential for data reproducibility. By providing direct access via the web-interface as well as a Python API, we seek to accelerate the discovery of catalytic materials for sustainable energy applications by enabling researchers to efficiently use the data as a basis for new calculations and model generation.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Robust Kagome Electronic Structure in Topological Quantum Magnets XMn 6 Sn 6 (X = Dy, Tb, Gd, Y)

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of XMn 6 Sn 6 (X=Dy,Tb,Gd,Y) family compounds. Here, we observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and 4f electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the XMn 6 Sn 6 family, thus providing an ideal platform for the search for, and investigation of, new emergent phenomena in magnetic topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structured electron beams from nano-engineered cathodes

The ability to engineer cathodes at the nano-scale have open new possibilities such as enhancing quantum eciency via surface-plasmon excitation, forming ultra-low-emittance beams, or producing structured electron beams. In this paper we present numerical investigations of the beam dynamics associated to this class of cathode in the weak- and strong-field regimes.We finally discuss the possible applications of some of the achievable cathode patterns when coupled with other phase space manipulations.

43 PARTICLE ACCELERATORS↗