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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Unusual nodal behaviors of the superconducting gap in the iron-based superconductor Ba ( F e 0.65 R u 0.35 ) 2 A s 2 : Effects of spin-orbit coupling

Here we have investigated the superconducting (SC) gap on hole Fermi surfaces (FSs) of optimally substituted Ba (Fe 0.65 Ru 0.35 ) 2 As 2 by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that, whereas the gap is isotropic in the k x - k y plane, the gap magnitudes of two resolved hole FSs show similar k z dependences and decrease as k z approaches ~ 2 π/c (i.e., around the Z point), unlike the other Fe-based superconductors reported so far, where the SC gap of only one hole FS shows a strong k z dependence. This unique gap structure can be understood in the scenario that the d z ₂ orbital character is mixed into both hole FSs due to finite spin-orbit coupling (SOC) and is reproduced by calculation within the random phase approximation including the SOC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Effects of partial La filling and Sb vacancy defects on CoS b 3 skutterudites

Over the past decade, the open frame ("cagey") structure of CoSb 3 skutterudite has invited intensive filling studies with various rare-earth elements for delivering state-of-the-art mid-temperature thermoelectric performance. In order to rationalize previously reported experimental results and provide new insight into the underexplored roles of La fillers and Sb vacancies, ab initio density functional theory studies, along with semi-classical Boltzmann transport theory calculations, are performed for pristine CoSb 3 of different lattice settings and La-filled CoSb 3 with and without Sb s mono- and di-vacancy defects. We examine the effects of van der Waals (vdW) interactions, spin-orbit coupling (SOC), spin polarization, partial La-filling, and Sb vacancy defects on the structural, electronic, and thermoelectric properties. The vdW interactions profoundly affect the lattice constant, which in turn affects the band gap. The SOC shows minor effects on the electronic and thermoelectric properties. The peculiar quasi-Dirac band in the pristine CoSb 3 largely survives La filling but not Sb vacancies, which instead introduce dispersive bands in the band gap region. Importantly, the band structure, density of states, and Fermi surface of La-filled CoSb 3 are significantly spin polarized, giving rise to spin-dependent thermoelectric properties. Seebeck coefficients directly calculated as a function of chemical potential are interpreted in connection with the electronic structures. Temperature-dependent Seebeck coefficients derived for the experimentally studied materials agree well with available experimental data. Seebeck coefficients obtained as a function of charge carrier concentration corroborate a thermoelectrically favorable role at high filling fractions played by the electron/hole pockets on the Fermi surface associated with the degenerate valleys/hills in the conduction/valence bands, respectively. Our results serve to advance the understanding of CoSb 3 skutterudite, a class of materials with important fundamental and application implications for thermoelectrics and spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Topological superconductor in quasi-one-dimensional Tl 2 - x Mo 6 Se 6

Here we propose that the quasi-one-dimensional molybdenum selenide compound Tl 2-x Mo 6 Se 6 is a time-reversal-invariant topological superconductor induced by intersublattice pairing, even in the absence of spin-orbit coupling (SOC). No noticeable change in superconductivity is observed in Tl-deficient (0 ≤ x ≤ 0.1) compounds. At weak SOC, the superconductor prefers the triplet d vector lying perpendicular to the chain direction and two-dimensional E 2u symmetry, which is driven to a nematic order by spontaneous rotation symmetry breaking. The locking energy of the d vector is estimated to be weak and hence the proof of its direction would rely on tunneling or phase-sensitive measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Absence of Dirac states in BaZnBi 2 induced by spin-orbit coupling

We report magnetotransport properties of BaZnBi 2 single crystals. Whereas electronic structure features Dirac states, such states are removed from the Fermi level by spin-orbit coupling (SOC) and consequently electronic transport is dominated by the small hole and electron pockets. Our results are consistent with not only three-dimensional, but also with quasi-two-dimensional portions of the Fermi surface. The SOC-induced gap in Dirac states is much larger when compared to isostructural SrMnBi 2 . This suggests that not only long-range magnetic order, but also mass of the alkaline-earth atoms A in ABX 2 ( A = alkaline-earth, B = transition-metal, and X = Bi/Sb) are important for the presence of low-energy states obeying the relativistic Dirac equation at the Fermi surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Saddle-like topological surface states on the T T ' X family of compounds ( T , T ' = Transition metal, X = Si , Ge)

Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting nodal lines in their bulk electronic spectrum and nontrivial drumhead states on the surface. Based on first-principles calculations and an effective model analysis, we can identify the presence of topological nodal-line semimetal states in the low crystalline symmetric TT'X family of compounds (T,T' = transition metal, X = Si or Ge) in the absence of spin-orbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to small lattice symmetry this material harbors a single nodal line on the k y = 0 plane with large energy dispersion and unique drumhead surface state with a saddlelike energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z 2 = (1;000). The topological surface state evolves from the drumhead surface state via the sharing of its saddlelike energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as Bi 2 Se 3 with Dirac-cone-like energy dispersions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spin-orbit coupling driven crossover from a starfruitlike nodal semimetal to Dirac and Weyl semimetal state in CaAuAs

Band crossings that occur on a mirror plane are compelled to form nodal loops in the momentum space in the absence of spin-orbit coupling (SOC). When other equivalent mirror planes are present, multiple such nodal loops can combine to form interesting topological structures involving crossed nodal lines. Here, based on first-principles calculations and an effective k ⋅ p model analysis, we show that CaAuAs hosts a unique starfruitlike crossed-nodal-line structure in the bulk electronic dispersion, which is comprised of three nodal loops that cross each other at the time-reversal-invariant momentum point A. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the Γ−A direction in the Brillouin zone. These Dirac points are protected by the combination of time-reversal, inversion, and C 3 rotation symmetries. Here we discuss how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Nature of the insulating ground state of the 5d postperovskite CaIrO 3

In this study, the insulating ground state of the 5d transition metal oxide CaIrO 3 has been classified as a Mott-type insulator. Based on a systematic density functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals, we reveal that the Ir t 2 g states exhibit large splittings and one-dimensional electronic states along the c axis due to a tetragonal crystal field. Our hybrid DFT calculation adequately describes the antiferromagnetic (AFM) order along the c direction via a superexchange interaction between Ir 4+ spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t 2 g states to open an insulating gap. These results indicate that CaIrO 3 can be represented as a spin-orbit Slater insulator, driven by the interplay between a long-range AFM order and the SOC. Such a Slater mechanism for the gap formation is also demonstrated by the DFT + dynamical mean field theory calculation, where the metal-insulator transition and the paramagnetic to AFM phase transition are concomitant with each other.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Valley-Polarized Quantum Anomalous Hall Effect in Ferrimagnetic Honeycomb Lattices

The valley-polarized quantum anomalous Hall effect (VP-QAHE), which combines valleytronics and topology in one material, is of significant fundamental and practical importance in condensed-matter physics and materials science. In previous model studies, VP-QAHE occurs under strong Rashba spin-orbit coupling (SOC), which is an extrinsic effect. In this report, using a low energy k • p model, we propose a different mechanism of VP-QAHE by introducing an intrinsic staggered magnetic exchange field and develop a general picture of valley dependent band inversion in honeycomb lattice. Using first-principles calculation, this new mechanism is further demonstrated in the Co decorated In-triangle adlayer on a Si(111) surface. This system is equivalent to a ferrimagnetic honeycomb lattice, and the supported adlayer is experimentally more feasible in synthesis, thus exhibiting advantages over the existing studies based on Rashba SOC and free-standing sheets. The underlying physical mechanism is generally applicable, opening a new avenue for exploration of substrate supported VP-QAHE.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Noncollinear Magnetic Structure and Multipolar Order in Eu 2 Ir 2 O 7

The magnetic properties of the pyrochlore iridate material Eu 2 Ir 2 O 7 (5d 5 ) have been studied based on first principles calculations, where the crystal field splitting Δ , spin-orbit coupling (SOC) λ, and Coulomb interaction U within Ir 5 d orbitals all play significant roles. The ground state phase diagram has been obtained with respect to the strength of SOC and Coulomb interaction U , where a stable antiferromagnetic ground state with all-in–all-out (AIAO) spin structure has been found. In addition, another antiferromagnetic state with energy close to AIAO has also been found to be stable. The calculated nonlinear magnetization of the two stable states both have the d -wave pattern but with a π/4 phase difference, which can perfectly explain the experimentally observed nonlinear magnetization pattern. Finally, compared with the results of the nondistorted structure, it turns out that the trigonal lattice distortion is crucial for stabilizing the AIAO state in Eu 2 Ir 2 O 7 . Furthermore, besides large dipolar moments, we also find considerable octupolar moments in the magnetic states.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Topological Hourglass Dirac Semimetal in the Nonpolar Phase of Ag 2 BiO 3

Materials with tunable charge and lattice degrees of freedom provide excellent platforms for investigating multiple phases that can be controlled via external stimuli. We show how the charge-ordered ferroelectric oxide Ag 2 BiO 3 , which has been realized experimentally, presents a unique exemplar of a metal-insulator transition under an external electric field. Our first-principles calculations, combined with a symmetry analysis, reveal the presence of a nearly ideal hourglass-Dirac-semimetal state in the nonpolar structure of Ag 2 BiO 3 . The low-energy band structure consists of two hourglasslike nodal lines located on two mutually orthogonal glide-mirror planes in the absence of spin-orbit coupling (SOC) effects. These lines cross at a common point and form an interlinked chainlike structure, which extends beyond the first Brillouin zone. Inclusion of the SOC opens a small gap in the nodal lines and results in two symmetry-enforced hourglasslike Dirac points on the $\tilde{C}$ 2y screw rotation axis. Our results indicate that Ag 2 BiO 3 will provide an ideal platform for exploring the ferroelectric-semiconductor to Dirac-semimetal transition by the application of an external electric field.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Planar Hall Effect in Antiferromagnetic MnTe Thin Films

We show that the spin-orbit coupling (SOC) in α-MnTe impacts the transport behavior by generating an anisotropic valence-band splitting, resulting in four spin-polarized pockets near Γ. A minimal k·p model is constructed to capture this splitting by group theory analysis, a tight-binding model, and ab initio calculations. The model is shown to describe the rotation symmetry of the zero-field planer Hall effect (PHE). The PHE originates from the band anisotropy given by SOC, and is quantitatively estimated to be 25%–31% for an ideal thin film with a single antiferromagnetic domain.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Preferred Spin Excitations in the Bilayer Iron-Based Superconductor $CaK(Fe_{0.96}Ni_{0.04})_4As_4$ with Spin-Vortex Crystal Order

Spin-orbit coupling (SOC) is a key to understand the magnetically driven superconductivity in iron-based superconductors, where both local and itinerant electrons are present and the orbital angular momentum is not completely quenched. Here, we report a neutron scattering study on the bilayer compound CaK(Fe 0.96 Ni 0.04 ) 4 As 4 with superconductivity coexisting with a noncollinear spin-vortex crystal magnetic order that preserves the tetragonal symmetry of the Fe-Fe plane. In the superconducting state, two spin resonance modes with odd and even L symmetries due to the bilayer coupling are found similar to the undoped compound CaKFe 4 As 4 but at lower energies. Polarization analysis reveals that the odd mode is c-axis polarized, and the low-energy spin anisotropy can persist to the paramagnetic phase at high temperature, which closely resembles other systems with in-plane collinear and c-axis biaxial magnetic orders. Overall, these results provide the missing piece of the puzzle on the SOC effect in iron-pnictide superconductors, and also establish a common picture of c-axis preferred magnetic excitations below T c regardless of the details of magnetic pattern or lattice symmetry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Prediction of threefold fermions in a nearly ideal Dirac semimetal BaAgAs

Materials with triply degenerate nodal points in their low-energy electronic spectrum produce crystalline-symmetry-enforced threefold fermions, which conceptually lie between the twofold Weyl and fourfold Dirac fermions. Here, we show how a silver-based Dirac semimetal BaAgAs realizes threefold fermions through our first-principles calculations combined with a low-energy effective k · p model Hamiltonian analysis. In this work, BaAgAs is shown to harbor triply degenerate nodal points, which lie on its C 3 rotation axis, and are protected by the C 6v (C 2 $\otimes$ C 3v ) point-group symmetry in the absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs transitions into a nearly ideal Dirac semimetal state with a pair of Dirac nodes lying on the C 3 rotation axis. We show that breaking inversion symmetry in the BaAgAs 1–x P x alloy yields a clean and tunable threefold fermion semimetal. Systematic relaxation of other symmetries in BaAgAs generates a series of other topological phases. BaAgAs materials thus provide another platform for exploring tunable topological properties associated with a variety of different fermionic excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Towards predictive band gaps for halide perovskites: Lessons from one-shot and eigenvalue self-consistent GW

Halide perovskites constitute a chemically diverse class of crystals with great promise as photovoltaic absorber materials, featuring band gaps between about 1 and 3.5 eV depending on composition. Their diversity calls for a general computational approach to predicting their band gaps. However, such an approach is still lacking. Here, we use density functional theory (DFT) and ab initio many-body perturbation theory within the GW approximation to compute the quasiparticle or fundamental band gap of a set of ten representative halide perovskites: CH 3 NH 3 PbI 3 ( MAPbI 3 ), MAPbBr 3 , CsSnBr 3 , ( MA ) 2 BiTlBr 6 , Cs 2 TlAgBr 6 , Cs 2 TlAgCl 6 , Cs 2 BiAgBr 6 , Cs 2 InAgCl 6 , Cs 2 SnBr 6 , and Cs 2 Au 2 I 6 . Comparing with recent measurements, we find that a standard generalized gradient exchange-correlation functional can significantly underestimate the experimental band gaps of these perovskites, particularly in cases with strong spin-orbit coupling (SOC) and highly dispersive band edges, to a degree that varies with composition. We show that these nonsystematic errors are inherited by one-shot G 0 W 0 and eigenvalue self-consistent GW 0 calculations, demonstrating that semilocal DFT starting points are insufficient for MAPbI 3 , MAPbBr 3 , CsSnBr 3 , ( MA ) 2 BiTlBr 6 , Cs 2 TlAgBr 6 , and Cs 2 TlAgCl 6 . On the other hand, we find that DFT with hybrid functionals leads to an improved starting point and G W 0 results in better agreement with experiment for these perovskites. Our results suggest that G W 0 with hybrid functional-based starting points are promising for predicting band gaps of systems with large SOC and dispersive bands in this technologically important class of semiconducting crystals.</

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spin-orbit coupling controlled ground states in the double perovskite iridates $\textit{A}_2\textit{B}$IrO$_6$ ($\textit{A}$ = Ba, Sr; $\textit{B}$ = Lu, Sc)

Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $\textit{J}$ = 0 electronic ground state for each Ir 5+ ion. The major focus of prior research has been on the double perovskite iridates Ba 2 YIrO 6 and Sr 2 YIrO 6 , where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, in this work, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba 2 LuIrO 6 , Sr 2 LuIrO 6 , Ba 2 ScIrO 6 , and Sr 2 ScIrO 6 that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $\textit{J}$ = 0 electronic ground states for the Ir 5+ ions in all cases, with similar values for Hund's coupling $J_H$ and the spin-orbit coupling constant $λ_{\text{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large $λ_{\text{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5d^4$ double perovskite iridates, which agrees well with conventional wisdom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Odd-Parity Magnetism Driven by Antiferromagnetic Exchange

Realizing odd-parity, time-reversal-preserving, nonrelativistic spin splitting is a central goal for spintronics applications. We propose a group-theory-based microscopic framework to induce odd-parity spin splitting from coplanar antiferromagnetic (AFM) states without spin-orbit coupling (SOC). We develop phenomenological models for 421 conventional period-doubling AFM systems in nonsymmorphic space groups and construct minimal microscopic models for 119 of these. We find that these AFM states can attain three possible competing ground states. These ground states all break symmetries in addition to those broken by the usual AFM order. Specifically, they give rise to either odd-parity spin-splitting, nematic order, or scalar odd-parity order related to multiferroicity. Our microscopic theories reveal that the odd-parity spin-splitting energy scale is generically large and further reveal that the scalar odd-parity order gives a nonzero Berry curvature dipole without SOC. We identify 67 materials in the Magndata database for which our theory applies. We provide density-functional theory (DFT) calculations on Fe-based materials that reveal an ℎ-wave spin splitting consistent with our symmetry arguments and apply our microscopic model to determine the nonrelativistic Edelstein response for CeNiAsO.

Antiferromagnets↗
DOE OSTI2016

Optimal Battery Utilization Over Lifetime for Parallel Hybrid Electric Vehicle to Maximize Fuel Economy

This paper presents a control strategy to maximize fuel economy of a parallel hybrid electric vehicle over a target life of the battery. Many approaches to maximizing fuel economy of parallel hybrid electric vehicle do not consider the effect of control strategy on the life of the battery. This leads to an oversized and underutilized battery. There is a trade-off between how aggressively to use and 'consume' the battery versus to use the engine and consume fuel. The proposed approach addresses this trade-off by exploiting the differences in the fast dynamics of vehicle power management and slow dynamics of battery aging. The control strategy is separated into two parts, (1) Predictive Battery Management (PBM), and (2) Predictive Power Management (PPM). PBM is the higher level control with slow update rate, e.g. once per month, responsible for generating optimal set points for PPM. The considered set points in this paper are the battery power limits and State Of Charge (SOC). The problem of finding the optimal set points over the target battery life that minimize engine fuel consumption is solved using dynamic programming. PPM is the lower level control with high update rate, e.g. a second, responsible for generating the optimal HEV energy management controls and is implemented using model predictive control approach. The PPM objective is to find the engine and battery power commands to achieve the best fuel economy given the battery power and SOC constraints imposed by PBM. Simulation results with a medium duty commercial hybrid electric vehicle and the proposed two-level hierarchical control strategy show that the HEV fuel economy is maximized while meeting a specified target battery life. On the other hand, the optimal unconstrained control strategy achieves marginally higher fuel economy, but fails to meet the target battery life.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2023

Design of a Multi-Chemistry Battery Pack System for Behind-the-Meter Storage Applications

Battery management systems (BMS) are essential for a battery pack's safe operation and longevity. This paper presents an active balancing method-based BMS for different cell chemistry structures to be used in behind-the-meter storage (BTMS) applications. The proposed system utilizes modular isolated dual active bridge (DAB) DC/DC converters to actively balance the battery pack through a low voltage (LV) bus. A supervisory controller monitors all the cell voltage, current, and state of charge (SOC) values. Based on the estimation of the SOCs, reference currents for the DAB converters are generated by the supervisory controller. Detailed modeling and the control approach of the modular DAB converters are presented in the paper. Moreover, the control strategy of the supervisory control is also analyzed. The proposed method and structure can be extended to any combination of the number of cells to design the battery pack. Simulation results are provided for a system consisting of three cells in parallel to form a cell block and three cell blocks in series to form the battery module. Experimental results are provided for three modular DAB converters operating with a LiFeMnPO4 prismatic cell with 3.2V, 20Ah rated values.

battery management systems↗