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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 OSTI2020

Spin-Orbit-Induced Topological Flat Bands in Line and Split Graphs of Bipartite Lattices

Topological flat bands, such as the band in twisted bilayer graphene, are becoming a promising platform to study topics such as correlation physics, superconductivity, and transport. In this Letter, we introduce a generic approach to construct two-dimensional (2D) topological quasiflat bands from line graphs and split graphs of bipartite lattices. A line graph or split graph of a bipartite lattice exhibits a set of flat bands and a set of dispersive bands. The flat band connects to the dispersive bands through a degenerate state at some momentum. We find that, with spin-orbit coupling (SOC), the flat band becomes quasiflat and gapped from the dispersive bands. By studying a series of specific line graphs and split graphs of bipartite lattices, we find that (i) if the flat band (without SOC) has inversion or C 2 symmetry and is nondegenerate, then the resulting quasiflat band must be topologically nontrivial, and (ii) if the flat band (without SOC) is degenerate, then there exists a SOC potential such that the resulting quasiflat band is topologically nontrivial. This generic mechanism serves as a paradigm for finding topological quasiflat bands in 2D crystalline materials and metamaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Topological electronic structure of YbMg 2 Bi 2 and CaMg 2 Bi 2

Zintl compounds have been extensively studied for their outstanding thermoelectric properties, but their electronic structure remains largely unexplored. Here, we present a detailed investigation of the electronic structure of the isostructural thermopower materials YbMg 2 Bi 2 and CaMg 2 Bi 2 using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The ARPES results show a significantly smaller Fermi surface and Fermi velocity in CaMg 2 Bi 2 than in YbMg 2 Bi 2 . Our ARPES results also reveal that in the case of YbMg 2 Bi 2 , Yb- 4 f states reside well below the Fermi level and likely have a negligible impact on transport properties. To properly model the position of 4f-states, as well as the overall electronic structure, a Hubbard U at the Yb sites and spin-orbit coupling (SOC) have to be included in the DFT calculations. The theoretical results reveal that both materials belong to a Z 2 topological class and host topological surface states around EF. Due to the intrinsic hole doping, the topological states reside above the Fermi level, inaccessible by ARPES. Our results also suggest that in addition to SOC, vacancies and the resulting hole doping play an important role in the transport properties of these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Kramers nodal lines and Weyl fermions in SmAlSi

Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Laser-induced-fluorescence imaging of a spin-polarized ultracold neutral plasma in a magnetic field

In this work, we report Doppler-sensitive laser-induced-fluorescence (LIF) imaging of an ultracold neutral plasma in a magnetic field. Local values of ion density, hydrodynamic fluid velocity, temperature, and spin polarization are obtained using a fluorescence model based on velocity-resolved rate equations (REs) including the transfer of ions between states due to laser coupling and spontaneous emission. The RE approach captures optical pumping of ions into states that are not driven by the LIF excitation laser, and this is validated with experimental data. Combined molecular-dynamics and quantum-trajectories simulations verify that velocity-changing collisions have a negligible impact on the state population evolution for typical experimental conditions. Relative intensities of Zeeman components of the LIF spectra provide clear evidence that the ions are electron-spin-polarized when created by photoionization of magnetically trapped 88 Sr atoms. This probe opens many possibilities for studying thermal transport and the equilibration of neutral plasmas in overlapping regimes of strong coupling and magnetization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Discrete quantum geometry and intrinsic spin Hall effect

We show that the quantum geometry of the Fermi surface can be numerically described by a three-dimensional discrete quantum manifold. This approach not only avoids singularities in the Fermi sea, but it also enables the precise computation of the intrinsic Hall conductivity resolved in spin, as well as any other local properties of the Fermi surface. The method assures numerical accuracy when the Fermi level is arbitrarily close to singularities, and it remains robust when Kramers degeneracy is protected by symmetry. Furthermore, the approach is demonstrated by calculating the anomalous Hall and spin Hall conductivities of a two-band lattice model of a Weyl semimetal and a full-band ab initio model of zinc-blende GaAs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic skyrmion diode: Unidirectional skyrmion motion via symmetry breaking of potential energy barriers

In this work, we realize a magnetic skyrmion diode operated by a unidirectional skyrmion transport that flows in only one direction, which is highly significant for information processing in spintronic and nanoelectronic devices. We easily control the skyrmion transport by engineering asymmetric shapes of geometric structures. Here, we present a simple method to describe the underlying mechanism behind the unidirectional skyrmion transport by characterizing the topography of potential energy surfaces from a purely geometric perspective. Our approach enables a deeper physical insight into skyrmion transport manipulation and efficient design of skyrmion-based devices in geometric structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Anisotropy of the magnetic and transport properties of EuZn 2 As 2

Several recent studies have shown that the anisotropy in the magnetic structure of EuCd 2 As 2 plays a significant role in stabilizing the Weyl nodes. Therefore, to investigate the relationship between magnetic anisotropy and Weyl physics, we present a comparative study between EuZn 2 As 2 and EuCd 2 As 2 that are isostructural but with different magnetic anisotropy. We performed structural analysis, electronic transport, and magnetization experiments on millimeter-sized single crystals of EuZn 2 As 2 , and compared the results to those of EuCd 2 As 2 . By combining the first principle calculations and neutron diffraction experiment, we identify the magnetic ground state of EuZn 2 As 2 as A-type antiferromagnetic order with a transition temperature (T_N = 19.6 K) twice that of EuCd 2 As 2 . Like EuCd 2 As 2 , the negative magnetoresistance of EuZn 2 As 2 is observed after suppressing the resistivity peak at T_N with increasing fields. However, the anisotropy in both transport and magnetization are much reduced in EuZn 2 As 2 . The difference could be ascribed to the weaker spin-orbit coupling, more localized d orbitals, and a larger contribution from the Eu s orbitals in the zinc compound, as suggested by the electronic band calculations. The same band structure effect could be also responsible for the observation of a smaller nonlinear anomalous Hall effect in EuZn 2 As 2 compared to EuCd 2 As 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

A -type antiferromagnetic order in the Zintl-phase insulator EuZn 2 P 2

Zintl phases, containing strongly covalently bonded frameworks with separate ionically bonded ions, have emerged as a critical materials family in which to couple magnetism and strong spin-orbit coupling to drive diverse topological phases of matter. Here we report the single-crystal synthesis, magnetic, thermodynamic, transport, and theoretical properties of the Zintl compound EuZn 2 P 2 that crystallizes in the anti-La 2 O 3 (CaAl 2 Si 2 ) P-3m1 structure, containing triangular layers of Eu 2+ ions. In-plane resistivity measurements reveal insulating behavior with an estimated activation energy of E g = 0.11eV. Specific heat and magnetization measurements indicate antiferromagnetic ordering at T N = 23K. Curie-Weiss analysis of in-plane and out of plane magnetic susceptibility from T = 150 to 300 K yields p eff = 8.61 for μ 0 H⊥c and p eff = 7.74 for μ 0 H//c, close to the expected values for the 4f 7 J = S = 7/2 Eu 2+ ion and indicative of weak anisotropy. Below T N , a significant anisotropy of χ ⊥ /χ // ≈ 2.3 develops, consistent with A-type magnetic order as observed in isostructural analogs and as predicted by the density functional theory calculations reported herein. The positive Weiss temperatures of θ W =19.2K for μ 0 H⊥c and θ W =41.9K for μ 0 H//c show a similar anisotropy and suggest competing ferromagnetic and antiferromagnetic interactions. Comparing Eu magnetic ordering temperatures across trigonal EuM 2 X 2 (M= divalent metal, X= pnictide) shows that EuZn 2 P 2 exhibits the highest ordering temperature, with variations in T N correlating with changes in expected dipolar interaction strengths within and between layers and independent of the magnitude of electrical conductivity. These results provide experimental validation of the crystochemical intuition that the cation Eu 2+ layers and the anionic (M 2 X 2 ) 2– framework can be treated as electronically distinct subunits, enabling further predictive materials design.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Hopping crossover and high-temperature superspin glass behavior in Ni films deposited on MoS 2

Realizing the potential of MoS 2 and other two-dimensional semiconductors in spintronic devices requires a fundamental understanding of their interaction with ferromagnetic metals. We have investigated the transport and magnetic properties of thin Ni films grown on single-crystal MoS 2 substrates. The Ni is found to have a granular morphology with an average grain size of approximately 2 nm. Like previously investigated granular Ni/insulator composites below the percolation threshold, transport is found to proceed via hopping. However, our Ni/MoS 2 sample exhibits a dual crossover: activated hopping to 𝑇−1/2 variable-range hopping at a higher temperature and then a transition to power-law behavior at a lower temperature. The magnetoresistance was used as a novel probe for uncovering superspin-glass-like behavior above room temperature in the Ni/MoS 2 nanostructure, which also exhibits a collective ferromagnetic state at very low temperatures, reminiscent of superferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1975

A new approach to radiative transfer theory using Jones's vectors. I Fundamentals

Radiative transfer of partially polarized radiation in an anisotropically scattering, inhomogeneous atmosphere containing an arbitrary polydispersion of particles is described using Jones's (1941) amplitude vectors and matrices. This approach exploits the close analogy between the quantum-mechanical states of spin 1/2 systems and the polarization states of electromagnetic radiation described by Jones's vector. The complete equivalence between the transport equation for Jones's vectors and the classical radiative-transfer equation for Stokes's intensity vectors is demonstrated in two independent ways after deriving the transport equations for the polarization coherency matrices and for the quaternions corresponding to the Jones's vectors. A compact-operator formulation of the theory is provided and used to derive the necessary equations for both a local and a global description of the transport of Jones's vectors. The integro-differential equations for the amplitude reflection and transmission matrices are derived and related to the usual corresponding equations.

Fymat, A. L.↗
NASA NTRS1985

An Idealized Short Period Comet Model: Surface Insolation, H2O Flux, Dust Flux and Mantle Evolution

The present cometary activity model integrates feedback processes that involve the transport of heat, gas, and dust, as well as dust mantle development, and includes the effects of latitude, rotation, and spin orientation. Attention is given to the development, structural change, and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. The results obtained suggest that an initially homogeneous, short period comet with a cosmic dust to water ice ratio, as well as typical orbit, rotation rate, and grain size distribution, would develop only a thin, less than 1 mm cyclic mantle at all points on the nucleus. Most H2O dust histories deduced from brightness data are noted to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. The exceptional case of Comet Encke is discussed.

Fanale, F. P.↗
NASA NTRS1984

An idealized short-period comet model - Surface insolation, H2O flux, dust flux, and mantle evolution

The present cometary activity model integrates feedback processes that involve the transport of heat, gas, and dust, as well as dust mantle development, and includes the effects of latitude, rotation, and spin axis orientation. Attention is given to the development, structural change, and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. The results obtained suggest that an initially homogeneous, short period comet with a cosmic dust-to-water ice ratio, as well as typical orbit, rotation rate, and grain size distribution, would develop only a thin, less than 1 mm cyclic mantle at all points on the nucleus. Most H2O dust histories deduced from brightness data are noted to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. The exceptional case of Comet Encke is discussed.

Fanale, F. P.↗
DOE OSTI2023

Radiation Transport Two-temperature GRMHD Simulations of Warped Accretion Disks

In many black hole (BH) systems, the accretion disk is expected to be misaligned with respect to the BH spin axis. If the scale height of the disk is much smaller than the misalignment angle, the spin of the BH can tear the disk into multiple, independently precessing "sub-disks." This is most likely to happen during outbursts in black hole X-Ray binaries (BHXRBs) and in active galactic nuclei (AGNs) accreting above a few percent of the Eddington limit, because the disk becomes razor-thin. Disk tearing has the potential to explain variability phenomena including quasi-periodic oscillations in BHXRBs and changing-look phenomena in AGNs. Here, we present the first radiative two-temperature general relativistic magnetohydrodynamic (GRMHD) simulation of a strongly tilted (65°) accretion disk around an M BH = 10 M ⊙ BH, which tears and precesses. This leads to luminosity swings between a few percent and 50% of the Eddington limit on sub-viscous timescales. Surprisingly, even where the disk is radiation-pressure-dominated, the accretion disk is thermally stable over t ≳ 14,000 r g /c. This suggests warps play an important role in stabilizing the disk against thermal collapse. The disk forms two nozzle shocks perpendicular to the line of nodes where the scale height of the disk decreases tenfold and the electron temperature reaches T e ~ 10 8 –10 9 K. In addition, optically thin gas crossing the tear between the inner and outer disk gets heated to T e ~ 108 K. This suggests that warped disks may emit a Comptonized spectrum that deviates substantially from idealized models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Endless Dirac nodal lines in kagome-metal Ni3In2S2

Abstract Topological semimetals are a frontier of quantum materials. In multiband electronic systems, topological band crossings can form closed curves, known as nodal lines. In the presence of spin–orbit coupling and/or symmetry-breaking operations, topological nodal lines can break into Dirac/Weyl nodes and give rise to interesting transport properties, such as the chiral anomaly and giant anomalous Hall effect. Recently, the time-reversal symmetry-breaking induced Weyl fermions are observed in a kagome-metal Co 3 Sn 2 S 2 , triggering interests in nodal-line excitations in multiband kagome systems. Here, using first-principles calculations and symmetry-based indicator theories, we find six endless nodal lines along the stacking direction of kagome layers and two nodal rings in the kagome plane in nonmagnetic Ni 3 In 2 S 2 . The linear dipsersive electronic structure, confirmed by angle-resolved photoemission spectroscopy, induces large magnetoresistance up to 2000% at 9 T. Our results establish a diverse topological landscape of multiband kagome metals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dynamics of rapidly spinning blob-filaments: Fluid theory with a parallel kinetic extension

Blob-filaments (or simply “blobs”) are coherent structures formed by turbulence and sustained by nonlinear processes in the edge and scrape-off layer (SOL) of tokamaks and other magnetically confined plasmas. The dynamics of these blob-filaments, in particular, their radial motion, can influence the scrape-off layer width and plasma interactions with both the divertor target and with the main chamber walls. Motivated by recent results from the XGC1 gyrokinetic simulation code reported on elsewhere [J. Cheng et al., Nucl. Fusion 63, 086015 (2023)], a theory of rapidly spinning blob-filaments has been developed for this work. The theory treats blob-filaments in the closed flux surface region or the region that is disconnected from sheaths in the SOL. It extends previous work by treating blob spin, arising from partially or fully adiabatic electrons, as the leading-order effect and retaining inertial (ion charge polarization) physics in next order. Spin helps to maintain blob coherency and affects the blob's propagation speed. Dipole charge polarization, treated perturbatively, gives rise to blob-filaments with relatively slow radial velocity, comparable to that observed in the simulations. The theory also treats the interaction of rapidly spinning blob-filaments with a zonal flow layer. It is shown analytically that the flow layer can act like a transport barrier for these structures. Finally, parallel electron kinetic effects are incorporated into the theory. Various asymptotic parameter regimes are discussed, and asymptotic expressions for the radial and poloidal motion of the blob-filaments are obtained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Altermagnetic polar metallic phase in ultrathin epitaxially strained RuO 2 films

Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in nontrivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic transition remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray, and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO 2 /TiO 2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features an altermagnetic metallic polar phase in epitaxially strained 2 nm films, suggesting a potential link between polar metals and altermagnetic materials. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO 2 single crystals. These results highlight the power of epitaxial heterostructure engineering to induce altermagnetism in systems initially nonmagnetic, opening avenues for realizing emergent quantum phases with multifunctional properties.

RuO2 thin film↗
DOE OSTI2021

Persistent Friedel oscillations in graphene due to a weak magnetic field

Two opposite chiralities of Dirac electrons in a two-dimensional (2D) graphene sheet modify the Friedel oscillations strongly: electrostatic potential around an impurity in graphene decays much faster than in 2D electron gas. At distances $\textit{r}$ much larger than the de Broglie wavelength, it decays as 1/$r^3$. Here we show that a weak uniform magnetic field affects the Friedel oscillations in an anomalous way. It creates a field-dependent contribution which is dominant in a parametrically large spatial interval $p^{–1}_0 ≲ r ≲ k_Fl^2$, where $\textit{l}$ is the magnetic length, $k_F$ is Fermi momentum, and $p^{–1}_0 = (k_Fl)^{4/3} /k_F$. Moreover, in this interval, the field-dependent oscillations do not decay with distance. The effect originates from a spin-dependent magnetic phase accumulated by the electron propagator. Further, the obtained phase may give rise to novel interaction effects in transport and thermodynamic characteristics of graphene and graphene-based heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Spin-orbit-proximitized ferromagnetic metal by monolayer transition metal dichalcogenide: Atlas of spectral functions, spin textures, and spin-orbit torques in Co/MoSe 2 , Co/WSe 2 and Co/TaSe 2 heterostructures

In this work, the heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are the principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. The key quantity in theoretical description of these effects is nonequilibrium spin density, which can appear on any monolayer of the heterostructure through which the current is flowing as long as the monolayer bands are affected by the native or proximity induced SO coupling. Here we demonstrate how hybridization of wave functions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)—such as semiconducting MoSe 2 and WSe 2 or metallic TaSe 2 —can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with a vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer—i.e., just the presence of the interface—even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations. However, switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-WSe 2 as the most optimal one, at least in the clean limit.

2-dimensional systems↗