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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

The Doping Effect on Ferromagnetic Arrangement and Electronic Structure of Cubic AlAs with Low Concentration of 3d (V, Cr, and Mn) Impurities

The doping effect of aluminum arsenide (AlAs) with 3d (TM = V, Cr, and Mn) of transition metal impurities gives new materials called diluted magnetic semiconductors, which have interesting electronic and magnetic properties for spintronics applications. We have used the full-potential linearized augmented plane wave (FP-LAPW) method to calculate the electronic band structures and magnetic properties of Al{sub 1−x}TM{sub x}As at low concentration x = 0.0625 of transition metal (TM = V, Cr, and Mn) atoms. We have found that the majority-spin states of Al{sub 0.9375}TM{sub 0.0625}As compounds are metallic due to large p-d hybridization between 3d levels of TM and the 4p levels of As around Fermi level, whereas the minority-spin states have semiconductor character. These compounds exhibit a half-metallic behavior with spin polarization of 100%, where the ferromagnetism is originated from double-exchange mechanism. Therefore, Al{sub 0.9375}TM{sub 0.0625}As (TM = V, Cr, and Mn) materials seem to be good candidates for spin injection in the field of spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Kinetics of spin relaxation in quantum wires and channels: Boundary spin echo and formation of a persistent spin helix

In this paper we use a spin kinetic equation to study spin-polarization dynamics in one-dimensional (1D) wires and 2D channels. The spin kinetic equation is valid in both diffusive and ballistic spin transport regimes and therefore is more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin-relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. We find that a propagating spin-polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. The Green's function of the spin kinetic equation is derived for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in specifically oriented 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS2010

Measuring Parameters of Massive Black Hole Binaries with Partially-Aligned Spins

It is important to understand how well the gravitational-wave observatory LISA can measure parameters of massive black hole binaries. It has been shown that including spin precession in the waveform breaks degeneracies and produces smaller expected parameter errors than a simpler, precession-free analysis. However, recent work has shown that gas in binaries can partially align the spins with the orbital angular momentum, thus reducing the precession effect. We show how this degrades the earlier results, producing more pessimistic errors in gaseous mergers. However, we then add higher harmonics to the signal model; these also break degeneracies, but they are not affected by the presence of gas. The harmonics often restore the errors in partially-aligned binaries to the same as, or better than/ those that are obtained for fully precessing binaries with no harmonics. Finally, we investigate what LISA measurements of spin alignment can tell us about the nature of gas around a binary,

Lang, Ryan N.↗
DOE OSTI2014

On the spin separation of algebraic two-component relativistic Hamiltonians: Molecular properties

The idea for separating the algebraic exact two-component (X2C) relativistic Hamiltonians into spin-free (sf) and spin-dependent terms [Z. Li, Y. Xiao, and W. Liu, J. Chem. Phys. 137, 154114 (2012)] is extended to both electric and magnetic molecular properties. Taking the spin-free terms (which are correct to infinite order in α ≈ 1/137) as zeroth order, the spin-dependent terms can be treated to any desired order via analytic derivative technique. This is further facilitated by unified Sylvester equations for the response of the decoupling and renormalization matrices to single or multiple perturbations. For practical purposes, explicit expressions of order α{sup 2} in spin are also given for electric and magnetic properties, as well as two-electron spin-orbit couplings. At this order, the response of the decoupling and renormalization matrices is not required, such that the expressions are very compact and completely parallel to those based on the Breit-Pauli (BP) Hamiltonian. However, the former employ sf-X2C wave functions, whereas the latter can only use nonrelativistic wave functions. As the sf-X2C terms can readily be interfaced with any nonrelativistic program, the implementation of the O(α{sup 2}) spin-orbit corrections to sf-X2C properties requires only marginal revisions of the routines for evaluating the BP type of corrections.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Real-Time Observation of Molecular Spinning with Angular High-Harmonic Spectroscopy

We demonstrate an angular high-harmonic spectroscopy method to probe the spinning dynamics of a molecular rotation wave packet in real time. With the excitation of two time-delayed, polarization-skewed pump pulses, the molecular ensemble is impulsively kicked to rotate unidirectionally, which is subsequently irradiated by another delayed probe pulse for high-order harmonic generation (HHG). The spatiotemporal evolution of the molecular rotation wave packet is visualized from the time-dependent angular distributions of the HHG yields and frequency shift measured at various polarization directions and time delays of the probe pulse. Here, the observed frequency shift in HHG is demonstrated to arise from the nonadiabatic effect induced by molecular spinning. Different from the previous spectroscopic and Coulomb explosion imaging techniques, the angular high-harmonic spectroscopy method can reveal additionally the electronic structure and multiple orbitals of the sampled molecule. All the experimental findings are well reproduced by numerical simulations. Further extension of this method would provide a powerful tool for probing complex polyatomic molecules with HHG spectroscopy.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Hole-like Fermi surface in the overdoped non-superconducting Bi 1.8 Pb 0.4 Sr 2 CuO 6+δ

In high-temperature cuprate superconductors, the anti-ferromagnetic spin fluctuations are thought to have a very important role in naturally producing an attractive interaction between the electrons in the d-wave channel. The connection between superconductivity and spin fluctuations is expected to be especially consequential at the overdoped end point of the superconducting dome. In some materials, that point seems to coincide with a Lifshitz transition, where the Fermi surface changes from the hole-like centered at $(\pi, \pi)$ to the electron-like, centered at the Γ point causing a loss of large momentum anti-ferromagnetic fluctuations. In this work, we study the doping dependence of the electronic structure of Bi 1.8 Pb 0.4 Sr 2 CuO$_{6+\delta}$ in angle-resolved photoemission and find that the superconductivity vanishes at lower doping than that at which the Lifshitz transition occurs. This requires a more detailed re-examination of a spin fluctuation scenario.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Discovery of Charge Order and Corresponding Edge State in Kagome Magnet FeGe

Kagome materials often host exotic quantum phases, including spin liquids, Chern gap, charge density wave, and superconductivity. Existing scanning microscopy studies of the kagome charge order have been limited to nonkagome surface layers. Here, we tunnel into the kagome lattice of FeGe to uncover features of the charge order. Our spectroscopic imaging identifies a 2×2 charge order in the magnetic kagome lattice, resembling that discovered in kagome superconductors. Spin mapping across steps of unit cell height demonstrates the existence of spin-polarized electrons with an antiferromagnetic stacking order. We further uncover the correlation between antiferromagnetism and charge order anisotropy, highlighting the unusual magnetic coupling of the charge order. Finally, we detect a pronounced edge state within the charge order energy gap, which is robust against the irregular shape fluctuations of the kagome lattice edges. Here, we discuss our results with the theoretically considered topological features of the kagome charge order including unconventional magnetism and bulk-boundary correspondence.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Topological Superconductivity in the Doped Chiral Spin Liquid on the Triangular Lattice

It has been established that the undoped, i.e., δ = 0 , system has a CSL ground state in the parameter region 0.32 ≤ J χ / J ≤ 0.56 . Upon light doping, we find that the ground state of the system is consistent with a Luther-Emery liquid with power-law superconducting and charge-density-wave correlations but short-range spin-spin correlations. In particular, the superconducting correlations, whose pairing symmetry is consistent with d ± i d wave, are dominant at all hole doping concentrations. Overall, our results provide direct evidences that doping the CSL on the triangular lattice can naturally give rise to topological superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Hybrid s -wave superconductivity in CrB 2

In a metal with multiple Fermi pockets, the formation of s-wave superconductivity can be conventional due to electron-phonon coupling or unconventional due to spin fluctuations. Here, we analyze the hexagonal diboride CrB 2 , which is an itinerant antiferromagnet at ambient conditions and turns superconducting upon increasing pressure. While the high-pressure behavior of T c suggests conventional s-wave pairing, we find that spin fluctuations promoting unconventional s-wave pairing become important in the vicinity of the antiferromagnetic dome. As the symmetry class of the s-wave state is independent of its underlying mechanism, we argue that CrB 2 is a realization of a hybrid s-wave superconductor where unconventional and conventional s-wave mechanisms team up to form a joint superconducting dome.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Doped Mott insulators in the triangular-lattice Hubbard model

Here we investigate the evolution of the Mott insulators in the triangular lattice Hubbard Model, as a function of hole doping δ in both the strong and intermediate coupling limits. Using the advanced density matrix renormalization group (DMRG) method, at light hole doping δ ≲ 10%, we find a significant difference between strong and intermediate couplings. Notably, at intermediate coupling an unusual metallic state emerges, with short ranged spin correlations but long ranged spin-chirality order. Moreover, no clear Fermi surface or wave vector is observed, this chiral metal also exhibits staggered loop current, which breaks the translational symmetry. These features disappear on increasing interaction strength or on further doping. At strong coupling, the 120 degree magnetic order of the insulating magnet persists for light doping, and produces hole pockets with a well-defined Fermi surface. On further doping, δ ≈ 10%~20% SDW order and coherent hole Fermi pockets are found at both strong and intermediate couplings. At even higher doping δ ≳ 20%, the SDW order is suppressed and the spin-singlet Cooper pair correlations are simultaneously enhanced. We also briefly comment on the strong particle-hole asymmetry of the model.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effective photon mass and exact translating quantum relativistic structures

Using a variation of the celebrated Volkov solution, the Klein-Gordon equation for a charged particle is reduced to a set of ordinary differential equations, exactly solvable in specific cases. The new quantum relativistic structures can reveal a localization in the radial direction perpendicular to the wave packet propagation, thanks to a non-vanishing scalar potential. The external electromagnetic field, the particle current density, and the charge density are determined. The stability analysis of the solutions is performed by means of numerical simulations. The results are useful for the description of a charged quantum test particle in the relativistic regime, provided spin effects are not decisive.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Short-period pulsar oscillations following a glitch

Following a glitch, the crust and magnetized plasma in the outer core of a neutron star are believed to rapidly establish a state of co-rotation within a few seconds by process analogous to classical Ekman pumping. However, in ideal magnetohydrodynamics, a final state of co-rotation is inconsistent with conservation of energy of the system. We demonstrate that, after the Ekman-like spin up is completed, magneto-inertial waves continue to propagate throughout the star, exciting torsional oscillations in the crust and plasma. The crust oscillation is irregular and quasi-periodic, with a dominant frequency of the order of seconds. Crust oscillations commence after an Alfvén crossing time, approximately half a minute at the magnetic pole, and are subsequently damped by the electron viscosity over approximately an hour. In rapidly rotating stars, the magneto-inertial spectrum in the core approaches a continuum, and crust oscillations are damped by resonant absorption analogous to quasi-periodic oscillations in magnetars. The oscillations predicted are unlikely to be observed in timing data from existing radio telescopes, but may be visible to next generation telescope arrays.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Mechanical form factors and densities of nonrelativistic fermions

The hadron physics community has been actively debating the interpretation of so-called mechanical properties of hadrons. Nonrelativistic quantum-mechanical systems like the hydrogen atom have been appealed to in these debates as analogies. Since such appeals are likely to continue, it is important to have Galilei-covariant expressions for matrix elements of the energy-momentum tensor. In this work, I obtain Galilei-covariant breakdowns of such matrix elements into mechanical form factors, with a special focus on spin-half states. I additionally study the spatial densities associated with these form factors, using the pilot wave interpretation to guide their breakdown into contributions from internal structure and from quantum-mechanical effects such as wave packet dispersion. For completeness, I also obtain nonrelativistic Breit frame densities.

form factors↗
DOE OSTI2014

HOW CAN NEWLY BORN RAPIDLY ROTATING NEUTRON STARS BECOME MAGNETARS?

In a newly born (high-temperature and Keplerian rotating) neutron star, r-mode instability can lead to stellar differential rotation, which winds the seed poloidal magnetic field (∼10{sup 11} G) to generate an ultra-high (∼10{sup 17} G) toroidal field component. Subsequently, by succumbing to the Tayler instability, the toroidal field could be partially transformed into a new poloidal field. Through such dynamo processes, the newly born neutron star with sufficiently rapid rotation could become a magnetar on a timescale of ∼10{sup 2} {sup –} {sup 3} s, with a surface dipolar magnetic field of ∼10{sup 15} G. Accompanying the field amplification, the star could spin down to a period of ∼5 ms through gravitational wave radiation due to the r-mode instability and, in particular, the non-axisymmetric stellar deformation caused by the toroidal field. This scenario provides a possible explanation for why the remnant neutron stars formed in gamma-ray bursts and superluminous supernovae could be millisecond magnetars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Tunable charge to spin conversion in strontium iridate thin films

Efficient charge to spin conversion is important for low-power spin logic devices. Spin and charge interconversion is commonly performed using heavy metals and topological insulators, while the field of oxides is not yet fully explored. Strontium iridate thin films were grown, where the different crystal structures form a perfect playground to understand the key factors in obtaining high charge to spin conversion efficiency (i.e.,large spin Hall angle). It was found that the semiconducting Sr 2 IrO 4 has a spin Hall angle of ~0.1 (depending on measurement technique), which is promising for a spin-orbit coupled electronic system and comparable to Pt. In contrast, the perovskite SrIrO 3 , reported to have a Dirac cone near the Fermi level, has a larger spin Hall angle of 0.3–0.4 degrees. The largest difference between the two materials is a large degree of spin-momentum locking in SrIrO 3 , comparable to known topological insulators. A simple semiclassical relationship is found where thespin Hall angle increases for higher degrees of spin-momentum locking and it also increases for lower Fermi wave vectors. Here this relationship is then able to explain the decreased spin Hall angle below 10 nm film thickness in SrIrO 3 , by relating it to the correspondingly higher carrier concentration (related to the higher Fermi wavevector). Breaking the commonly believed anticorrelation between resistivity and carrier concentration paves apathway to lower power losses due to resistance while keeping large spin Hall angles.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Quantifying charge resonance and multiexciton character in coupled chromophores by charge and spin cumulant analysis

We extend excited-state structural analysis to quantify the charge-resonance and multi-exciton character in wave functions of weakly interacting chromophores such as molecular dimers. The approach employs charge and spin cumulants which describe inter-fragment electronic correlations in molecular complexes. We introduce indexes corresponding to the weights of local, charge resonance, and biexciton (with different spin structure) configurations that can be computed for general wave functions thus allowing one to quantify the character of doubly excited states. The utility of the approach is illustrated by applications to several small dimers, e.g., He-H{sub 2}, (H{sub 2}){sub 2}, and (C{sub 2}H{sub 4}){sub 2}, using full and restricted configuration interaction schemes. In addition, we present calculations for several systems relevant to singlet fission, such as tetracene, 1,6-diphenyl-1,3,5-hexatriene, and 1,3-diphenylisobenzofuran dimers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of Charge Localization on the Effective Hyperfine Interaction in Organic Semiconducting Polymers

Hyperfine interaction (HFI), originating from the coupling between spins of charge carriers and nuclei, has been demonstrated to strongly influence the spin dynamics of localized charges in organic semiconductors. Nevertheless, the role of charge localization on the HFI strength in organic thin films has not yet been experimentally investigated. In this study, the statistical relation hypothesis that the effective HFI of holes in regioregular poly(3-hexylthiophene) (P3HT) is proportional to 1/N 0.5 has been examined, where N is the number of the random nuclear spins within the envelope of the hole wave function. First, by studying magnetoconductance in hole-only devices made by isotope-labeled P3HT we verify that HFI is indeed the dominant spin interaction in P3HT. Second, assuming that holes delocalize fully over the P3HT polycrystalline domain, the strength of HFI is experimentally demonstrated to be proportional to 1/N 0.52 in excellent agreement with the statistical relation. Third, the HFI of electrons in P3HT is about 3 times stronger than that of holes due to the stronger localization of the electrons. Finally, the effective HFI in organic light emitting diodes is found to be a superposition of effective electron and hole HFI. Such a statistical relation may be generally applied to other semiconducting polymers. Lastly, this Letter may provide great benefits for organic optoelectronics, chemical reaction kinetics, and magnetoreception in biology.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Neutron scattering study of fluctuating and static spin correlations in the anisotropic spin glass Fe 2 Ti O 5

The anisotropic spin-glass transition, in which spin freezing is observed only along the c axis in pseudobrookite Fe 2 TiO 5 , has long been perplexing because the Fe 3+ moments (d 5 ) are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature T g ≈55K, and a model was proposed in which the freezing of the surfboard magnetization fluctuations leads to the anisotropic spin-glass state. Given this model, we have carried out high-resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intrasurfboard spin dynamics on neutron (picosecond) timescales. Here, in this paper, we report on the temperature-dependence of the spin fluctuations measured from single-crystal Fe 2 TiO 5 . Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above T g . The spin fluctuations possess a steep energy–wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches T g from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of most of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intrasurfboard spin correlations to at least T/T g ≈0.1 , consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗