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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Spin dynamics in permalloy nano-ellipses for honeycomb and square lattices

We report experimental and theoretical studies of spin dynamics in lattice structures of permalloy (Ni 80 Fe 20 ) nano-ellipses, with four different types of networks including honeycomb and square lattices. The lattices are patterned at the center line of the co-planar wave guide and consist of non-contacting or contacting ellipses. Micromagnetic simulations show excellent agreement with the broadband ferromagnetic resonance (FMR) experimental results. We find the existence of a spin-wave mode localized in the vertex region of the contacting nano-ellipse network. Our finding has important implications when designing an artificial spin ice (ASI) network for functional magnonics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Giant momentum-dependent spin splitting in centrosymmetric low-$\textit{Z}$ antiferromagnets

The energy vs crystal momentum $\textit{E(k)}$ diagram for a solid (band structure) constitutes the road map for navigating its optical, magnetic, and transport properties. By selecting crystals with specific atom types, composition, and symmetries, one could design a target band structure and thus desired properties. A particularly attractive outcome would be to design energy bands that are split into spin components with a momentum-dependent splitting, as envisioned by Pekar and Rashba, enabling spintronic application. The current paper provides “design principles” for wave-vector dependent spin splitting (SS) of energy bands that parallel the traditional Dresselhaus and Rashba spin-orbit coupling (SOC)–induced splitting, but originates from a fundamentally different source—antiferromagnetism. Here, we identify a few generic antiferromagnetic (AFM) prototypes with distinct SS patterns using magnetic symmetry design principles. These tools allow also the identification of specific AFM compounds with SS belonging to different prototypes. A specific compound—centrosymmetric tetragonal MnF 2 —is used via density functional band-structure calculations to quantitatively illustrate one type of AFM SS. Unlike the traditional SOC-induced effects restricted to noncentrosymmetric crystals, we show that antiferromagnetic-induced spin splitting broadens the playing field to include even centrosymmetric compounds, and gives SS comparable in magnitude to the best known (“giant’” SOC effects, even without traditional SOC, and consequently does not rely on the often-unstable high atomic number elements required for high SOC. We envision that use of the current design principles to identify an optimal antiferromagnet with spin-split energy bands would be beneficial for efficient spin-charge conversion and spin-orbit torque applications without the burden of requiring compounds containing heavy elements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic properties of metastable honeycomb KCoAsO 4

We present comprehensive neutron scattering data on polycrystalline samples of a new metastable honeycomb material KCoAsO 4 . Below 𝑇 𝑁 = 14 K, the system orders into a zigzag antiferromagnetic state, with spins ordered into alternating ferromagnetic chains similar to the isostructural sister compound KNiAsO 4 . In the case of KCoAsO 4 , we find the moments are robustly canted out of plane closer to the crystallographic 𝑐 axis. A combination of weak interlayer coupling, lattice strain, and inhomogeneities lead to the coexistence of two magnetic ordering wave vectors 𝑘 1 = (1.5 0 0) and 𝑘 2 = (0.5 0 0.5), where the two structures differ only in their layer stacking. Inelastic data show the presence of a spin orbital mode at 24 meV, supporting a pseudospin $\tilde{𝑆}$ = 1/2 Kramer's doublet ground state of the Co 2+ ions. We model the low energy excitations using both a conventional XXZ Hamiltonian and generalized Kitaev Heisenberg Hamiltonian within a linear spin wave limit. While either model can qualitatively reproduce the observed spectra, the lack of fine features and observation of disorder prevent a clear-cut determination of the low energy Hamiltonian. In the case of an XXZ-type model, a large easy-axis anisotropy is necessary to reproduce the gapped spectra and canting of the magnetic moments. For the generalized Kitaev model, despite the large canting of the moments away from the honeycomb layers, we find a noticeable if nondominant Kitaev term persists. In conclusion, the contrast of the magnetic properties of KCoAsO 4 to other cobalt honeycombs highlights the sensitivity of the low energy magnetic properties of Co 2+ to fine details of its crystalline environment.

Honeycomb lattice↗
NASA NTRS2000

Co-Investigator Proposal for Enstrophy - - Filamentation of Auroral Currents

Cornell University provided three instruments for the Enstrophy experiment: an electric field meter, a plasma wave receiver, and a magnetometer for measuring FAC. The electric field meter consisted of a 6 m Weitzmann boom system with analog signal processing and 12 bit ADC, which yielded one electric field component instantaneously and a two dimensional electric field every half spin. The plasma wave receiver used the same sensing system with the addition of pre-amplifiers in the spheres to sense plasma waves up to and including the electron Langmuir frequency. Signal processing employed a variety of continuous and snap shot techniques depending on the frequency range and band width. The science magnetometer provided by Cornell University was a Billingsly design fluxgate previously used on spacecraft missions but without radiation hardening. The magnetometer was mounted on a one meter, stiff aluminum "flop-down" boom. The Enstrophy payload was launched on february 11, 1999. Because of a design flaw in the event timers, the magnetometer boom was deployed before the payload despun. As a result the magnetometer separated mechanically from the boom but maintained electrical connection. This was confirmed by the calculation of the scalar magnetic field from all three vector components of the magnetic field. However, the individual vector values had no scientific value. The electric field and plasma wave instrumentation worked as designed. The data from these instruments was provided to the University of New Hampshire and to the Principal Investigator, as proposed.

Kintner, Paul M.↗
DOE OSTI2020

Spin Splitting Energy of Transition Metals: A New, More Affordable Wave Function Benchmark Method and Its Use to Test Density Functional Theory

Accurately predicting the spin splitting energy of chemical species is important for understanding their reactivity and magnetic properties, but it is very challenging, especially for molecules containing transition metals. One impediment to progress is the scarcity of accurate benchmark data. Here we report a set of calculations designed to yield reliable benchmarks for simple transition-metal complexes that can be used to test density functional methods that are affordable for large systems of more practical interest. Various wave function methods are tested against experiment for Fe 2+ , Fe 3+ , and Co 3+ , including CASSCF, CASPT2, CASPT3, MRCISD, MRCISD+Q, ACPF, AQCC, CCSD(T), and CASPT2/CCSD(T) and also a new method called CASPT2.5, which is performed by taking the average of the CASPT2 and CASPT3 energies. Furthermore, we find that MRCISD+Q, ACPF, and AQCC require smaller active spaces for good accuracy than are required by CASPT2 and CASPT3, and this aspect may be important for calculations on larger molecules; here we find that CASPT2.5 extrapolated to a complete basis set is the most suitable method—in terms of computational cost and in terms of accuracy on monatomic systems—and therefore we chose this method for molecular benchmarks. Then Kohn–Sham density functional calculations with 60 exchange-correlation functionals are tested for FeF 2 , FeCl 2 , and CoF 2 . We find that MN15-L, M06-SX, and revM06 have very good agreement with CASPT2.5 benchmarks in terms of both the spin splitting energy and the optimized geometry for each spin state. In addition, we recommend def2-TZVP as the most suitable basis set to perform density functional calculations for molecular spin splitting energies; extra polarization functions in the basis set do not help to increase the accuracy of the spin splitting energy in KS calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2021

GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run

We report on gravitational wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo in the first half of the third observing run (O3a) between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near real-time through GCN Notices and Circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of ~ 0.8, as well as events whose components could not be unambiguously identified as black holes or neutron stars. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational wave data alone. The range of candidate event masses which are unambiguously identified as binary black holes (both objects ≥ 3 M⨀) is increased compared to GWTC-1, with total masses from ∼ 14M⨀ for GW190924 021846 to ∼ 150M⨀ for GW190521. For the first time, this catalog includes binary systems with significantly asymmetric mass ratios, which had not been observed in data taken before April 2019. We also find that 11 of the 39 events detected since April 2019 have positive effective inspiral spins under our default prior (at 90% credibility), while none exhibit negative effective inspiral spin. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in ∼26 weeks of data (∼1.5 per week) is consistent with GWTC-1.

Gravitational wave detection↗
DOE OSTI2021

Theory and phase-field simulations of electrical control of spin cycloids in a multiferroic

Cycloidal spin orders are common in multiferroics. One of the prototypical examples is BiFeO 3 (BFO) which shows a large polarization and a cycloidal antiferromagnetic (AFM) order at room temperature. Here, we employ Landau theory and phase-field simulations to analyze the coupled switching dynamics of polarization and cycloidal AFM orders in BFO. We identify 14 types of transitional spin structures between two cycloids and nine electric-field-induced spin switching paths. We demonstrate the electric-field-induced rotation of wave vectors of the cycloidal spins and discover two types of cycloidal spin switching dynamics: fast local spin flips and slow rotation of wave vectors. Also, we construct roadmaps to achieve the switching between any two spin cycloids through multistep applications of electric fields. Furthermore, this paper provides a theoretical framework for the phenomenological description of spin cycloids and a fundamental understanding of the switching mechanisms to achieve electrical control of magnetic orders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Pair density wave in the doped three-band Hubbard model on two-leg square cylinders

A pair density wave (PDW) is a superconducting (SC) state with a spatially modulated order parameter. Although much is known about the properties of the PDW state, its realization in microscopic models with divergent susceptibility has been challenging. Here, in this work, we report a density-matrix renormalization group study of a three-band Hubbard model (also known as the Emery model) for cuprates on long two-leg square cylinders. Upon light doping, we find that the ground state of the system is consistent with that of a PDW state with mutually commensurate and power-law SC, charge (CDW), and spin (SDW) density wave correlations. The SC correlations are dominant between neighboring Cu sites with d-wave pairing symmetry. Interestingly, we find that the near-neighbor interactions, especially the near-neighbor attractive V pd interaction between neighboring Cu and oxygen sites, can notably enhance the SC correlations while simultaneously suppressing the CDW correlations. For a modestly strong attractive V pd , the SC correlations become quasi-long-ranged with a divergent PDW susceptibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2014

First All-Sky Search for Continuous Gravitational Waves from Unknown Sources in Binary Systems

We present the first results of an all-sky search for continuous gravitational waves from unknown spinning neutron stars in binary systems using LIGO and Virgo data. Using a specially developed analysis program, the TwoSpect algorithm, the search was carried out on data from the sixth LIGO science run and the second and third Virgo science runs. The search covers a range of frequencies from 20 Hz to 520 Hz, a range of orbital periods from 2 to ∼2,254 h and a frequency- and period-dependent range of frequency modulation depths from 0.277 to 100 mHz. This corresponds to a range of projected semimajor axes of the orbit from ∼0.6 × 10(exp −3) ls to ∼6,500 ls assuming the orbit of the binary is circular. While no plausible candidate gravitational wave events survive the pipeline, upper limits are set on the analyzed data. The most sensitive 95% confidence upper limit obtained on gravitational wave strain is 2.3 × 10(exp −24) at 217 Hz, assuming the source waves are circularly polarized. Although this search has been optimized for circular binary orbits, the upper limits obtained remain valid for orbital eccentricities as large as 0.9. In addition, upper limits are placed on continuous gravitational wave emission from the low-mass x-ray binary Scorpius X-1 between 20 Hz and 57.25 Hz.

systems↗
DOE OSTI2025

Orbital-spin locking and its optical signatures in altermagnets

Altermagnets, magnetic materials with zero magnetization and spin-split band structure, have gained tremendous attention recently for their rich physics and potential applications. Here, we report on a microscopic tight-binding model that unveils a unique coupling between orbitals and spins in d-wave altermagnets, which gives rise to momentum-dependent and spin-selective optical absorption. This coupling promotes the controlled optical excitation of up or down spins depending on the polarization direction of linearly polarized light. Such an effect originates from the coupling of orbitals to the sublattice degree of freedom through the crystal field, which is then coupled to spins through the antiferromagnetic interaction. Our crystal field analysis, which is general to any type of altermagnet, helps understand the onset of altermagnetism from a microscopic point of view, and we use our results to propose clear magneto-optical signatures of our predictions. Our findings shine light on the interplay between orbitals and spins in altermagnets, thus paving the way towards novel orbitronic and optospintronic devices.

Vila, Marc↗
DOE OSTI2023

Scattering in black hole backgrounds and higher-spin amplitudes. Part I

The scattering of massless waves of helicity |$h$| = $0$, $\frac{1}{2}$, $1$ in Schwarzschild and Kerr backgrounds is revisited in the long-wavelength regime. Using a novel description of such backgrounds in terms of gravitating massive particles, we compute classical wave scattering in terms of 2 → 2 QFT amplitudes in flat space, to all orders in spin. The results are Newman-Penrose amplitudes which are in direct correspondence with solutions of the Regge-Wheeler/Teukolsky equation. By introducing a precise prescription for the point-particle limit, in Part I of this work we show how both agree for $h$ = 0 at finite values of the scattering angle and arbitrary spin orientation. Associated classical observables such as the scattering cross sections, wave polarizations and time delay are studied at all orders in spin. The effect of the spin of the black hole on the polarization and helicity of the waves is found in agreement with previous analysis at linear order in spin. In the particular limit of small scattering angle, we argue that wave scattering admits a universal, point-particle description determined by the eikonal approximation. We show how our results recover the scattering eikonal phase with spin up to second post-Minkowskian order, and match it to the effective action of null geodesics in a Kerr background. Using this correspondence we derive classical observables such as polar and equatorial scattering angles. This study serves as a preceding analysis to Part II, where the Gravitational Wave ($h$ = 2) case will be studied in detail.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

i -Wave Symmetry Altermagnetism and Anomalous Hall Conductivity in Monolayer FeCl 3

In this work, the FeCl 3 monolayer is proposed to host a rare and exotic i -wave symmetry altermagnetism. Using first-principles calculations, we performed a detailed investigation of the structural stability and electronic, magnetic, and spintronic properties of monolayer FeCl 3 . Our results suggest the system is semiconducting in nature and exhibits i-wave symmetry nonrelativistic spin splitting (NRSS), originating from its hexagonal crystal structure with roto-inversion symmetries and anisotropic crystal field. The momentum-resolved spin-splitting further supports the presence of spin-dependent topological features. Further, the monolayer exhibits sizable conventional spin-Hall and anomalous Hall conductivities, highlighting its promise as a versatile platform for next-generation flexo-spintronic applications.

altermagnetism↗
DOE OSTI2025

Protected fermionic zero modes in periodic gauge fields

It is well known that macroscopically normalizable zero-energy wave functions of spin-$\frac{1}{2}$ particles in a two-dimensional inhomogeneous magnetic field are spin-polarized and exactly calculable with degeneracy equaling the number of flux quanta linking the whole system. Here, extending this argument to massless Dirac fermions subjected to magnetic fields that have zero net flux but are doubly periodic in real space, we show that there exist only two Bloch-normalizable zero-energy eigenstates, one for each spin flavor. This result is immediately relevant to graphene multilayer systems subjected to doubly periodic strain fields, which at low energies enter the Hamiltonian as periodic pseudogauge vector potentials. Furthermore, we explore various related settings including nonlinearly dispersing band structure models and systems with singly periodic magnetic fields.

36 MATERIALS SCIENCE↗
NASA NTRS1975

The propagation of plane waves and higher order acoustic modes in multisectioned ducts

A theoretical and experimental study of acoustic propagation in an anechoically terminated multisectioned duct was performed. A unique source array consisting of two concentric rings of sources, providing phase and amplitude control in the radial as well as circumferential direction, was developed to generate plane waves and both spinning and non-spinning higher order modes. Measurement of attenuation and radial mode shapes were taken with various finite length liners inserted between the hard wall sections of a duct with an anechoic termination. A search technique was developed to find the complex eigenvalues for a liner under the assumption of a locally reacting boundary condition. The experimental results were compared with a theoretical analysis which includes the modal transmission and reflection at the interface of each duct section, and this indicated that the local reaction boundary condition is valid for three liner configurations.

Wyerman, B. R.↗
NASA NTRS1976

A Theoretical and Experimental Study of Acoustic Propagation in Multisectioned Circular Ducts

The propagation of plane waves and higher order acoustic modes in a circular multisectioned duct was studied. A unique source array consisting of two concentric rings of sources, providing phase and amplitude control in the radial, as well as circumferential direction, was developed to generate plane waves and both spinning and nonspinning higher order modes. Measurements of attenuation and radial mode shapes were taken with finite length liners inserted between the hard wall sections of an anechoically terminated duct. Materials tested as liners included a glass fiber material and both sintered fiber metals and perforated sheet metals with a honeycomb backing. The fundamental acoustic properties of these materials were studied with emphasis on the attenuation of sound by the liners and the determination of local versus extended reaction behavior for the boundary condition. A search technique was developed to find the complex eigenvalues for a liner under the assumption of a locally reacting boundary condition.

Wyerman, B. R.↗
NASA NTRS1976

The propagation and attenuation of complex acoustic waves in treated circular and annular ducts

The propagation of plane waves and higher order acoustic modes in a circular multisectioned duct was studied. A unique source array consisting of two concentric rings of sources, providing phase and amplitude control in the radial, as well as circumferential direction, was developed to generate plane waves and both spinning and nonspinning higher order modes. Measurements of attenuation and radial mode shapes were taken with finite length liners between the hard wall sections of an anechoically terminated duct. Materials tested as liners included a glass fiber material and both sintered fiber metals and perforated sheet metals with a honeycomb backing. The fundamental acoustic properties of these materials were studied with emphasis on the attenuation of sound by the liners and the determination of local versus extended reaction behavior for the boundary condition. The experimental results were compared with a mathematical model for the multisectioned duct.

Reethof, G.↗
DOE OSTI2021

Coherent Gate Operations in Hybrid Magnonics

Electromagnonics-the hybridization of spin excitations and electromagnetic waves-has been recognized as a promising candidate for coherent information processing in recent years. Among its various implementations, the lack of available approaches for real-time manipulation on the system dynamics has become a common and urgent limitation. In this work, by introducing a fast and uniform modulation technique, we successfully demonstrate a series of benchmark coherent gate operations in hybrid magnonics, including semiclassical analogies of Landau-Zener transitions, Rabi oscillations, Ramsey interference, and controlled mode swap operations. Finally, our approach lays the groundwork for dynamical manipulation of coherent signals in hybrid magnonics and can be generalized to a broad range of applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗