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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Spin and lattice dynamics of the two-dimensional van der Waals ferromagnet CrI3

Chromium tri-iodide (CrI 3 ) is a prototypical ferromagnetic van der Waals insulator with its genuinely two-dimensional (2D) long-range magnetic order below 45 K demonstrated recently. The underlying magnetic anisotropy has not been completely understood while both the Dzyaloshinskii-Moriya (DM) interaction and the Kitaev—Γ type interaction have been considered as the relevant magnetic Hamiltonian. In addition, the relation between the crystal structure and the magnetic order needs to be further elucidated concerning their possible coupling in few-layer samples and in the topmost surface layers of bulk samples. Here, we investigate these issues via temperature- and magnetic field-dependent terahertz spectroscopy on bulk CrI 3 single crystals, focusing on the dynamics of ferromagnetic resonances (FMRs) and optical phonons in the terahertz (THz) region from 4 to 120 cm -1 (from 0.5 to 15 meV). We narrow down the possible ranges of the interaction parameters such as the off-diagonal symmetric terms and the single-ion anisotropy. The accurate values of these parameters significantly constrain the magnitude of possible Kitaev—Γ exchange interaction and the topological magnon gap. Moreover, the structure-magnetism relationship was critically analyzed based on the temperature- and field-dependences of two E u in-plane optical phonon modes, which shows that the commonly believed structural phase diagram of CrI 3 , derived from surface-preferential data, has to be seriously modified.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Classical signatures of quenched and thermal disorder in the dynamics of correlated spin systems

Neutron scattering is frequently used to look for evidence of features indicative of quantum-entangled phases of matter such as continua from fractionalisation or quantised excitations. However, the non-specificity of these features and difficulty of both fully quantum treatments and semiclassical models of disorder, make the diagnosis of such states problematic. Here, we demonstrate the feasibility of semiclassical treatments of disordered systems for supercells of ~10 000 spins. By examining a number of classically disordered models we show the presence of quantised excitations, broad continua and anomalous damping originating from quenched disorder or large classical degeneracies.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Phonon dynamics in quantum spin liquid and valence bond crystal states in the barlowite family of kagome magnets

The barlowite [Cu 4 ⁢(OD) 6⁢ FBr] family of materials is characterized by antiferromagnetic spin-1/2 kagome layers and are believed to host interesting ground states of quantum magnetism. Substituting the interlayer sites with zinc ions yields Zn-barlowite (Zn 𝑥 ⁢Cu 4−𝑥 ⁢(OD) 6 ⁢FBr, 𝑥 ≈ 0.80) which is a strong candidate to have a quantum spin liquid (QSL) state. Experimentally distinguishing between valence bond crystal (VBC) and QSL states in the absence of magnetic ordering can prove challenging as both states exhibit similar average spin-spin correlations. Using high-resolution inelastic x-ray scattering with 1.4 meV energy resolution, we probe acoustic phonons in these structurally similar materials at various temperatures. Our measurements uncover robust differences possibly driven by different spin-phonon physics in VBC and QSL states, particularly at the 𝑀-point Brillouin zone boundary. Over a large 𝑇=[15,200]⁢ K temperature range, where neither material undergoes magnetic ordering nor a structural transition, we observe phonon softening in Zn-barlowite in contrast to a hardening of the same phonon in barlowite. Further, these results indicate that acoustic phonons in the barlowite system can be used to study and possibly distinguish between QSL and disordered VBC states in quantum magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1972

Dynamics of a spinning space station with a counterweight connected by multiple cables.

A unique and relatively simple approach is presented for obtaining the linearized equations of motion. A conservative system consisting of two rigid bodies connected by any number of massless cables with linear axial stiffness is assumed. The cable forces and torques are expanded in a Taylor series about the equilibrium values of the system coordinates which results in a cable stiffness matrix. The method of obtaining the equilibrium values of the coordinates is discussed and results are presented. The range of validity of the linear model is determined by comparing results with a digital simulation of the nonlinear system.

Nixon, D. D.↗
NASA NTRS1973

Attitude dynamic of spin-stabilized satellites with flexible appendages

Equations of motion and computer programs have been developed for analyzing the motion of a spin-stabilized spacecraft having long, flexible appendages. Stability charts were derived, or can be redrawn with the desired accuracy for any particular set of design parameters. Simulation graphs of variables of interest are readily obtainable on line using program FLEXAT. Finally, applications to actual satellites, such as UK-4 and IMP-1 have been considered.

Renard, M. L.↗
NASA NTRS1977

Effect of gravity-gradient torques on the dynamics of a spinning spacecraft with telescoping appendages

The effects of gravity-gradient torques during boom deployment maneuvers of a spinning spacecraft are examined with a combination of numerical and analytical methods. It is assumed that the spacecraft consists of a rigid central hub and movable rigid telescoping booms where the booms extend only along the hub principal axes. For the special case of symmetric deployment the stability boundaries are determined and a stability chart is used to study the system behavior. Possible cases of instability during this type of maneuver are identified.

Rajan, M.↗
NASA NTRS1986

Formulation of numerical procedures for dynamic analysis of spinning structures

The paper presents the descriptions of recently developed numerical algorithms that prove to be useful for the solution of the free vibration problem of spinning structures. First, a generalized procedure for the computation of nodal centrifugal forces in a finite element owing to any specified spin rate is derived in detail. This is followed by a description of an improved eigenproblem solution procedure that proves to be economical for the free vibration analysis of spinning structures. Numerical results are also presented which indicate the efficacy of the currently developed procedures.

Gupta, K. K.↗
DOE OSTI2021

Temperature evolution of the phonon dynamics in the Kitaev spin liquid

In this work, we present a study of the phonon dynamics in the honeycomb Kitaev spin model at finite temperatures. We show that the fractionalized spin excitations of the Kitaev spin liquid, the itinerant Majorana fermions, and static Z 2 fluxes have distinct effects on the phonon dynamics, which makes the phonon dynamics a promising tool for exploring the Kitaev spin liquid candidate materials. In particular, we focus on the signature of the fractionalized excitations in the thermodynamic behavior of the sound attenuation and the phonon Hall viscosity: the former describes the phonon decay into the fractionalized excitations, and the latter is the leading order time-reversal symmetry breaking effect on the acoustic phonon. We find that the angular dependence of the attenuation coefficient and its magnitude is modified by the thermal excitation of the Z 2 fluxes. The strength of this effect strongly depends on the relative magnitude of the sound velocity and the Fermi velocity characterizing the low-energy Majorana fermions. We also show that the Hall viscosity is strongly suppressed by the increase of the density of the Z 2 fluxes at finite temperatures. All these observations reflect the effects of the emergent disorder on the Majorana fermions introduced by the Z 2 fluxes. Our analysis is based on the complementary analytical calculations in the low-temperature zero-flux sector, and numerical calculations in the inhomogeneous flux sectors at intermediate and high temperatures with stratified Monte Carlo (strMC) method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Spin–spin interactions in defects in solids from mixed all-electron and pseudopotential first-principles calculations

Abstract Understanding the quantum dynamics of spin defects and their coherence properties requires an accurate modeling of spin-spin interaction in solids and molecules, for example by using spin Hamiltonians with parameters obtained from first principles calculations. We present a real-space approach based on density functional theory for the calculation of spin-Hamiltonian parameters, where only selected atoms are treated at the all-electron level, while the rest of the system is described with the pseudopotential approximation. Our approach permits calculations for systems containing more than 1000 atoms, as demonstrated for defects in diamond and silicon carbide. We show that only a small number of atoms surrounding the defect needs to be treated at the all-electron level, in order to obtain an overall all-electron accuracy for hyperfine and zero-field splitting tensors. We also present results for coherence times, computed with the cluster correlation expansion method, highlighting the importance of accurate spin-Hamiltonian parameters for quantitative predictions of spin dynamics.

36 MATERIALS SCIENCE↗