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At least 271 records · Page 15

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DOE OSTI2026

Nuclear spin coherence and RKKY interaction in superconducting Nb 3 Sn

We have investigated the normal and superconducting states of the technologically important compound Nb$_3$Sn using $^{93}$Nb nuclear magnetic resonance. From spin-lattice relaxation we find strong suppression of the zero-temperature superconducting order parameter by magnetic field. Additionally we have identified an anomalously large electron-nuclear exchange interaction from spin-spin relaxation measurements, an order of magnitude beyond that of the nuclear dipolar coupling. This RKKY interaction evolves from normal to superconducting states, becoming essentially Lorentzian in the low temperature limit.

Zhai, Gan [Northwestern U.] (ORCID:000000016623956↗

DOE OSTI2018

Structure and Magnetism Evolution from FeCo Nanoparticles to Hollow Nanostructure Conversion for Magnetic Applications

Hollow nanostructures have witnessed an increasing interest in emerging fields, including energy storage electrodes, nanocatalysis, photonics, and biomedical carriers, etc. The fundamental mechanisms for hollowing conversion chemistry of complex nanoalloys is indispensable to grow such nanostructures with controlled compositions and functionalities. In this contribution, the Kirkendall effect on magnetic Fe-Co bimetallic alloys with the formation of hollow nanostructures for exchange interactions was studied. The rich diffusion kinetics of binary Fe-Co alloys accompanied with its conversion from solid, core-shell to hollow nanostructures were been captured through X-ray spectroscopy and electron microscopy. We further report the cation exchange improved magnetic performance of hollow nanostructures with increasing the crystallinity.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Functional Renormalization Group Study of Superconductivity in Rhombohedral Trilayer Graphene

We employ a functional renormalization group approach to ascertain the pairing mechanism and symmetry of the superconducting phase observed in rhombohedral trilayer graphene. Superconductivity in this system occurs in a regime of carrier density and displacement field with a weakly distorted annular Fermi sea. We find that repulsive Coulomb interactions can induce electron pairing on the Fermi surface by taking advantage of momentum-space structure associated with the finite width of the Fermi sea annulus. Furthermore, the degeneracy between spin-singlet and spin-triplet pairing is lifted by valley-exchange interactions that strengthen under the RG flow and develop nontrivial momentum-space structure. We find that the leading pairing instability is d-wave-like and spin-singlet, and that the theoretical phase diagram versus carrier density and displacement field agrees qualitatively with experiment.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Experimental demonstration and analysis of random field effects in ferromagnet/antiferromagnet bilayers

More than 30 years ago, Malozemoff [Phys. Rev. B 35, 3679 (1987)] hypothesized that exchange interaction at the interface between a ferromagnet (F) and an antiferromagnet (AF) can act as an effective random field, which can profoundly affect the magnetic properties of the system. However, until now this hypothesis has not been directly experimentally tested. We utilize magnetoelectronic measurements to analyze the effective exchange fields at permalloy/CoO interface. Our results cannot be explained in terms of quasiuniform effective exchange fields but are in agreement with the random-field hypothesis of Malozemoff. Finally, the presented approach opens a new route for the quantitative analysis of effective exchange fields and anisotropies in magnetic heterostructures for memory, sensing and computing applications.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Magnetic evolution induced by freezing of Ag + fluctuations in an intercalated transition metal dichalcogenide

Here, we present a comprehensive study on the magnetic evolution processes in AgCrSe 2 using both neutron scattering and theoretical calculations. Additionally, the development of magnetic short-range correlation with decoupled interlayer interactions is confirmed between 55 and 200 K. Significant magneto-crystalline anisotropy energy around 0.1 meV of Cr 3+ is identified that mainly contributes to the robust existence of the in-plane magnetic correlations in the magnetic short-order regime. Furthermore, the indirect superexchange interactions through Cr-Se-Ag-Se-Cr path between the adjacent CrSe 6 octahedral layers is also demonstrated. In combination with the mutual coupling among multiple degrees of freedom (spin, phonon, lattice, and dipole), we reveal the key role of the freezing of Ag + fluctuations in preventing interlayer exchange interactions from decoupling.

36 MATERIALS SCIENCE↗

NASA NTRS1980

Solar wind absorption by Venus

The portion of solar wind interacting with the dayside ionosphere and atmosphere of Venus was determined based on magnetic field fluctuations in the ionosheath and the interaction with the upper neutral atmosphere above the ionopause. Fluctuations with the ratio of the number of particles intersecting the daytide ionopause to the total number of particles of 0.3 suggest that about 0.3% of solar wind may be absorbed. Most of fast H atoms resulting from the charge exchange interactions with the atmosphere escape; some of the energy deposition processes produce observable signatures (such as a narrow Lyman alpha emission region), but penetrating solar wind particles do not control the physical and/or chemical structure of the daytime Venus ionosphere.

Gombosi, T. I.↗

DOE OSTI2025

Thermodynamic origin of the pressure-induced Invar effect: General criterion and experimental study of Fe 68 ⁢Pd 32

Synchrotron X-ray diffraction measurements were performed on 57 Fe 68 Pd 32 at multiple pressures and two temperatures in a diamond-anvil cell. Between 4 and 11GPa, the thermal expansion was zero or slightly negative. This pressure-induced Invar effect was studied further with 57 Fe nuclear forward scattering and nuclear resonant inelastic X-ray scattering to obtain information on the pressure-induced changes in both the magnetization and the phonon density of states. Magnetic entropy and phonon entropy were obtained from these results, the latter with additional measurements from inelastic neutron scattering to account for the contributions from Pd atoms. The dependencies of these entropies on pressure gave the magnetic and phonon contributions to thermal expansion. These canceled in the region of pressure-induced Invar behavior, even though they individually increased by more than a factor of 2 below the Curie pressure. The behavior of phonons gives evidence for spin-phonon interactions in Fe 68 Pd 32 . A general explanation of the pressure-induced Invar effect is presented, showing that Invar behavior is typically expected at a pressure P* below a magnetic transition at pressure PC. The difference in pressure P C − P* scales with the fractional reduction in magnetic exchange interaction divided by the average Gr¨uneisen parameter.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Electronic Origin of Delicate Antiferromagnetism in Fe 𝑥 ⁢NbS 2

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe 𝑥 ⁢NbS 2 is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of 𝑥 𝑐 = 1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the 𝑥𝑐. Moment-carrying Fe 3⁢𝑑 𝑧 2 electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in the TMD layer and rapidly lose coherence above 𝑥 𝑐 due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. Furthermore, these observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.

Angle-resolved photoemission spectroscopy↗

DOE OSTI2026

Edge Reconstruction in a Quantum Spin Hall Insulator

We study interaction-driven edge reconstruction in a quantum spin Hall insulator described by the Bernevig-Hughes-Zhang model with Kanamori-Hubbard interactions using the real-space density matrix renormalization group method in both the grand-canonical and canonical ensembles. For a two-dimensional cylinder with a smooth edge, we identify discrete particle-number transitions that lead to a spin-polarized edge state stabilized by an emergent ferromagnetic exchange interaction. The reconstruction is orbital-selective, occurring predominantly in the 𝑠-orbital channel. Our results reveal a microscopic mechanism for emergent fluctuating moments at the edge that could compromise the topological protection of helical edge states by time-reversal symmetry.

Soni, Rahul [ORNL] (ORCID:0000000317714299)↗

NASA NTRS1986

The importance of atomic and molecular correlation on the bonding in transition metal compounds

The determination of accurate spectroscopic parameters for molecular systems containing transition metal atoms is shown to require extensive data sets and a high level correlation treatment, and techniques and their limitations are considered. Extensive results reported on the transition metal atoms, hydrides, oxides, and dimers makes possible the design of a calculation to correctly describe the mixing of different atomic asymptotes, and to give a correct balance between molecular bonding and exchange interactions. Examples considered include the dipole moment of the 2Delta state of NiH, which can help determine the mixture of 3d(8)4s(2) and 3d(9)4s(1) in the NiH wavefunction, and the bonding in CrO, where an equivalent description of the relative energies associated with the Cr 3d-3d atomic exchange and the Cr-O bond is important.

Bauschlicher, Charles W., Jr.↗

DOE OSTI2020

Suppression of superconducting parameters by correlated quasi-two-dimensional magnetic fluctuations

In this work, we consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside a superconducting state. We assume that the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. A representative example of such material is Rb Eu Fe 4 As 4 . We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is very sensitive to the relation between the magnetic correlation length ξ h and superconducting coherence length ξ s defining the ‘scattering’ ( ξ h < ξ s ) and ‘smooth’ ( ξ h > ξ s ) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this ‘scattering-to-smooth’ crossover for the case of quasi-two-dimensional magnetic fluctuations realized in Rb Eu Fe 4 As 4 . Since the magnetic-scattering energy scale is comparable with the gap in the crossover region, the standard quasiclassical approximation is not applicable and full microscopic treatment is required. We find that the corrections to both the gap and superfluid density increase proportionally to ξ h until it remains much smaller than ξ s . In the opposite limit, when the correlation length exceeds the coherence length both parameters have much weaker dependence on ξ h . Moreover, the gap correction may decrease with increasing of ξ h in the immediate vicinity of the magnetic transition if it is located at temperature much lower than the superconducting transition. We also find that the crossover between the two regimes is unexpectedly broad: The standard scattering approximation becomes sufficient only when ξ h is substantially smaller than ξ s .

36 MATERIALS SCIENCE↗

DOE OSTI2022

Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6

Abstract Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe 2 , a two-dimensional quantum spin Hall insulator, and few-layer Cr 2 Ge 2 Te 6 , an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe 2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.

36 MATERIALS SCIENCE↗

NASA NTRS1991

Effect of patch borders on coercivity in amorphous rare earth-transition metal thin films

The coercivity at the micron scale is a very important property of magneto-optical media. It is a key factor that determines the magnetic domain wall movement and domain reversal. How the coercivity is influenced by a special type of patch borders is discussed. Patch formation is a general phenomenon in growth processes of amorphous rare earth transition metal thin films. Different patches may stem from different seeds and the patch borders are formed when they merge. Though little is known about the exact properties of the borders, we may expect that the exchange interaction at the patch border is weaker than that within a patch, since there is usually a spatial gap between two patches. Computer simulations were performed on a 2-D hexagonal lattice consisting of 37 complete patches with random shape and size. From the series of simulations we may conclude that the domain in the patch with borders of 30 percent exchange strength can expand most easily to the whole lattice, because the exchange strength can expand most easily to the whole lattice, because the exchange strength of the border is not too high to prevent the domain from growing within the patch and it is not too low to prevent the domain from expanding beyond the patch.

Patterson, G.↗

DOE OSTI2022

Engineering infinite-range SU($\textit{n}$) interactions with spin-orbit-coupled fermions in an optical lattice

Here, we study multilevel fermions in an optical lattice described by the Hubbard model with on-site SU($\textit{n}$) -symmetric interactions. We show that in an appropriate parameter regime this system can be mapped onto a spin model with all-to-all SU($\textit{n}$)-symmetric couplings. Raman pulses that address internal spin states modify the atomic dispersion relation and induce spin-orbit coupling, which can act as a synthetic inhomogeneous magnetic field that competes with the SU($\textit{n}$) exchange interactions. We investigate the mean-field dynamical phase diagram of the resulting model as a function of $\textit{n}$ and different initial configurations that are accessible with Raman pulses. Consistent with previous studies for $\textit{n}$ = 2 , we find that for some initial states the spin model exhibits two distinct dynamical phases that obey simple scaling relations with $\textit{n}$. Moreover, for $\textit{n}$ > 2 we find that dynamical behavior can be highly sensitive to initial intraspin coherences. Our predictions are readily testable in current experiments with ultracold alkaline-earth-metal(-like) atoms.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Quantum Spin Torque Driven Transmutation of an Antiferromagnetic Mott Insulator

The basic model of spin-transfer torque (STT) in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncollinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize Néel order in equilibrium, ↑ ↓ ∙∙∙ ↑ ↓, and their STT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) are strongly electron-correlated antiferromagnetic Mott insulators (AFMIs) whose localized spins form a ground state quite different from the unentangled Néel state | ↑ ↓ ∙∙∙ ↑ ↓$\rangle$. The true ground state is entangled by quantum spin fluctuations, leading to the expectation value of all localized spins being zero, so that LLG dynamics of classical vectors of fixed length rotating due to STT cannot even be initiated. Instead, a fully quantum treatment of both conduction electrons and localized spins is necessary to capture the exchange of spin angular momentum between them, denoted as quantum STT. We use a recently developed time-dependent density matrix renormalization group approach to quantum STT to predict how injection of a spin-polarized current pulse into a normal metal layer coupled to an AFMI overlayer via exchange interaction and possibly small interlayer hopping—mimicking, e.g., topological-insulator/NiO bilayer employed experimentally—will induce a nonzero expectation value of AFMI localized spins. This new nonequilibrium phase is a spatially inhomogeneous ferromagnet with a zigzag profile of localized spins. The total spin absorbed by AFMI increases with electron-electron repulsion in AFMIs, as well as when the two layers do not exchange any charge.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Emerging many-body effects in semiconductor artificial graphene with low disorder

Abstract The interplay between electron–electron interactions and the honeycomb topology is expected to produce exotic quantum phenomena and find applications in advanced devices. Semiconductor-based artificial graphene (AG) is an ideal system for these studies that combines high-mobility electron gases with AG topology. However, to date, low-disorder conditions that reveal the interplay of electron–electron interaction with AG symmetry have not been achieved. Here, we report the creation of low-disorder AG that preserves the near-perfection of the pristine electron layer by fabricating small period triangular antidot lattices on high-quality quantum wells. Resonant inelastic light scattering spectra show collective spin-exciton modes at the M-point's nearly flatband saddle-point singularity in the density of states. The observed Coulomb exchange interaction energies are comparable to the gap of Dirac bands at the M-point, demonstrating interplay between quasiparticle interactions and the AG potential. The saddle-point exciton energies are in the terahertz range, making low-disorder AG suitable for contemporary optoelectronic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Derivation of low-energy Hamiltonians for heavy-fermion materials

Here, by utilizing a multiorbital periodic Anderson model with parameters obtained from ab initio band structure calculations, combined with degenerate perturbation theory, we derive effective Kondo-Heisenberg and spin Hamiltonians that capture the interaction among the effective magnetic moments. This derivation encompasses fluctuations via both nonmagnetic 4⁢𝑓 0 and magnetic 4⁢𝑓 2 virtual states, and its accuracy is confirmed through comparison with experimental data obtained from CeIn 3 . The significant agreement observed between experimental results and theoretical predictions underscores the potential of deriving minimal models from first-principles calculations for achieving a quantitative description of 4⁢𝑓 materials. Moreover, our microscopic derivation unveils the underlying origin of anisotropy in the exchange interaction between Kramers doublets, shedding light on the conditions under which this anisotropy may be weak compared to the isotropic contribution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

High-field magnetic phase diagrams of the 𝑅⁢Mn 6 ⁢Sn 6 (𝑅=Gd–Tm) kagome metals

𝑅⁢Mn 6 ⁢Sn 6 (𝑅=Y, Gd–Lu) kagome metals are promising materials hosting flat electronic bands and Dirac points that interact with magnetism. The coupling between the two magnetic 𝑅 and Mn sublattices can drive complex magnetic states with potential consequences for spin and charge transport and other topological properties. Here, in this work, we use a detailed magnetic Hamiltonian to calculate and predict the magnetic phase diagrams for 𝑅⁢Mn 6 ⁢Sn 6 kagome metals within the mean-field approximation. These calculations reveal a variety of collinear, noncollinear, and noncoplanar phases that arise from competition between various interlayer magnetic exchange interactions and magnetic anisotropies of the 𝑅 and Mn ions. We enumerate these phases and their magnetic space groups for future analysis of their impact on topological and trivial bands near the Fermi surface.

kagome metal↗