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At least 181 records · Page 10

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DOE OSTI2024

How spin relaxes and dephases in bulk halide perovskites

Abstract Spintronics in halide perovskites has drawn significant attention in recent years, due to their highly tunable spin-orbit fields and intriguing interplay with lattice symmetry. Here, we perform first-principles calculations to determine the spin relaxation time ( T 1 ) and ensemble spin dephasing time ( $${T}_{2}^{*}$$ T 2 * ) in a prototype halide perovskite, CsPbBr 3 . To accurately capture spin dephasing in external magnetic fields we determine the Landé g -factor from first principles and take it into account in our calculations. These allow us to predict intrinsic spin lifetimes as an upper bound for experiments, identify the dominant spin relaxation pathways, and evaluate the dependence on temperature, external fields, carrier density, and impurities. We find that the Fröhlich interaction that dominates carrier relaxation contributes negligibly to spin relaxation, consistent with the spin-conserving nature of this interaction. Our theoretical approach may lead to new strategies to optimize spin and carrier transport properties.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2024

Chiral spin-liquid-like state in pyrochlore iridate thin films

The pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topological phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all-out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y 2 Ir 2 O 7 thin films (thickness ≤30 nm) persisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 125 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

NaRuO2: Kitaev-Heisenberg exchange in triangular-lattice setting

Abstract Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90° metal-ligand-metal links, $${t}_{2g}^{5}$$ t 2 g 5 transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the $${t}_{2g}^{5}$$ t 2 g 5 triangular-lattice material NaRuO 2 , recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. Quantum chemical analysis combined with subsequent effective spin model simulations provide perspective onto the nature of the experimentally observed quantum spin liquid—it seemingly implies fairly large antiferromagnetic second-neighbor isotropic exchange, and the atypical proximity to ferromagnetic order is related to ferromagnetic nearest-neighbor Heisenberg coupling.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Using magnetic dynamics to measure the spin gap in a candidate Kitaev material

Spin-orbit entangled materials have attracted widespread interest due to the novel magnetic phenomena arising from the interplay between spin-orbit coupling and electronic correlations. However, the intricate nature of spin interactions within Kiteav materials complicates the precise measurement of low-energy spin excitations. Using Na 2 Co 2 TeO 6 as an example, we study these low-energy spin excitations using the time-resolved resonant elastic x-ray scattering (tr-REXS). Our observations unveil remarkably slow spin dynamics at the magnetic peak, whose recovery timescale is several nanoseconds. This timescale aligns with the extrapolated spin gap of ~1 μeV, obtained by density matrix renormalization group (DMRG) simulations in the thermodynamic limit. The consistency demonstrates the efficacy of tr-REXS in discerning low-energy spin gaps inaccessible to conventional spectroscopic techniques.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Beyond Kitaev physics in strong spin-orbit coupled magnets

We review the recent advances and current challenges in the field of strong spin-orbit coupled Kitaev materials, with a particular emphasis on the physics beyond the exactly-solvable Kitaev spin liquid point. To this end, we present a comprehensive overview of the key exchange interactions in candidate materials with a specific focus on systems featuring effective J eff = 1/2 magnetic moments. This includes, but not limited to, 5d 5 iridates, 4d 5 ruthenates and 3d 7 cobaltates. Our exploration covers the microscopic origins of these interactions, along with a systematic attempt to map out the most intriguing correlated regimes of the multi-dimensional parameter space. Our approach is guided by robust symmetry and duality transformations as well as insights from a wide spectrum of analytical and numerical studies. We also survey higher spin Kitaev models and recent exciting results on quasi-one-dimensional models and discuss their relevance to higher-dimensional models. Lastly, we highlight some of the key questions in the field as well as future directions.

Kitaev materials↗

DOE OSTI2025

Correlation-driven topological band inversion in VSe 2

Recent spectroscopic studies have uncovered topological surface states and band inversion in 1⁢T−VSe 2 , positioning this material at the intersection of correlated electron physics and nontrivial band topology. While previous interpretations attribute these features to surface strain, the microscopic origin of the topological band structure remains unresolved. Here, we present an alternative explanation based on electronic correlations, showing that a negative effective Hubbard interaction (𝑈 eff < 0) applied to the Se 4p orbitals can reproduce the experimentally observed band inversion at both the Γ and 𝑀 points. Using density functional theory (DFT) calculations with orbital-selective interactions, we demonstrate that this approach naturally gives rise to topological surface states without invoking structural distortions. In conclusion, our results highlight the crucial role of ligand orbital correlations in shaping band topology and provide a novel framework for understanding and engineering topological phases in chalcogenide-based quantum materials.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Tuning the magnetic properties of the spin-split antiferromagnet MnTe through pressure

The hexagonal antiferromagnet MnTe has attracted enormous interest as a prototypical example of a spin-compensated magnet in which the combination of crystal and spin symmetries lifts the spin degeneracy of the electron bands without the need for spin-orbit coupling, a phenomenon called nonrelativistic spin splitting (NRSS). Subgroups of NRSS are determined by the specific spin-interconverting symmetry that connects the two opposite-spin sublattices. In MnTe, this symmetry is rotation, leading to the subgroup with spin splitting away from the Brillouin zone center, often called altermagnetism. MnTe also has the largest spontaneous magnetovolume effect of any known antiferromagnet, implying strong coupling between the magnetic moment and volume. This magnetostructural coupling offers a potential knob for tuning the spin-splitting properties of MnTe. Here, we use neutron diffraction with in situ applied pressure to determine the effects of pressure on the magnetic properties of MnTe and further explore this magnetostructural coupling. We find that applying pressure significantly increases the Néel temperature, but decreases the ordered magnetic moment. We explain this as a consequence of strengthened magnetic exchange interactions under pressure, resulting in higher 𝑇 N , with a simultaneous reduction of the local moment of individual Mn atoms, described here via density functional theory. This reflects the increased orbital hybridization and electron delocalization with pressure. In conclusion, these results shed light on the competition between magnetic exchange interactions and the strength of individual magnetic moments and show that the magnetic properties of MnTe can be controlled by pressure, opening the door to improved properties for spintronic applications through tuning via physical or chemical pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Dipolar and quadrupolar correlations in the 5⁢𝑑 2 Re-based double perovskites Ba 2 ⁢ YReO 6 and Ba 2 ⁢ ScReO 6

Double perovskites containing heavy transition-metal ions are an important family of compounds for the study of the interplay between electron correlation and spin-orbit coupling. Here, by combining magnetic susceptibility, heat capacity, and neutron scattering measurements, we investigate the dipolar and quadrupolar correlations in two prototypal rhenium-based double perovskite compounds Ba 2 ⁢YReO 6 and Ba 2 ⁢ ScReO 6 . A type-I dipolar antiferromagnetic ground state with a propagation vector 𝐪 = (0,0,1) is observed in both compounds. At temperatures above the magnetic transitions, a quadrupolar ordered phase is identified. Weak spin excitations, which are gapped at low temperatures and softened in the correlated paramagnetic phase, are explained using a minimal model that considers both the dipolar and quadrupolar interactions. In conclusion, at larger wave vectors, we observe dominant phonon excitations that are well described by density functional calculations.

Omar, Otkur [University of Science and Technology ↗

DOE OSTI2025

Weak Ferromagnetism in Altermagnets from Alternating 𝑔-Tensor Anisotropy

Altermagnets are magnetic materials with antiferromagnetic spin ordering but exhibit ferromagnetic properties. Understanding the microscopic origin of the latter is a central problem. Ferromagnetlike properties such as the anomalous Hall effect are linked with weak ferromagnetism, whose microscopic origin in altermagnets remains unclear however. We show theoretically that the alternating 𝑔-tensor anisotropy in altermagnets can induce weak orbital ferromagnetism even when the Dzyaloshinskii-Moriya interaction is forbidden. We demonstrate this mechanism to explain weak ferromagnetism for both collinear and noncollinear spin altermagnets. Our findings provide new insights into the origin of weak ferromagnetism and suggest orbital-based ways for manipulating magnetic configurations in altermagnets.

Altermagnetism↗

DOE OSTI2024

Disorder-driven localization and electron interactions in Bi x ⁢ TeI thin films

Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the density of states. Bi x TeI films grown at room temperature and 230 K exhibit dramatic magnetotransport effects due to disorder, localization, and electron correlation effects, including a metal-insulator transition at a composition that depends on growth temperature. The increased disorder caused by growth at 230 K causes the conductivity to decrease by several orders of magnitude for several compositions of Bi x TeI. The transition from metal to insulator with decreasing composition x is accompanied by a decrease in the dephasing length, which leads to the disappearance of the weak-antilocalization effect. Electron-electron interactions cause low temperature conductivity corrections on the metallic side and Efros-Shklovskii variable range hopping on the insulating side, effects which are absent in single crystalline Bi x TeI. Finally, the observation of a tunable metal-insulator transition and the associated strong localization and quantum effects in Bi x TeI shows the possibility of tuning spin transport in quantum materials via disorder.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Shubnikov–de Haas oscillations reaching the quantum limit in two-dimensional electron systems at SrTi⁢O 3 (111) interfaces

Transition metal oxides in the (111) orientation have been predicted to harbor topological phases and unconventional quantum states because of their hexagonal crystal symmetry and strong interactions between charge, spin, and orbital degrees of freedom. We report Shubnikov–de Haas oscillations into the quantum limit at magnetic fields up to 35 T in high-mobility (>20 000 c⁢m 2 V −1 s −1 ) two-dimensional electron liquids at (111)-oriented SrTi⁢O 3 interfaces with controllable carrier densities. Spin splitting is observed at low Landau levels, which is attributed to the interplay between the Zeeman splitting and Rashba spin-orbit coupling according to our theoretical modeling, yielding Landé factor 𝑔 = 0.29 and Rashba coefficient 𝛼 = 0.6 meV nm. Furthermore, at high magnetic field after the system reaches the lowest Landau level, the temperature dependence of the resistance shows a metallic to insulating state transition with the magnetoresistance changing significantly from a primarily quadratic to a large linear field dependence.

Magnetoresistance↗

DOE OSTI2025

Electronic anisotropy and rotational symmetry breaking at a Weyl semimetal/spin ice interface

In magnetic pyrochlore materials, the interplay of spin-orbit coupling, electronic correlations, and geometrical frustration gives rise to exotic quantum phases, including topological semimetals and spin ice. While these phases have been observed in isolation, the interface-driven phenomena emerging from their interaction have never been realized previously. Here, we report on the discovery of interfacial electronic anisotropy and rotational symmetry breaking at a heterostructure consisting of the Weyl semimetal Eu 2 Ir 2 O 7 and spin ice Dy 2 Ti 2 O 7 . Subjected to magnetic fields, we unveil a sixfold anisotropic transport response that is theoretically accounted by a Kondo-coupled heterointerface, where the spin ice’s field-tuned magnetism induces electron scattering in the Weyl semimetal’s topological Fermi-arc states. Furthermore, at elevated magnetic fields, we reveal a twofold anisotropic response indicative of the emergence of a symmetry-broken many-body state. This discovery showcases the potential of pyrochlore frustrated magnet/topological semimetal heterostructures in search of emergent interfacial phenomena.

Science & Technology - Other Topics↗

DOE OSTI2025

Sunny.jl

Sunny is a Julia package designed to serve the needs of the quantum magnetism community. It supports the specification of a very broad class of spin models and a diverse suite of numerical solvers. These include powerful methods for simulating spin dynamics both in and out of equilibrium. Uniquely, it features a broad generalization of classical and semiclassical approaches to SU(N) coherent states, which is useful for studying systems exhibiting strong spin-orbit coupling or local entanglement effects. Sunny also offers a well-developed framework for calculating the dynamical spin structure factor, enabling direct comparison with scattering experiments. Ease of use is a priority, with tools for symmetry-guided modeling and interactive visualization.

Dahlbom, David↗

DOE OSTI2025

Sunny.jl: A Julia Package for Spin Dynamics

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2024

Theoretical Studies of Polar Systems near Ferroelectrics Quantum Critical Points (Final Report)

The PI has studied emergent quantum phases near polar quantum critical points. A first challenge was to develop a theoretical description of the observation that many polar materials undergo classical first-order transitions while displaying quantum criticality. The possibility of novel metallic states near polar quantum critical points was then studied; identification and characterization of a non-Fermi liquid phase was made in a multiband system. In dilute quantum critical polar metals, electronic coupling to polar energy fluctuations was shown to result in attractive electron-electron interactions and to superconductivity. A spin-phonon resonance measurement in applied magnetic field was proposed to determine the magnitude of spin-orbit mediated electron-phonon coupling; furthermore in the polar phase new field-induced phonon collective modes were identified with specific signatures for experiment. In order to keep current, the PI has also worked on other types of strongly correlated problems. With experimental groups, she has developed phenomenologies to identify and explain observed dynamical critical behaviors. The PI has studied the interplay of topology, fractionalization and deconfinement in a minimalist spin model. An emergent phase transition in a 1 + 1 frustrated spin nanotube has also been identified and characterized. The PI has also begun exploring criticality out of equilibrium, characterizing rich dynamical phases associated with photoinduced polar transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1977

Mercury's core - The effect of obliquity on the spin-orbit constraints

Constraints were previously placed on various properties of a Mercurian liquid core for compatibility with Mercury's escape from the stable spin-orbit resonance with the spin angular velocity equal to twice the orbital mean motion (the 2n resonance), under the assumption that the planet's obliquity was nearly zero at the time of resonance passage. Capture probabilities at arbitrary nonzero obliquities for the 2n resonance are determined for the cases where the core is strongly or weakly coupled to the mantle. It is found that the capture probabilities are reduced to below unity for all core-fluid viscosities in the weak-coupling limit, but are almost unchanged in the strong-coupling limit. The reduction in capture probability is attributed to reduction of the mantle's spin angular velocity by the core-mantle interaction, which would also reduce the obliquity to negligibly small values before the 2n resonance was even reached. It is concluded that the constraints on the core may still be maintained since Mercury most likely passed through the 2n resonance with nearly zero obliquity.

Peale, S. J.↗

NASA NTRS1988

Recent advances at NASA in calculating the electronic spectra of diatomic molecules

Advanced entry vehicles, such as the proposed Aero-assisted Orbital Transfer Vehicle, provide new and challenging problems for spectroscopy. Large portions of the flow field about such vehicles will be characterized by chemical and thermal nonequilibrium. Only by considering the actual overlap of the atomic and rotational lines emitted by the species present can the impact of radiative transport within the flow field be assessed correctly. To help make such an assessment, a new computer program is described that can generate high-resolution, line-by-line spectra for any spin-allowed transitions in diatomic molecules. The program includes the matrix elements for the rotational energy and distortion to the fourth order; the spin-orbit, spin-spin, and spin-rotation interactions to first order; and the lambda splitting by a perturbation calculation. An overview of the Computational Chemistry Branch at Ames Research Center is also presented.

Whiting, Ellis E.↗

NASA NTRS1993

Point form relativistic quantum mechanics and relativistic SU(6)

The point form is used as a framework for formulating a relativistic quantum mechanics, with the mass operator carrying the interactions of underlying constituents. A symplectic Lie algebra of mass operators is introduced from which a relativistic harmonic oscillator mass operator is formed. Mass splittings within the degenerate harmonic oscillator levels arise from relativistically invariant spin-spin, spin-orbit, and tensor mass operators. Internal flavor (and color) symmetries are introduced which make it possible to formulate a relativistic SU(6) model of baryons (and mesons). Careful attention is paid to the permutation symmetry properties of the hadronic wave functions, which are written as polynomials in Bargmann spaces.

Klink, W. H.↗