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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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245 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Twistronics of Janus transition metal dichalcogenide bilayers

Twisted multilayers of two-dimensional (2D) materials are an increasingly important platform for investigating quantum phases of matter, and in particular, strongly correlated electrons. The moiré pattern introduced by the relative twist between layers creates effective potentials of long wavelength, leading to electron localization. However, in contrast to the abundance of 2D materials, few twisted heterostructures have been studied until now. Here, in this study, we develop a first-principles continuum theory to study the electronic bands introduced by moire patterns of twisted Janus transition metal dichalcogenide (TMD) homo- and heterobilayers. The model includes lattice relaxation, stacking-dependent effective mass, and Rashba spin-orbit coupling. We then perform a high-throughput generation and characterization of DFT-extracted continuum models for more than a hundred possible combinations of materials and stackings. Our model predicts that the moiré physics and emergent symmetries depend on chemical composition, vertical layer orientation, and twist angle, so that the miniband wave functions can form triangular, honeycomb, and kagome networks. Rashba spin-orbit effects, peculiar of these systems, can dominate the moiré bandwidth at small angles. Our work enables detailed investigation of Janus twisted heterostructures, allowing the discovery and control of novel electronic phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Effects of Na deficiency on spin dynamics in the Mott insulating Na 4−x Ir 3 O 8

Here, we report on Raman scattering and 23 Na nuclear magnetic resonance (NMR) investigations of j eff = 1/2 hyperkagome antiferromagnet Na 4−x Ir 3 O 8 , which lies in the Mott insulating state. Our Raman scattering experiments unveil remarkable parallels between the magnetic excitations of the pristine Na 4 Ir 3 O 8 and the Kitaev honeycomb material Na 2 IrO 3 , which are characterized by dominant fractional spinon excitations. In the case of moderate Na-deficient Na 4−x Ir 3 O 8 , however, a substantial suppression of the magnetic excitations is observed, alluding to the notable influence of charge fluctuations on spin dynamics. In addition, our site-specific 23 Na NMR measurements offer further insights into the spin dynamics when a minor concentration of holes is introduced into a spin-orbit coupled Mott insulator. Specifically, the spin-lattice relaxation rate 1/ T 1 reveals the emergence of pseudogaplike correlations at the Na(2) site, alongside a critical slowing down behavior at the Na(1) and Na(3) sites. These findings showcase the intricate interplay between itinerant holes and magnetic correlations in a spin-liquid-like background.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic structure and spin dynamics of the quasi-two-dimensional antiferromagnet Zn-doped copper pyrovanadate

We report magnetic properties of the antiferromagnet Zn x Cu 2-x V 2 O 7 (ZnCVO) with x ≈ 0.06 have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that our ZnCVO samples are isostructural with β-Cu 2 V 2 O 7 (β-CVO) with a small deviation in the lattice parameters. Macroscopic magnetic property measurements also confirm the similarity between the two compounds. The Cu 2+ spins were found to align along the crystallographic c axis, antiparallel to their nearest neighbors connected by the leading exchange interaction J 1 . Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the bc plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composed of J 1 , J 5 , and J 6 interactions, four bonds per one spin site, coupled by weak interplane interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

CH 4 combustion over a commercial Pd/CeO 2 -ZrO 2 three-way catalyst: Impact of thermal aging and sulfur exposure

Thermal aging and sulfur poisoning are major problems influencing the efficiency and lifetime of natural gas three-way catalysts (TWCs) during real-world operation. In the present study, thermal aging and sulfur-induced deactivations on CH 4 combustion were investigated over commercial monolithic honeycombs, i.e., Pd/CexZr 1-x O 2 (CZ)-based TWCs that are thermally aged in the bench (HTA), followed by being operated behind the stoichiometric natural gas engine in high sulfur fuel (HTA + S). In this work, both HTA and HTA + S samples show degraded performance in CH 4 combustion relative to the fresh one, which are caused by the reconstruction of Pd species induced by thermal aging and the decreased reducibility of the support. The 18 O 2 labelling experiments show that in all cases CH 4 combustion proceeds via a MvK mechanism in which lattice O (OL) plays a key role. The sulfation aging (HTA + S sample) promotes the activation of CH 4 and O 2 by forming Pd δ+ -(SO$_4^{2-}$) δ- and Ce 3+ -VO couples at the interface as compared to HTA sample. However, such a promotion effect could be compromised by the blocking effect of sulfate on the exchange of O L from the bulk to the surface and across the surface. The fundamental understanding of thermal and S deactivation mechanisms from this study could help to predict a real-world lifetime use curve of natural gas TWCs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Magnetic order in the van der Waals magnet VCl 3

Here, we investigated the structural and magnetic properties of single-crystalline VCl 3 , a newly synthesized member of the vanadium trihalide family. High-quality single crystals were grown by the chemical vapor transport method, and their behavior was characterized using neutron diffraction and thermodynamic measurements. We show that VCl 3 crystallizes in the BiI 3 -type structure at room temperature and undergoes a structural phase transition at 𝑇 𝑆 = 103.7⁢(5)⁢K that lowers the lattice symmetry, followed by a zigzag antiferromagnetic order with a propagation vector 𝑘 = (0,0.5,1) below 𝑇 𝑁 = 21.8⁢(1)⁢K. Neutron diffraction experiments indicate that the ordered moments are canted by approximately 21° away from the 𝑐 axis toward the 𝑎 axis, yielding a total moment of approximately 1.09⁢(2) ⁢𝜇 B /V 3+ . Field-dependent magnetization along the 𝑐 axis exhibits a half magnetization plateau, indicative of a field-stabilized fractional state. These results establish VCl 3 as a new platform for exploring structural transitions, anisotropic magnetism, and field-induced phases in vanadium-based honeycomb magnets.

Kao, Zeyu [Fudan Univ., Shanghai (China)]↗
DOE OSTI2025

Hidden frustration in the triangular-lattice antiferromagnet NdCd 3 ⁢P 3

We report a study of the magnetic ground state and crystal electric field (CEF) scheme in the triangular-lattice antiferromagnet NdCd 3 ⁢P 3 . Combined neutron scattering, magnetization, and heat capacity measurements demonstrate that the Nd 3+ moments occupying the triangular lattice in this material harbor hidden signs of frustration not detected in typical Curie-Weiss-based parametrization of the frustration index (𝑓 = Θ 𝐶⁢𝑊 /𝑇 𝑁 ). This is evidenced by a zero-field splitting of the Kramers' ground state and first excited state doublets at temperatures far in excess of 𝑇 𝑁 as well as signatures of low-energy fluctuations for 𝑇 ≫ 𝑇 𝑁 . As a result, a suppression of the zero-field ordered moment relative to its field saturation value is observed, and the impacts of this magnetic frustration as well as the coexisting bond frustration in the CdP honeycomb network on the physical properties of NdCd 3 ⁢P3 are discussed.

Frustrated magnetism↗
DOE OSTI2022

Quantum spin liquids bootstrapped from Ising criticality in Rydberg arrays

Arrays of Rydberg atoms constitute a highly tunable, strongly interacting venue for the pursuit of exotic states of matter. We develop a strategy for accessing a family of fractionalized phases known as quantum spin liquids in two-dimensional Rydberg arrays. We specifically use effective field theory methods to study arrays assembled from Rydberg chains tuned to an Ising phase transition that famously hosts emergent fermions propagating within each chain. This highly entangled starting point allows us to naturally access spin liquids familiar from Kitaev's honeycomb model—albeit from an entirely different framework. In particular, we argue that finite-range repulsive Rydberg interactions, which frustrate nearby symmetry-breaking orders, can enable coherent propagation of emergent fermions between the chains in which they were born. Delocalization of emergent fermions across the full two-dimensional Rydberg array yields a gapless Z 2 spin liquid with a single massless Dirac cone. Here, the Rydberg occupation numbers exhibit universal power-law correlations that provide a straightforward experimental diagnostic of this phase. We further show that explicitly breaking symmetries perturbs the gapless spin liquid into gapped, topologically ordered descendants: Breaking lattice symmetries generates toric-code topological order, whereas introducing Floquet-mediated chirality generates non-Abelian Ising topological order. In the toric-code phase, we analytically construct microscopic incarnations of non-Abelian defects, which can be created and transported by dynamically controlling the atom positions in the array. Our work suggests that appropriately tuned Rydberg arrays provide a cold-atoms counterpart of solid-state “Kitaev materials” and, more generally, it spotlights a different angle for pursuing experimental platforms for Abelian and non-Abelian fractionalization.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Complex-Concentrated Anion Doping Enables Ultra-Stable Lattice Oxygen and Structural Integrity in Lithium-Rich Layered Oxide Cathodes

Lithium- and manganese-rich layered oxides (LMR) stand out as next-generation lithium-ion cathode chemistries, which harness both transition-metal and lattice-oxygen redox processes to deliver exceptional capacity and energy density. However, their full potential is hindered by intrinsic oxygen instability and structural degradation, resulting in pronounced voltage fade and capacity decay. Here, we present a complex-concentrated anion-doping paradigm in which multiple anions, F, Br, and S, are incorporated into the oxygen sublattice to enhance oxygen-redox and structural stability. X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy confirm ultra-stable local oxygen coordination environments during long-term cycling, with detrimental phase transformations and oxygen-loss-induced cavitation dramatically inhibited. Notably, we show that the characteristic LiTM6 transition metal (TM) honeycomb ordering is preserved even after electrochemical cycling. Concurrently, this strategy yields an unprecedented volume change of only 0.63% upon charging to 4.8 V vs. Li+/Li, achieving the first zero-strain LMR cathode. The resulting LMR cathode delivers ultralow voltage fade (1 mV per cycle during the first 100 cycles and becomes negligible in subsequent cycles) and outstanding energy retention (93% after 200 cycles) in a pouch cell configuration. Our complex-concentrated anion-doping concept establishes a broadly applicable strategy for resolving chemo-mechanical failure mechanisms in ceramic intercalation electrodes for next-generation energy storage.

Li-ion batteries↗
DOE OSTI2021

Higher-order topology induced by structural buckling

Abstract Higher-order topological insulator (HOTI) states, such as two-dimension (2D) HOTI featured with topologically protected corner modes at the intersection of two gapped crystalline boundaries, have attracted much recent interest. However, the physical mechanism underlying the formation of HOTI states is not fully understood, which has hindered our fundamental understanding and discovery of HOTI materials. Here we propose a mechanistic approach to induce higher-order topological phases via structural buckling of 2D topological crystalline insulators (TCIs). While in-plane mirror symmetry is broken by structural buckling, which destroys the TCI state, the combination of mirror and rotation symmetry is preserved in the buckled system, which gives rise to the HOTI state. We demonstrate that this approach is generally applicable to various 2D lattices with different symmetries and buckling patterns, opening a horizon of possible materials to realize 2D HOTIs. The HOTIs so generated are also shown to be robust against buckling height fluctuation and in-plane displacement. A concrete example is given for the buckled $\beta $-Sb monolayer from first-principles calculations. Our finding not only enriches our fundamental understanding of higher-order topology, but also opens a new route to discovering HOTI materials.

42 ENGINEERING↗
DOE OSTI2025

Electronic structure of the honeycomb iridate Cu 2 ⁢IrO 3 at high pressure

Cu 2 IrO 3 has attracted recent interest due to its proximity to the Kitaev quantum spin liquid state and the complex structural response observed at high pressures. We use x-ray spectroscopy and scattering as well as electrical transport techniques to unveil the electronic structure of Cu 2 IrO 3 at ambient and high pressures. Despite featuring a Ir 4+ J eff = 1/2 state at ambient pressure, Ir L 3 -edge resonant inelastic x-ray scattering reveals broadened electronic excitations that point to the importance of Ir 5d-Cu 3d interaction. High pressure first drives an Ir-Ir dimer state with collapsed < L · S > and < L z >/< S z >, signaling the formation of 5d molecular orbitals. A novel Cu → Ir charge transfer is observed above 30 GPa at low temperatures, leading to an approximate Ir 3+ and Cu 1.5+ valence, with persistent insulating electrical transport seemingly driven by charge segregation of Cu 1+ /Cu 2+ ions into distinct sites. Concomitant x-ray spectroscopy and diffraction measurements through different thermodynamic paths demonstrate a strong electron-lattice coupling, with J eff = 1/2 and Ir 3+ /Cu 1.5+ electronic states occurring only in phases 1 and 5, respectively. Remarkably, the charge-transfer state can only be reached if Cu 2 IrO 3 is pressurized at low temperature, suggesting that phonons play an important role in the inhibiting this phase. Furthermore, these results point to the choice of thermodynamic path across interplanar collapse transition as a key parameter to access novel states in intercalated iridates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Ultrafast formation of a transient two-dimensional diamondlike structure in twisted bilayer graphene

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, AB', or ABC stacking when directly converting graphite into sp 3 bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and AB' stacking configurations, we report the ultrafast formation of a transient two-dimensional diamondlike structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photoinduced phase transition is evidenced by the appearance of bond lengths of 1.94 and 3.14 Å in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first-principles calculation indicate that sp 3 bonds nucleate at AA and AB' stacked areas in a moiré pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗