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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 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Possible Quantum Paramagnetism in Compressed Sr 2 IrO 4

The result of compression on the magnetic ground state of Sr 2 IrO 4 is studied with x-ray resonant techniques in the diamond anvil cell. The weak interlayer exchange coupling between square-planar 2D IrO 2 layers is easily modified upon compression, with a crossover between magnetic structures around 7 GPa mimicking the effect of an applied magnetic field at ambient pressure. Higher pressures drive an order-disorder magnetic phase transition with no magnetic order detected above 17–20 GPa. The persistence of strong exchange interactions between J eff = 1 / 2 magnetic moments within the insulating IrO 2 layers up to at least 35 GPa points to a highly frustrated magnetic state in compressed Sr 2 IrO 4 , opening the door for realization of novel quantum paramagnetic phases driven by extended 5 d orbitals with entangled spin and orbital degrees of freedom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn 3 P

Antiferromagnet Mn 3 P with Neel temperature T N =30 K is composed of Mn tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at ~1.6 GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn 3 P, which is most likely effected by the underlying frustration effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Helical Edge States and Quantum Phase Transitions in Tetralayer Graphene

Helical conductors with spin-momentum locking are promising platforms for Majorana fermions. Here we report observation of two topologically distinct phases supporting helical edge states in charge neutral Bernal-stacked tetralayer graphene in Hall bar and Corbino geometries. As the magnetic field B ⊥ and out-of-plane displacement field D are varied, we observe a phase diagram consisting of an insulating phase and two metallic phases, with 0, 1, and 2 helical edge states, respectively. These phases are accounted for by a theoretical model that relates their conductance to spin-polarization plateaus. Transitions between them arise from a competition among interlayer hopping, electrostatic and exchange interaction energies. Finally, our work highlights the complex competing symmetries and the rich quantum phases in few-layer graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Observing the Suppression of Superconductivity in RbEuFe 4 As 4 by Correlated Magnetic Fluctuations

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe 4 As 4 in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. Furthermore, these results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe 4 As 4 , with important implications for future investigations of physical phenomena arising from the interplay between them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Flat-Band-Enabled Triplet Excitonic Insulator in a Diatomic Kagome Lattice

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure of narrow-gap semiconductors towards exciton formation. Here we show that the flat valence and conduction bands of a Yin-Yang Kagome lattice, as exemplified in a superatomic graphene lattice, conspire to enable an interesting state of triplet EI, based on first-principles Density Functional Theory (DFT) calculations combined with many-body GW and Bethe-Salpeter Equation (BSE). As an intrinsic property of topological flat bands, highly localized electron and hole wavefunctions significantly reduce the screening and enhance the exchange interaction, leading to an unusually high triplet exciton binding energy (~1.2 eV) exceeding the GW band gap by ~0.2 eV and a huge singlet-triplet splitting of ~0.4 eV. Here, the flat-bands-enabled triplet EI state also points to the possibility of complete population inversion between the two topological flat bands for the realization of excited quantum spin Hall effect. Our findings enrich once again the intriguing physics of flat bands, which has drawn broad interest.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Dynamical Scaling as a Signature of Multiple Phase Competition in Yb 2⁢ Ti 2 ⁢O 7

Yb 2 ⁢Ti 2 ⁢O 7 is a celebrated example of a pyrochlore magnet with highly frustrated, anisotropic exchange interactions. To date, attention has largely focused on its unusual, static properties, many of which can be understood as coming from the competition between different types of magnetic order. Here, in this work, we use inelastic neutron scattering with exceptionally high energy resolution to explore the dynamical properties of Yb 2 ⁢Ti 2 ⁢O 7 . We find that spin correlations exhibit dynamical scaling, analogous to behavior found near to a quantum critical point. We show that the observed scaling collapse can be explained within a phenomenological theory of multiple-phase competition, and confirm that a scaling collapse is also seen in semiclassical simulations of a microscopic model of Yb 2 ⁢Ti 2 ⁢O 7 . These results suggest that dynamical scaling may be general to systems with competing ground states.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Line-Graph Approach to Spiral Spin Liquids

Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by subdimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is nonbipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases. As examples, we examine neutron scattering experiments performed on two spinel compounds, ZnCr 2 Se 4 and CuInCr 4 Se 8 , to demonstrate the feasibility of this new approach and expose its possible limitations in experimental realizations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Continuous Spin Excitations in the Three-Dimensional Frustrated Magnet K 2 Ni 2 ⁢(SO 4 ) 3

Continuous spin excitations are widely recognized as one of the hallmarks of novel spin states in quantum magnets, such as quantum spin liquids (QSLs). Here, we report the observation of such kind of excitations in K 2 Ni 2 (SO 4 ) 3 , which consists of two sets of intersected spin-1 (Ni 2+ ) trillium lattices. Our inelastic neutron scattering measurement on single crystals clearly shows a dominant excitation continuum, which exhibits a distinct temperature-dependent behavior from that of spin waves and is rooted in strong quantum spin fluctuations. Further using the self-consistent-Gaussian-approximation method, we determine that the fourth- and fifth nearest-neighbor exchange interactions are dominant. These two bonds together form a unique three-dimensional network of corner-sharing tetrahedra, which we name as a “hypertrillium” lattice. In conclusion, our results provide direct evidence for the existence of QSL features in K 2 Ni 2 (SO 4 ) 3 and highlight the potential for the hypertrillium lattice to host frustrated quantum magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Spin Polarons and Ferromagnetism in Doped Dilute Moiré-Mott Insulators

Moiré heterostructures of transition metal dichalcogenides (TMDs) exhibit Mott-insulating behavior both at half filling as well as at fractional fillings, where electronic degrees of freedom form self-organized Wigner crystal states. An open question concerns magnetic states obtained by lifting the pseudospin-1/2 degeneracy of these states at lowest temperatures. While at half filling virtual hopping is expected to induce (weak) antiferromagnetic exchange interactions, these are strongly suppressed when considering dilute filling fractions. We argue that, instead, a small concentration of doped electrons leads to the formation of spin polarons, inducing ferromagnetic order at experimentally relevant temperatures, consistent with recently observed ferromagnetic states in moiré TMD systems. Here, we predict explicit signatures of polaron formation in the magnetization profile.

Kondo lattice model↗
DOE OSTI2025

Giant Nonreciprocity and Gyration through Modulation-Induced Hatano-Nelson Coupling in Integrated Photonics

Asymmetric energy exchange interactions, also known as Hatano-Nelson type couplings, enable the study of non-Hermitian physics and associated phenomena like the non-Hermitian skin effect and exceptional points (EPs). Since these interactions are by definition nonreciprocal, there have been very few options for implementations in integrated photonics. Here, in this work, we show that asymmetric couplings are readily achievable in integrated photonic systems through time-domain dynamic modulation. We experimentally study this concept using a two-resonator photonic molecule produced in a lithium niobate on insulator platform that is electro-optically modulated by rf stimuli. We demonstrate the dynamic tuning of the Hatano-Nelson coupling between the resonators, surpassing the asymmetry that has been achieved in previous work, to reach an EP for the first time. We are additionally able to flip the relative sign of the couplings for opposite directions by going past the EP. Using this capability, we show that the through-chain transport can be configured to exhibit both giant (~60 dB) optical contrast as well as photonic gyration or nonreciprocal π phase contrast.

42 ENGINEERING↗
DOE OSTI2018

Performance of the strongly constrained and appropriately normed density functional for solid-state materials

Constructed to satisfy 17 known exact constraints for a semilocal density functional, the strongly constrained and appropriately normed (SCAN) meta-generalized-gradient-approximation functional has shown early promise for accurately describing the electronic structure of molecules and solids. One open question is how well SCAN predicts the formation energy, a key quantity for describing the thermodynamic stability of solid-state compounds. To answer this question, we perform an extensive benchmark of SCAN by computing the formation energies for a diverse group of nearly 1000 crystalline compounds for which experimental values are known. Due to an enhanced exchange interaction in the covalent bonding regime, SCAN substantially decreases the formation energy errors for strongly bound compounds, by approximately 50% to 110 meV/atom, as compared to the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE). However, for intermetallic compounds, SCAN performs moderately worse than PBE with an increase in formation energy error of approximately 20%, stemming from SCAN's distinct behavior in the weak bonding regime. The formation energy errors can be further reduced via elemental chemical potential fitting. We find that SCAN leads to significantly more accurate predicted crystal volumes, moderately enhanced magnetism, and mildly improved band gaps as compared to PBE. Altogether, SCAN represents a significant improvement in accurately describing the thermodynamics of strongly bound compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Complex magnetic structure in Ba 5 Ru 3 O 12 with isolated Ru 3 O 12 trimer

We report detailed magnetic, transport, heat-capacity, and neutron diffraction measurements of Ba 5 Ru 3 O 12 , a compound consisting of isolated Ru 3 O 12 trimers. We show that this system develops long-range antiferromagnetic ordering at T N ~ 60 K without structural distortion or metal-insulator-type transition, which is in sharp contrast to other Barium Ruthenate trimer systems such as 9R-BaRuO 3 and Ba 4 Ru 3 O 10 . A complex magnetic structure is revealed which is attributable to the magnetic frustration due to competing exchange interactions between Ru ions on different crystallographic sites within the Ru 3 O 12 trimer and different degree of orbital hybridization on different Ru-sites. In conclusion, we have also investigated the magnetic structure of Ba 4 Ru 3 O 10 for comparisons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Epitaxial stabilization of thin films of the frustrated Ge-based spinels

Frustrated magnets can host numerous exotic many-body quantum and topological phenomena. GeNi 2 O 4 is a three-dimensional S=1 frustrated magnet with an unusual two-stage transition to the two-dimensional antiferromagnetic ground state, while GeCu 2 O 4 is a high-pressure phase with a strongly tetragonally elongated spinel structure and magnetic lattice formed by S=1/2 CuO 2 linear chains with frustrated interchain exchange interactions and exotic magnetic behavior. Here, we report on the thin-film epitaxial stabilization of these two compounds. The developed growth mode, surface morphology, crystal structure, and copper valence state were characterized by in situ reflection high-energy electron diffraction, atomic force microscopy, x-ray reflectivity, x-ray diffraction, x-ray photoelectron spectroscopy, and resonant x-ray absorption spectroscopy. Our results pave an alternative route to the comprehensive investigation of the puzzling magnetic properties of these compounds and the exploration of emergent features driven by strain.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semilocal density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of CrNi 2 , the only experimentally observed intermetallic phase. This structure's ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. In conclusion, implications for related commercial and high-entropy alloys are discussed for each case.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Relationship between magnetoresistance behavior and magnetic states in intercalated compounds Fe x TiS 2

Here, temperature and field-dependent neutron powder diffraction (NPD) measurements have been performed to reveal the nature of the unusual evolution of the magnetoresistance behavior with increasing Fe content in the intercalated compounds Fe x TiS 2 (with x=0.25,0.33,0.50,0.55) synthesized by solid-phase reaction method with prolonged homogenization heat treatment. As derived from neutron diffraction measurements, both the Fe 0.25 TiS 2 and the Fe 0.50 TiS 2 compound exhibit an antiferromagnetic (AFM) order below their respective Néel temperatures T N ≈52 K and T N ≈140K, which results in the presence of a large magnetoresistance accompanying the field-induced phase transition from AFM to the ferromagnetic (FM) state. At low temperatures, this AFM-FM transition is irreversible, confirmed by the irreversibility of changes in the NPD patterns and the presence of remnant magnetoresistance. In contrast, Fe 0.33 TiS 2 shows short-range magnetic order at T f ≈44 K due to a triangular network of intercalated Fe atoms and frustrations of exchange interactions with a field-induced FM alignment of Fe magnetic moments in an applied magnetic field as revealed by NPD measurements. The field-induced transformations of the cluster glass magnetic state in this compound lead to a significant decrease in electrical resistivity. According to NPD data, a reduced impact of an external magnetic field on the electrical resistivity of the compound Fe 0.55 TiS 2 can be ascribed to the presence of ferromagnetic or ferrimagnetic order in compounds with the Fe concentrations above x=0.50. The results obtained indicate that the distribution of the Fe atoms, along with their concentration in Fe x TiS 2 layered compounds, plays a decisive role in the formation of the magnetic state and the behavior of the magnetoresistance.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Single crystal growth, chemical defects, magnetic and transport properties of antiferromagnetic topological insulators ( Ge 1 – δ – x Mn x ) 2 Bi 2 Te 5 ( x ≤ 0.47 , 0.11 ≤ δ ≤ 0.20 )

Magnetic topological insulators provide a platform for emergent phenomena arising from the interplay between magnetism and band topology. Here we report the single crystal growth, crystal structure, magnetic and transport properties, as well as the neutron scattering studies of topological insulator series (Ge 1–δ–x Mn x ) 2 ⁢Bi 2 ⁢Te 5 (x ≤ 0.47, 0.11 ≤ δ ≤ 0.20). Upon doping up to x = 0.47, the lattice parameter c decreases by 0.8%, while the lattice parameter a remains nearly unchanged. Significant Ge vacancies and Ge/Bi site mixing are revealed via elemental analysis as well as refinements of the neutron and x-ray diffraction data, resulting in holes dominating the charge transport. At x = 0.47, below 10.8 K, a bilayer A-type antiferromagnetic ordered state emerges, featuring an ordered moment of 3.0(3) μ B /Mn at 5 K, with the c axis as the easy axis. Magnetization data unveils a much stronger effective interlayer antiferromagnetic exchange interaction and a much smaller uniaxial anisotropy compared to MnBi 2 ⁢Te 4 . We attribute the former to the shorter nearest-neighbor Mn-Mn interlayer superexchange path and the latter to the smaller ligand-field splitting in (Ge 1–δ–x ⁢Mn x ) 2 ⁢Bi 2 ⁢Te 5 . Finally, our study demonstrates that this series of materials holds promise for the investigation of the layer Hall effect and quantum metric nonlinear Hall effect.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Equilibrium Dynamics of Infinite-Range Quantum Spin Glasses in a Field

We determine the low-energy spectrum and Parisi replica-symmetry-breaking function for the spin-glass phase of the quantum Ising model with infinite-range random exchange interactions and transverse and longitudinal ( h ) fields. We show that, for all h , the spin-glass state has full replica symmetry breaking and the local spin spectrum is gapless, with a spectral density that vanishes linearly with frequency. These results are obtained using an action functional—argued to yield exact results at low frequencies—that expands in powers of a spin-glass order parameter, which is bilocal in time, and a matrix in replica space. We also present the exact solution of the infinite-range spherical quantum p -rotor model at nonzero h : here, the spin-glass state has one-step replica symmetry breaking and gaplessness only appears after imposition of an additional marginal stability condition. Possible connections to experiments on random arrays of trapped Rydberg atoms are noted. Published by the American Physical Society 2024

Tikhanovskaya, Maria↗
DOE OSTI2025

Large-𝑁 SU(4) Schwinger boson theory for coupled-dimer antiferromagnets

Here, we develop a systematic large-𝑁 expansion based on the Schwinger boson representation of SU(4) coherent states of dimers for the paradigmatic spin-1/2 bilayer square lattice Heisenberg antiferromagnet. This system exhibits a quantum phase transition between a quantum paramagnetic state and a Néel order state, driven by the coupling constant 𝑔 = 𝐽′/𝐽, which is defined as the ratio between the interdimer 𝐽′ and intradimer 𝐽 exchange interactions. We demonstrate that this approach accurately describes static and dynamic properties on both sides of the quantum phase transition. The critical coupling constant 𝑔 𝑐 ≈ 0.42 and the dynamic spin structure factor reproduce quantum Monte Carlo results with high precision. Notably, the 1/𝑁 corrections reveal the longitudinal mode of the magnetically ordered phase along with the overdamping caused by its decay into the two-magnon continuum. The present large-𝑁 SU(4) Schwinger boson theory can be extended to more general cases of quantum paramagnets that undergo a quantum phase transition into magnetically ordered states.

Schwinger boson method↗