Local Moments Research Articles

Protracted Kondo coherence with dilute carrier density in cerium based nickel pnictides

Nozières' exhaustion theory argues that the temperature for coherently screening of all local moments in Kondo lattice could be much lower than the temperature of single moment screening in case of insufficient conduction electrons. Experiments indicate that cerium based nickel pnictides CeNi2−δAs2(δ≈0.28) is an ideal material to exam such protracted Kondo screening. Using density functional theory plus dynamical mean-field theory, we found CeNi2As2 presents a transition from the local moment states to the coherent Kondo screening states under compression, accompanied by topological changes of the Fermi surface. Similar phase transition could be driven by chemical pressure via the isovalence As→P substitution. The derived coherent Kondo temperature Tcoh is much lower than the single-impurity Kondo temperature TK due to the diluted Ni-3d conduction electrons, indicating the protraction of Kondo screening in crystal. In contrast to structurally analogous layered iron pnictides, the electronic structures of the present systems show strong three-dimensionality.

Dipole polarization modulating of vinylene-linked covalent organic frameworks for efficient photocatalytic hydrogen evolution

Photocatalytic hydrogen (H2) evolution using covalent organic frameworks (COFs) is an attractive and promising avenue for exploration, but one of its big challenges is low photo-induced charge separation. In this study, we present a straightforward and facile dipole polarization engineering strategy to enhance charge separation efficiency, achieved through atomic modulation (O, S, and Se) of the COF monomer. Our findings demonstrate that incorporating atoms with varying electronegativities into the COF matrix significantly influences the local dipole moment, thereby affecting charge separation efficiency and photostability, which in turn affects the rates of photocatalytic H2 evolution. As a result, the newly developed TMT-BO-COF, which contains highly electronegative O atoms, exhibits the lowest exciton binding energy, the highest efficiency in charge separation and transportation, and the longest lifetime of the active charges. This leads to an impressive average H2 production rate of 23.7 mmol g−1 h−1, which is 2.5 and 24.5 times higher than that of TMT-BS-COF (containing S atoms) and TMT-BSe-COF (containing Se atoms), respectively. A novel photocatalytic hydrogen evolution mechanism based on proton-coupled electron transfer on N in the structure of triazine rings in vinylene-linked COFs is proposed by theoretical calculations. Our findings provide new insights into the design of highly photoactive organic framework materials for H2 evolution and beyond.

Imaging magnetic transition of magnetite to megabar pressures using quantum sensors in diamond anvil cell

High-pressure diamond anvil cells have been widely used to create novel states of matter. Nevertheless, the lack of universal in-situ magnetic measurement techniques at megabar pressures makes it difficult to understand the underlying physics of materials’ behavior at extreme conditions, such as high-temperature superconductivity of hydrides and the formation or destruction of the local magnetic moments in magnetic systems. Here, we break through the limitations of pressure on quantum sensors by modulating the uniaxial stress along the nitrogen-vacancy axis and develop the in-situ magnetic detection technique at megabar pressures with high sensitivity (~1μT/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1{{{\\rm{\\mu }}}}{{{\\rm{T}}}}/\\sqrt{{{{\\rm{Hz}}}}}$$\\end{document}) and sub-microscale spatial resolution. By directly imaging the magnetic field and the evolution of magnetic domains, we observe the macroscopic magnetic transition of Fe3O4 in the megabar pressure range from ferrimagnetic (α-Fe3O4) to weak ferromagnetic (β-Fe3O4) and finally to paramagnetic (γ-Fe3O4). The scenarios for magnetic changes in Fe3O4 characterized here shed light on the direct magnetic microstructure observation in bulk materials at high pressure and contribute to understanding magnetism evolution in the presence of numerous complex factors such as spin crossover, altered magnetic interactions and structural phase transitions.

Carbonatoperoxovanadates with Strong Second-Harmonic Generation: Insights from First-Principles Calculations on Anisotropic Structures and Optical Parameters.

Carbonatoperoxovanadates are considered as promising functional materials in optoelectronic devices due to their excellent optical properties, particularly strong second-harmonic generation (SHG) response. However, the relationship between their geometric structures and optical properties remains unclear. Herein, the structural, electronic, and optical properties of carbonatoperoxovanadates A3VO(O2)2CO3 (A=K, Rb, and Cs) were investigated using first-principles calculation. Results suggest that high density and parallel arrangement of nonlinear optical active [VO(O2)2CO3] units are conducive to generating large SHG response in A3[V(O2)2O]CO3. Optical anisotropy was observed. Birefringence values for A3[V(O2)2O]CO3 were comparable to those of commonly used infrared nonlinear optical materials. Specifically, results of tiny optical characteristics (local dipole moments, HUMO-LUMO gap, polarizability anisotropy, and hyperpolarizability) indicate that asymmetry [VO(O2)2CO3] is an excellent nonlinear optical active functional unit, owing to the synergistic effect between its non-centrosymmetric nonlinear optical elements. This study elucidates the structure-property relationship of carbonatoperoxovanadates, offering valuable insights for designing novel high-performance SHG materials.

Impact of composition on the properties of full-Heusler Ti2FexMn1-xAl alloys in spintronics

This study explores the structural, electronic, and magnetic characteristics of full-Heusler Ti2FexMn1-xAl alloys for spintronic applications. The regular Heusler structure is identified as the most stable across all x concentrations. The inverse Heusler structure exhibits half-metallic behavior with a finite energy band gap in the spin-up states, while the regular structure shows metallic behavior for both spin directions. Dirac-like points along the M→Γ direction are observed, particularly in alloys with x = 0 and 0.25 (inverse structure) and x = 0.5, 0.75, and 1 (regular structure), indicating advanced electronic properties. Magnetic analysis reveals that Ti atoms' local magnetic moments are antiparallel to those of Mn and Fe atoms. The total magnetic moment is highest for x = 1 (Ti2MnAl) and nearly zero for x = 0 (Ti2FeAl). Additionally, the inverse Heusler structure achieves 100 % spin polarization at the Fermi energy, underscoring its suitability for spintronic applications. This study highlights the potential of Ti2FexMn1-xAl alloys for future spintronic devices.

Molten Salt-Assisted Synthesis of Ferric Oxide/M-N-C Nanosheet Electrocatalysts for Efficient Oxygen Reduction Reaction.

Efficient, stable, and low-cost oxygen reduction catalysts are the key to the large-scale application of metal-air batteries. Herein, high-dispersive Fe2O3 nanoparticles (NPs) with abundant oxygen vacancies uniformly are anchored on lignin-derived metal-nitrogen-carbon (M-N-C) hierarchical porous nanosheets as efficient oxygen reduction reaction (ORR) catalysts (Fe2O3/M-N-C, M═Cu, Mn, W, Mo) based on a general and economical KCl molten salt-assisted method. The combination of Fe with the highly electronegative O induces charge redistribution through the Fe-O-M structure, thereby reducing the adsorption energy of oxygen-containing substances. The coupling effect of Fe2O3 NPs with M-N-C expedites the catalytic activity toward ORR by promoting proton generation on Fe2O3 and transfer to M-N-C. Experimental and theoretical calculation further revealed the remarkable electronic structure evolution of the metal site during the ORR process, where the emission density and local magnetic moment of the metal atoms change continuously throughout their reaction. The unique layered porous structure and highly active M-N4 sites resulted in the excellent ORR activity of Fe2O3/Cu-N-C with the onset potential of 0.977V, which is superior to Pt/C. This study offers a feasible strategy for the preparation of non-noble metal catalysts and provides a new comprehension of the catalytic mechanism of M-N-C catalysts.

Effect of substitutional Sb doping on the structural stability, half-metallicity, elastic properties, electronic properties, and magnetism of the Co₂MnSn full Heusler compound

We conducted Density Functional Theory (DFT) calculations using the full potential linearized augmented plane wave (FP-LAPW) method to modify the Fermi level by introducing Sb doping into Co2MnSn.This was done through the Generalized Gradient Approximation (GGA) and modified Becke-Johnson (mBJ-GGA) formalisms to enhance spin polarization and suggest signs of half-metallicity. For both ferromagnetic and paramagnetic phases, lattice optimization was performed to identify the stable magnetic structure. The results designate that the doped alloys are stable in a ferromagnetic phase with L21 structure. The calculated elastic constants suggest that the doped alloys meet the criteria for mechanical stability. The magnetism in these compounds is primarily due to the localized moments on the Mn and Co atoms. According to mBJ-GGA calculations, the total magnetic moment slightly increases with Sb doping. Analysis of the optical constants revealed the optoelectronic properties, confirming semiconducting behavior. Doping elements to achieve half-metallicity is demonstrated to be an effective method for discovering new materials suitable for spintronics applications.

Observation of a V-Shape Superconductivity Evolution on Tungsten-Intercalated 2H-Type Niobium Diselenide.

van der Waals-layered niobium diselenide (NbSe2) intercalated by d-electron transition metals is an ideal test bed for the exploration of their diversiform evolution of ground states. These intercalations are mostly viewed as ordered structures aligned with periodicities of their host materials that enable control of the electronic phases via gradually changing of intercalation ratios. Here, we present the structure and superconductivity in tungsten (W)-intercalated 2H-NbSe2 crystals, which reveals an order to disorder distribution of W atoms with increasing confined intercalating amounts, leading to an approximate V-shape suppression of superconductivity. Aided by density functional theory calculations, we demonstrate that the local magnetic moment around W intercalants induced by the charge redistribution gives rise to the quick superconductivity suppression in 2H-NbSe2 below a certain dilute amount (W% = 0.06). Simultaneously, W intercalants also induce structural aberration due to aggregation effects and inhibit the generation of an ordered structure in 2H-NbSe2, resulting in a recovery of its superconductivity. The alteration of structure and electronic phases in 2H-NbSe2 via intercalation of nonmagnetic transition metals in the van der Waals gap enables the exploration of combined magnetic quantum criticality, superconductivity, and other related electronic correlations.

Emergent Berezinskii-Kosterlitz-Thouless and Kugel-Khomskii Physics in the Triangular Lattice Bilayer Colbaltate.

Motivated by the experiments on the triangular lattice bilayer colbaltate K_{2}Co_{2}(SeO_{3})_{3}, we consider an extended XXZ model to explore the underlying physics. The model is composed of interacting Co^{2+} dimers on the triangular lattice, where the Co^{2+} ion provides an effective spin-1/2 local moment via the spin-orbit coupling and the crystal field effect. The intradimer interaction is dominant and would simply favor the local spin singlet, and the interdimer interactions compete with the interdimer interaction, leading to rich behaviors. With the easy-axis anisotropy, it is shown that, in the ground state manifold of the intradimer Ising interaction, the system realizes an effective quantum Ising model, where the ground state is either a three-sublattice order with a mixture of antiferromagnetic Ising order and a valence-bond spin singlet or Ising disordered. The finite temperature regime realizes the Berezinskii-Kosterlitz-Thouless physics. To explore the full excitations, we incorporate the excited state manifold of the intradimer Ising interaction and establish the emergent Kugel-Khomskii physics. Thus, the triangular lattice bilayer colbaltate is an excellent platform to explore the interplay between geometrical frustration and anisotropic interactions as well as the emergent effective models and the resulting physics.

Controlling Ultrafast Magnetization Dynamics via Coherent Phonon Excitation in a Ferromagnet Monolayer.

Exploring ultrafast magnetization control in 2D magnets via laser pulses is established, yet the interplay between spin dynamics and the lattice remains underexplored. Utilizing real-time time-dependent density functional theory (rt-TDDFT) coupled with Ehrenfest dynamics and nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigate the laser-induced spin-nuclei dynamics with pre-excited A1g and E2g coherent phonons in the 2D ferromagnet Fe3GeTe2 (FGT) monolayer. Selective pre-excitation of coherent phonons under ultrafast laser irradiation significantly alters the local spin moment of FGT, consequently inducing additional spin loss attributed to the nuclear motion-induced asymmetric interatomic charge transfer. Excited spin-resolved charge undergoes a bidirectional spin-flip between spin-down and spin-up states, characterized by a subtle change in the spin moment within approximately 100 fs, followed by unidirectional spin-flip, which will further contribute to the spin moment loss of FGT within tens of picoseconds. Our results shed light on the coupling of coherent phonons with magnetization dynamics in 2D limit.

Aerodynamics of a three-dimensionally deformed rigid wing

A flexible wing with a large aspect ratio emerges in many modern engineering applications (e.g. solar planes and super large wind turbine blades) and its interaction with incident flow differs markedly from conventional fluid–structure interactions (FSI), exhibiting frequently a large three-dimensional (3D) deformation. Yet, there is little information on how such deformation may change wing aerodynamics. This work investigates the aerodynamic performance of deformed and cantilever-supported NACA0012 rigid wings with an aspect ratio of 9, using ANSYS Fluent with the SST-κ-ω turbulence model at a chord-length-based Reynolds number Rec of 1.5 × 105. Numerical simulation is validated experimentally. The wing tip bending displacement is up to 4.14c, and the maximum twisted angle is up to 7°. The angle α of attack varies from 0° to 20° at mid span of the wing. It has been found that the torsional deformation can significantly advance the local flow separation, reattachment, bubble length, and transition from laminar to turbulence, resulting in a drop in the critical angle αcr of attack, at which the separation bubble size reaches the maximum. Accordingly, the lift and drag coefficients increase, as well as the bending and pitching-up moments, though the stall is advanced due to a change in local α. The tip vortex is also enhanced, inducing strong downwash postponing the separation bubble on the wing and resulting in redistributed force near the wing tip that increases markedly the local bending moment but decrease the local pitching-up moment. On the other hand, the bending deformation tends to produce an effect opposite to the torsion on the flow structure and causing little change in the lift coefficient, though reducing the induced drag and moments appreciably. With both deformations in place, the torsion overwhelms the bending in general in terms of its impact upon aerodynamics and flow structures, the latter acting to cancel at least partially the effect of the former.

Strong Pairing Originated from an Emergent Z_{2} Berry Phase in La_{3}Ni_{2}O_{7}.

The recent discovery of high-temperature superconductivity in La_{3}Ni_{2}O_{7} offers a fresh platform for exploring unconventional pairing mechanisms. Starting with the basic argument that the electrons in d_{z^{2}} orbitals nearly form local moments, we examine the effect of the Hubbard interaction U on the binding strength of Cooper pairs based on a single-orbital bilayer model with intralayer hopping t_{∥} and interlayer superexchange J_{⊥}. By extensive density matrix renormalization group calculations, we observe a remarkable enhancement in binding energy as much as 10-20 times larger with U/t_{∥} increasing from 0 to 12 at J_{⊥}/t_{∥}∼1. We demonstrate that such a substantial enhancement stems from a kinetic-energy-driven mechanism. Specifically, a Z_{2} Berry phase will emerge at large U due to the Hilbert space restriction (Mottness), which strongly suppresses the mobility of single particle propagation as compared to U=0. However, the kinetic energy of the electrons (holes) can be greatly restored by forming an interlayer spin-singlet pairing, which naturally results in a superconducting state even for relatively small J_{⊥}. An effective hard-core bosonic model is further proposed to estimate the superconducting transition temperature at the mean-field level.

Magnetic excitation in the hyperkagome antiferromagnet Mn3RhSi

In a paramagnetic state of a hyperkagome lattice antiferromagnet Mn3RhSi, magnetic diffuse scattering by magnetic short-range order is observed over a wide temperature range of approximately 500 K above the Néel temperature of 190 K. We have studied the magnetic excitation with a single crystal in a wide energy range from 0.3 to 140 meV by inelastic neutron scattering, where the prominent inelastic neutron-scattering signal is observed at Q=(2,0,0). The inelastic scattering intensity integrated for two of twelve magnon modes leads to a localized magnetic moment of about 5 μB per Mn site, although the long-range ordered magnetic moment is only 2.61 μB at 4 K. The result suggests that a large part of the magnetic moment does not order in Mn3RhSi, possibly due to the geometrical spin frustration of the hyperkagome lattice. Published by the American Physical Society 2024

A novel methodology to determine tensile properties in the heat-affected zone of tailor welded blanks with different base material thicknesses

When testing the mechanical properties of a tailor welded blank (TWB) under uniaxial tension, the localized bending moment generated by eccentric loading, due to the change in thickness between the welded sheets, causes the experimentally measured stress-strain behavior to not match the actual stress-strain behavior. In this paper, a new method to calibrate the tensile stress-strain behavior in the heat affected zone (HAZ) of a TWB with different thicknesses is proposed based on indentation testing and tensile test with multi-camera digital image correlation (DIC). The nominal stress-strain curve is calculated for calibration purposes. The nominal strain is obtained by performing multiple linear regression analysis on the front and back surface strains of the HAZ provided by the multi-camera DIC system. Experiments were conducted using this method on a TWB of the same material (DP1180) and different thicknesses (1 mm and 1.4 mm). The validation of the corrected stress-strain curves demonstrated the reliability and effectiveness of the proposed method.

Light-induced orbital magnetism in metals via inverse Faraday effect

We present a microscopic calculation of the inverse Faraday effect in metals. We derive a static local magnetic moment induced on the application of high-frequency light, using the Eilenberger formulation of quasiclassical theory. We include the effect of disorder and formulate a theory applicable across the entire temperature range, in the absence of external applied fields. For light-induced electric fields of amplitude ∼100kV/cm, the induced fields are large ∼0.1T for metallic Nb. The predictions of our theory agree with recent experimental and theoretical results [O. H.-C. Cheng , and J. Hurst , ]. An extension of this approach to superconductors would open a new route of inducing orbital magnetic field and potentially vortices in superconductors. Published by the American Physical Society 2024

Local magnetic moment formation and Kondo screening in the presence of Hund exchange: Two-band Hubbard model analysis

Local magnetic moment formation and Kondo screening in the presence of Hund exchange: Two-band Hubbard model analysis

Signatures of polarized chiral spin disproportionation in rare earth nickelates

In rare earth nickelates (RENiO3), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO3 thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO3 (101)ortho films and RENiO3 single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

Van der Waals Electrides.

ConspectusElectrides make up a fascinating group of materials with unique physical and chemical properties. In these materials, excess electrons do not behave like normal electrons in metals or form any chemical bonds with atoms. Instead, they "float" freely in the gaps within the material's structure, acting like negatively charged particles called anions (see the graph). Recently, there has been a surge of interest in van der Waals (vdW) electrides or electrenes in two dimensions. A typical example is layered lanthanum bromide (LaBr2), which can be taken as [La3+(Br1-)2]+•(e-). Each excess free electron is trapped within a hexagonal pore, forming dense dots of electron density. These anionic electrons are loosely bound, giving vdW electrides some unique properties such as ferromagnetism, superconductivity, topological features, and Dirac plasmons. The high density of the free electron makes electrides very promising for applications in thermionic emission, organic light-emitting diodes, and high-performance catalysts.In this Account, we first discuss the discovery of numerous vdW electrides through high-throughput computational screening of over 67,000 known inorganic crystals in Materials Project. A dozen of them have been newly discovered and have not been reported before. Importantly, they possess completely different structural prototypes and properties of anionic electrons compared to widely studied electrides such as Ca2N. Finding these new vdW electrides expands the variety of electrides that can be made in the experiment and opens up new possibilities for studying their unique properties and applications.Then, based on the screened vdW electrides, we delve into their various emerging properties. For example, we developed a new magnetic mechanism specific to atomic-orbital-free ferromagnetism in electrides. We uncover the dual localized and extended nature of the anionic electrons in such electrides and demonstrate the formation of the local moment by the localized feature and the ferromagnetic interaction by the direct overlapping of their extended states. We further show the effective tuning of the magnetic properties of vdW electrides by engineering their structural, electronic, and compositional properties. Besides, we show that the complex interaction between the multiple quantum orderings in vdW electrides leads to many interesting properties including valley polarization, charge density waves, a topological property, a superconducting property, and a thermoelectrical property.Moreover, we discuss strategies to leverage the unique intrinsic properties of vdW electrides for practical applications. We show that these properties make vdW electrides potential candidates for advanced applications such as spin-orbit torque memory devices, valleytronic devices, K-ion batteries, and thermoelectricity. Finally, we discuss the current challenges and future perspectives for research using these emerging materials.

Magnetocaloric effect of topological excitations in Kitaev magnets

Traditional magnetic sub-Kelvin cooling relies on the nearly free local moments in hydrate paramagnetic salts, whose utility is hampered by the dilute magnetic ions and low thermal conductivity. Here we propose to use instead fractional excitations inherent to quantum spin liquids (QSLs) as an alternative, which are sensitive to external fields and can induce a very distinctive magnetocaloric effect. With state-of-the-art tensor-network approach, we compute low-temperature properties of Kitaev honeycomb model. For the ferromagnetic case, strong demagnetization cooling effect is observed due to the nearly free Z2 vortices via spin fractionalization, described by a paramagnetic equation of state with a renormalized Curie constant. For the antiferromagnetic Kitaev case, we uncover an intermediate-field gapless QSL phase with very large spin entropy, possibly due to the emergence of spinon Fermi surface and gauge field. Potential realization of topological excitation magnetocalorics in Kitaev materials is also discussed, which may offer a promising pathway to circumvent existing limitations in the paramagnetic hydrates.

Distinguishing surface and bulk electromagnetism via their dynamics in an intrinsic magnetic topological insulator.

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.