Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Lattice Field Theory”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Strongly anisotropic electronic and magnetic structures in oxide dichlorides RuOCl 2 and OsOCl 2

The van der Waals oxide dichlorides MOX 2 (M=V, Ta, Nb, Ru, and Os; X=halogen element), with different electronic densities, are attracting considerable attention. Ferroelectricity, spin-singlet formation, and orbital-selective Peierls phases were reported in this family with d 1 or d 2 electronic configurations, all believed to be caused by the strongly anisotropic electronic orbital degree of freedom. Here, using density functional theory and density matrix renormalization group methods, we investigate the electronic and magnetic properties of RuOCl 2 and OsOCl 2 with d 4 electronic configurations. Different from a previous study using VOI 2 with d 1 configuration, these systems with 4d4 or 5d4 do not exhibit a ferroelectric instability along the a axis. Due to the fully occupied dxy orbital in RuOCl 2 and OsOCl 2 , the Peierls instability distortion disappears along the b axis, leading to an undistorted Immm phase (No. 71). Furthermore, we observe strongly anisotropic electronic and magnetic structures along the a axis. For this reason, the materials of our focus can be regarded as “effective one-dimensional” systems even when they apparently have a dominant two-dimensional lattice geometry. The large crystal-field splitting energy (between dxz/yz and dxy orbitals) and large hopping between nearest-neighbor Ru and Os atoms suppresses the J = 0 singlet state in MOCl 2 (M=Ru or Os) with electronic density n = 4, resulting in a spin-1 system. Moreover, we find staggered antiferromagnetic order with π wave vector along the M-O chain direction (a axis) while the magnetic coupling along the b axis is weak. Based on Wannier functions from first-principles calculations, we calculated the relevant hopping amplitudes and crystal-field splitting energies of the t 2g orbitals for the Os atoms to construct a multiorbital Hubbard model for the M-O chains. Staggered AFM with ↑|-|↓-↑|-|↓ spin structure dominates in our density matrix renormalization group calculations, in agreement with density functional theory calculations. Our results for RuOCl 2 and OsOCl 2 provide guidance to experimentalists and theorists working on this interesting family of oxide dichlorides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Continuous phase transition between bosonic integer quantum Hall liquid and a trivial insulator: Evidence for deconfined quantum criticality

The deconfined quantum critical point, a prototype Landau-forbidden transition, could exist, in principle, in the phase transitions involving a symmetry-protected topological phase; however, examples of such kinds of transitions in physical systems are rare beyond one-dimensional systems. Here, using a density-matrix renormalization-group calculation, we unveil a bosonic integer quantum Hall phase in a two-dimensional correlated honeycomb lattice, by full identification of its internal structure from the topological K matrix. Furthermore, we demonstrate that imbalanced periodic chemical potentials can destroy the bosonic integer quantum Hall state and drive it into a featureless trivial (Mott) insulator, where all physical observables evolve smoothly across the critical point. At the critical point, the entanglement entropy reveals a characteristic scaling behavior, which is consistent with the critical field theory as an emergent QED3 with two flavors of Dirac fermions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Heterobilayer moiré magnets: Moiré skyrmions and commensurate-incommensurate transitions

In this work, we study untwisted heterobilayers of ferromagnetic and antiferromagnetic van der Waals materials, with in particular a Dzyaloshinskii-Moriya interaction in the ferromagnetic layer. A continuum low-energy field theory is utilized to study such systems. We develop a phase diagram as a function of the strength of interlayer exchange and Dzyaloshinskii-Moriya interactions, combining perturbative and strong coupling analyses with numerical simulations using Landau-Lifshitz-Gilbert equations. Various moiré-periodic commensurate phases are found, and the commensurate-incommensurate transition is discussed. Among the commensurate phases, we observe an interesting skyrmion lattice phase wherein each moiré unit-cell hosts one skyrmion.

2-dimensional systems↗
DOE OSTI2026

Quantum simulation of QED in Coulomb gauge

A recent work considered quantum simulation of Quantum Electrodynamics on a lattice in the Coulomb gauge with gauge degrees of freedom represented in the occupation basis in momentum space. In this work, we consider the more efficient representation of the gauge degrees of freedom in field basis in position space and develop a quantum algorithm for real-time simulation. We show that the continuum Coulomb gauge Hamiltonian is equivalent to the temporal gauge Hamiltonian when acting on physical states consisting of fermion and transverse gauge fields. The Coulomb gauge Hamiltonian is discretized by using the Green’s function of the discrete Laplacian operator under the Dirichlet boundary conditions. Both the continuum Coulomb gauge Hamiltonian and the discretized one proposed here guarantee that the unphysical longitudinal gauge fields are decoupled and commute with the corresponding Hamiltonian. Thus there is no need to impose any constraint. The local gauge field basis and the canonically conjugate variable basis are swapped efficiently using the quantum Fourier transform. We prove that the qubit cost to represent physical states and the gate count for real-time simulation scale polynomially with the lattice size, energy, time, accuracy, and Hamiltonian parameters in lattice units. The gate cost here for implementing the time evolution of the gauge field is reduced at least by a factor on the order of 10 8 for modest lattice size and accuracy level compared with the previous work.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Evolution of magnetic field induced ordering in the layered quantum Heisenberg triangular-lattice antiferromagnet Ba 3 CoSb 2 O 9

Quantum fluctuations in the effective spin-12 layered triangular-lattice quantum Heisenberg antiferromagnet Ba 3 CoSb 2 O 9 lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane, including a celebrated collinear “up-up-down” spin ordering with magnetization equal to 1/3 of the saturation magnetization over an extended field range. Theoretically unresolved, however, are the effects of interlayer antiferromagnetic coupling and transverse magnetic fields on the ground states of this system. Additional magnetic field induced phase transitions are theoretically expected and in some cases have been experimentally observed, but details regarding their number, location, and physical character appear inconsistent with the predictions of existing models. Conversely, an absence of experimental measurements as a function of magnetic-field orientation has left other key predictions of these models untested. To address these issues, in this study we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic ab plane. For H||ab, we have discovered an additional magnetic field induced phase transition at low temperature and an unexpected tetracritical point in the high-field phase diagram, which coupled with the apparent second-order nature of the phase transitions eliminates several theoretically proposed spin structures for the high-field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel (H||a) but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition are not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field induced phases, provide guidance on the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal two-dimensional spin-12 triangular-lattice quantum Heisenberg antiferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

All about the neutron from lattice QCD

I describe how simulations of lattice QCD using the path integral formulation provide the two basic quantum mechanical properties of QCD, its ground state in which correlation functions are calculated, and Fock state wavefunctions between which matrix elements of operators are calculated. Both constructs are stochastic, so unfortunately one gets no intuitive picture or even qualitative understanding of what they look like, nevertheless they contain and display all the subtleties of the quantum field theory. Today, these simulations provide many quantities that are impacting phenomenology and experiments. I illustrate the methods and the steps in the analysis using, as examples, three observables: the isovector charges of the nucleon, the contribution of the quark’s intrinsic spin to the nucleon spin, and the pion-nucleon sigma term.

Gupta, Rajan (ORCID:0000000317843058)↗
DOE OSTI2023

Half‐Integer Topological Charge Polarization of Quasi‐Dirac Bound States in the Continuum

Abstract The non‐trivial polarization topology of bound states in the continuum (BICs) provides new strategies in nanophotonics. The polarization topology depends on the geometric parameters and energy‐momentum dispersion of the system and can be engineered to add specific functionalities for light molding. Herein, such a possibility is investigated by studying the topology of the polarization states associated with the optical field radiated by BICs when Dirac‐cone‐degeneracy is lifted. The opening of a pseudogap in the Dirac cone dispersion of square‐lattice dielectric photonic crystal slabs is achieved by tuning the slab thickness. First, the emergence of half‐integer topological charges without the requirement of BIC annihilation is theoretically shown, which instead occurs when in‐plane inversion symmetry is broken. Then, using spin‐to‐orbital angular momentum conversion, the theory of half‐integer topological charges mediated by BICs is demonstrated and experimentally proved. The same device is able to give rise to vortices with different orbital angular momentum depending on the way it is illuminated, thus improving the potential of optical multiplexing. In addition, the additive character of the topology‐induced phase‐vortex generation is finally demonstrated for both integer and half‐integer charges using also vortex states as input beams, which is of relevance for information delivery.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Three-pion effects in $K^0-\bar{K}^0$ mixing

The rate of mixing between a neutral kaon and an anti-kaon ($K^0$-$\bar{K}^0$) is given, in part, by a long-range matrix element, defined with two insertions of the weak Hamiltonian separated by physical, Minkowski time evolution. For physical quark masses, the kaon mass lies above the two- and three-pion thresholds and, as a result, this long-range matrix element receives contributions from intermediate on-shell $2\pi$ and $3\pi$ states. These contributions cannot easily be captured in a finite Euclidean spacetime, meaning that such matrix elements are not directly accessible via lattice QCD. In this talk, we present a strategy for combining quantities that can be extracted in numerical lattice QCD calculations in order to reproduce the physical, infinite-volume long-range amplitude for $K^0-\bar{K}^0$. The key novelty relative to published work is that we fully include the effects of three-particle states that were previously neglected. The strategy is built on existing formalism for long-range matrix elements with two-particle intermediate states, together with the relativistic-field-theory finite-volume formalism for extracting three-hadron weak decays.

Jackura, Andrew↗
DOE OSTI2020

Possible role of grain-boundary and dislocation structure for the magnetic-flux trapping behavior of niobium: A first-principles study

In this work, first-principles methods were used to understand magnetic flux trapping at vacancies, dislocations, and grain boundaries in high-purity superconducting niobium. Full-potential linear augmented plane-wave methods were applied in progressively greater complexity, starting at simple vacancies and extending to screw dislocations and tilt grain boundaries to analyze the effects of magnetic field on the superconducting state surrounding these defects. Density-functional theory calculations identified changes in electronic structure at the dislocation core and different types of symmetric tilt grain boundaries relative to bulk niobium. Electron redistribution enhanced nonparamagnetic effects and thus perturb superconductivity, resulting in local conditions suitable for premature flux penetration and subsequently flux pinning. Since the coherence length of superconducting niobium at 0 K is significantly larger than the lattice parameter, the effects of line and planar defects in niobium are predicted to be stronger for defect clusters than single defects in isolation, which is consistent with recent experimental observations. Controlling accumulation or depletion of charge at the defects, e.g., by segregation of an impurity atom, can mitigate these tendencies thus increasing the quality of superconducting niobium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Slow ferromagnetic fluctuations in the kagome metal Sc 3 ⁢ Mn 3 ⁢Al 7 ⁢ Si 5 revealed by 27 Al NMR

Static and dynamical magnetic and electronic properties of the kagome metal Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 have been investigated by 27 Al nuclear magnetic resonance (NMR) measurements. Two distinct 27 Al -NMR signals with two different values of quadrupolar frequencies of 𝜈 Q = 1.55(2) and 1.07(2) MHz are observed, which are assigned to Al(1) and Al(2), respectively. From the detailed NMR spectrum measurements under three different magnetic field directions and the density functional theory calculations, the principal axes of the electric field gradient for each Al site have been determined. The temperature dependence of Knight shift (𝐾) shows a similar temperature dependence of the DC magnetic susceptibility 𝜒 except for the low-temperature region below ∼50 K where 𝐾 is almost constant while 𝜒 keeps increasing, which suggests that the increase in 𝜒 at low temperatures is not intrinsic. 27 Al spin-lattice relaxation rate divided by temperature (1/𝑇 1 ⁢𝑇) is found to be constant, confirming the metallic state of Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 from a microscopic point of view. Based on a Korringa ratio analysis using the 𝑇 1 and 𝐾 data, ferromagnetic fluctuations are found to dominate in Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 . In conclusion, these fluctuations are suggested to be very slow with frequencies on the order of kilohertz or lower.

Ding, Qing-Ping [Ames Laboratory (AMES), Ames, IA ↗
DOE OSTI2025

Ab Initio Molecular Dynamics Insights into Stress Corrosion Cracking and Dissolution of Metal Oxides

Oxide phases such as α-Fe2O3 (hematite) and α-Al2O3 (corundum) are highly insoluble in water; however, subcritical crack growth has been observed in humidity nonetheless. Chemically induced bond breaking at the crack tip appears unlikely due to sterically hindered molecular transport. The molecular mechanics of a crack in corundum with a reactive force field reveal minimal lattice trapping, leading to bond breaking before sufficient space opens for water transport. To address this, we model a pre-built blunt crack with space for H2O molecule adsorption at the tip and show that it reduces fracture toughness by lowering the critical J-integral. Then, we explore stress-enhanced dissolution to understand the mechanism of crack tip blunting in the oxide/water system. Density functional theory combined with metadynamics was employed to describe atomic dissolution from flat hematite and corundum surfaces in pure water. Strain accelerates dissolution, stabilizing intermediate states with broken bonds before full atom detachment, while the free energy profile of unstrained surfaces is almost monotonic. The atomistic calculations provided input for a kinetic model, predicting the shape evolution of a blunt crack tip, which displays three distinct regimes: (i) dissolution primarily away from the tip, (ii) enhanced blunting near but not at the apex, and (iii) sharpening near the apex. The transition between regimes occurs at a low strain, highlighting the critical role of water in the subcritical crack growth of oxide scales, with dissolution as the fundamental microscopic mechanism behind this process.

Chemistry↗
NASA NTRS1974

Finite element solution theory for three-dimensional boundary flows

A finite element algorithm is derived for the numerical solution of a three-dimensional flow field described by a system of initial-valued, elliptic boundary value partial differential equations. The familiar three-dimensional boundary layer equations belong to this description when diffusional processes in only one coordinate direction are important. The finite element algorithm transforms the original description into large order systems of ordinary differential equations written for the dependent variables discretized at node points of an arbitrarily irregular computational lattice. The generalized elliptic boundary conditions is piecewise valid for each dependent variable on boundaries that need not explicitly coincide with coordinate surfaces. Solutions for sample problems in laminar and turbulent boundary flows illustrate favorable solution accuracy, convergence, and versatility.

Baker, A. J.↗
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 OSTI2020

Direct time-domain determination of electron-phonon coupling strengths in chromium

In this work, we report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. Furthermore, for the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. In conclusion, these results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay times measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Ferromagnets, a new anomaly, instantons, and (noninvertible) continuous translations

We discuss a large class of classical field theories with continuous translation symmetry. In the quantum theory, a new anomaly explicitly breaks this translation symmetry to a discrete symmetry. Furthermore, this discrete translation symmetry is extended by a d – 2-form global symmetry. All these theories can be described as U(1) gauge theories where Gauss law states that the system has nonzero charge density. Special cases of such systems can be phrased as theories with a compact phase space. Examples are ferromagnets and lattices in the lowest Landau level. In some cases, the broken continuous translation symmetry can be resurrected as a noninvertible symmetry. We clarify the relation between the discrete translation symmetry of the continuum theory and the discrete translation symmetry of an underlying lattice model. Our treatment unifies, clarifies, and extends earlier works on the same subject.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Antiferroelectric Ceramics for Energy–Efficient Capacitors by Theory–Guided Discovery

Antiferroelectric ceramics, via the electric-field-induced antiferroelectric (AFE)–ferroelectric (FE) phase transitions, show great promise for high-energy-density capacitors. Yet, currently, only 70–80% energy release is found during a charge–discharge cycle. Here, for PbZrO 3 -based oxides, geometric nonlinear theory of martensitic phase transitions is applied (first used to guide supercompatible shape-memory alloys) to predict the reversibility of the AFE–FE transition by using density-functional theory to assess AFE/FE interfacial lattice-mismatch strain that assures ultralow electric hysteresis and extended fatigue lifetime. A good correlation of mismatch strain with electric hysteresis, hence, with energy efficiency of AFE capacitors is observed. Here, guided by theory, high-throughput material search is conducted and AFE compositions with a near-perfect charge–discharge energy efficiency (98.2%), i.e., near-zero hysteresis are discovered. And the fatigue life of the capacitor reaches 79.5 million charge–discharge cycles, a factor of 80 enhancement over AFE ceramics with large electric hysteresis.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga 2 O 3

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in β-Ga 2 O 3 , an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10 -4 Ω.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga 2 O 3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy measurements confirm this finding and the interpretation of the experimental results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time, it demonstrates for the first time p-type and remarkable n-type conductivity in Ga 2 O 3 which should usher in the development of Ga 2 O 3 devices and advance optoelectronics and high-power devices.

36 MATERIALS SCIENCE↗