A new scaling approach for the mesoscale simulation of magnetic domain structures using Monte Carlo simulations
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Radhakrishnan, B.↗
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Results for “EXCHANGE INTERACTIONS”
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Simulating the Geological Fate of Terrestrial Organic Matter: Lignin vs Cellulose
While shale gas has become a major source of energy, a more sustainable recovery requires a better understanding of the gas/kerogen matrix interactions. Here, we use replica exchange molecular dynamics to investigate the geological conversion of two important classes of gas-forming constituents of the terrestrial organic matter: lignin and cellulose. In agreement with results from pyrolysis experiments, we show that lignin produces twice as much kerogen and 5 times more methane than cellulose. In addition, while ex-cellulose kerogen is relatively stiff and almost nonporous, ex-lignin kerogen, despite having a very similar composition and bonding, is an order of magnitude more compliant due to the presence of large micropores. The obtained results can potentially improve the nanoscale brick of bottom-up models of shale gas recovery.
58 GEOSCIENCES↗
Modeling Liquid Water by Climbing up Jacob’s Ladder in Density Functional Theory Facilitated by Using Deep Neural Network Potentials
Within the framework of Kohn–Sham density functional theory (DFT), the ability to provide good predictions of water properties by employing a strongly constrained and appropriately normed (SCAN) functional has been extensively demonstrated in recent years. Here, we further advance the modeling of water by building a more accurate model on the fourth rung of Jacob’s ladder with the hybrid functional, SCAN0. In particular, we carry out both classical and Feynman path-integral molecular dynamics calculations of water with the SCAN0 functional and the isobaric–isothermal ensemble. To generate the equilibrated structure of water, a deep neural network potential is trained from the atomic potential energy surface based on ab initio data obtained from SCAN0 DFT calculations. For the electronic properties of water, a separate deep neural network potential is trained by using the Deep Wannier method based on the maximally localized Wannier functions of the equilibrated trajectory at the SCAN0 level. The structural, dynamic, and electric properties of water were analyzed. The hydrogen-bond structures, density, infrared spectra, diffusion coefficients, and dielectric constants of water, in the electronic ground state, are computed by using a large simulation box and long simulation time. For the properties involving electronic excitations, we apply the GW approximation within many-body perturbation theory to calculate the quasiparticle density of states and bandgap of water. Compared to the SCAN functional, mixing exact exchange mitigates the self-interaction error in the meta-generalized-gradient approximation and further softens liquid water toward the experimental direction. For most of the water properties, the SCAN0 functional shows a systematic improvement over the SCAN functional. However, some important discrepancies remain. The H-bond network predicted by the SCAN0 functional is still slightly overstructured compared to the experimental results.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Assessment of Quantum Mechanical Methods for Copper and Iron Complexes by Photoelectron Spectroscopy
Broken-symmetry density functional theory (BS-DFT) calculations are assessed for redox energetics [Cu(SCH3)2]1–/0, [Cu(NCS)2]1–/0, [FeCl4]1–/0, and [Fe(SCH3)4]1–/0 against vertical detachment energies (VDE) from valence photoelectron spectroscopy (PES), as a prelude to studies of metalloprotein analogs. The M06 and B3LYP hybrid functionals give VDE that agree with the PES VDE for the Fe complexes, but both underestimate it by 400 meV for the Cu complexes; other hybrid functionals give VDEs that are an increasing function of the amount of Hartree–Fock (HF) exchange and so cannot show good agreement for both Cu and Fe complexes. Range-separated (RS) functionals appear to give a better distribution of HF exchange since the negative HOMO energy is approximately equal to the VDEs but also give VDEs dependent on the amount of HF exchange, sometimes leading to ground states with incorrect electron configurations; the LRC-ωPBEh functional reduced to 10% HF exchange at short-range give somewhat better values for both, although still 150 meV too low for the Cu complexes and 50 meV too high for the Fe complexes. Overall, the results indicate that while HF exchange compensates for self-interaction error in DFT calculations of both Cu and Fe complexes, too much may lead to more sensitivity to nondynamical correlation in the spin-polarized Fe complexes.
Environmental Molecular Sciences Laboratory↗
The final stage of gravitational collapse for high density fluid medium
The High density high density fluids can be represented by a stiff matter state equation P={rho} and also by the Hagedorn state equation. The first is constructed using a lagrangian that allows bare nucleons to interact attractively via scalar meson exchange, and repulsively by a more massive vector meson exchange; the second consider that for large mass the spectrum of hadrons grows exponentially, namely {rho}(m) {approx}exp(m/T{sub H}), where T{sub H} is the Hagedorn temperature, resulting the state equation P = P{sub 0}+{rho}{sub 0}ln({rho}/{rho}{sub 0}). We study the gravitational collapse for a high density fluid, considering a Hagedorn state equation in a presence of a vacuum component.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Influence of emitted electrons transiting between surfaces on plasma-surface interaction
Emitted electrons are accelerated back into the plasma by the sheath. If their mean free path is large, they can propagate directly to another surface without suffering collisions. We analyze the effects of “transit” on plasma-surface interaction. When transit occurs, surfaces exchanging electrons are intricately coupled. All surfaces float more negatively than they would if the emission collisionally remixed with the bulk plasma. Asymmetries of the system drive a net “transit current” between the surfaces, which influences their potential difference. The larger the initial energy spread of the emitted electrons, the larger the potential difference.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Gilbert damping of ferromagnetic metals incorporating inhomogeneous spin dynamics
The effects of inhomogeneous spin dynamics on magnetic damping in ferromagnetic metals are studied. On the basis of linear response theory, we derive the microscopic expression for the Gilbert damping term in a two-dimensional electron gas interacting with the magnetization via exchange coupling in the presence of Rashba spin-orbit coupling (SOC). In the spin wave propagating with the wave vector, q, the behavior of q-dependent damping can be explained in terms of both inter- and intra-band spin excitations. The spatially dependent damping torques originating from Rashba SOC that cancel out in a uniform precession system distort the circular orbit of a magnetization-precession trajectory in the presence of inhomogeneous spin dynamics.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Towards quantifying the role of exact exchange in the prediction hydrogen bond spin-spin coupling constants involving fluorine
The effect of a fraction of Hartree-Fock exchange on the calculated spin-spin coupling constants involving fluorine through a hydrogen bond is analyzed in detail. Coupling constants calculated using wavefunction methods are revisited in order to get high-level calculations using the same basis set. Accurate MCSCF results are obtained using an additive approach. These constants and their contributions are used as a reference for density functional calculations. Within the density functional theory, the Hartree-Fock exchange functional is split in short- and long-range using a modified version of the Coulomb-attenuating method with the SLYP functional as well as with the original B3LYP. Results support the difficulties for calculating hydrogen bond coupling constants using density functional methods when fluorine nuclei are involved. Coupling constants are very sensitive to the Hartree-Fock exchange and it seems that, contrary to other properties, it is important to include this exchange for short-range interactions. Best functionals are tested in two different groups of complexes: those related with anionic clusters of type [F(HF){sub n}]{sup −} and those formed by difluoroacetylene and either one or two hydrogen fluoride molecules.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Helical spin structure in iron chains with hybridized boundaries
In this paper, we have compared the magnetic properties of well-controlled ultra-short (≤50 nm) atomic iron (Fe) chains embedded in Fe-phthalocyanine films with those in Fe–hydrogen (H2) phthalocyanine superlattices. Surprisingly, we found that the coercivity of the atomic chains with free boundary conditions is independent of the chain length, whereas the one subject to hybridization of the chain ends exhibits an unexpected length dependence. These findings suggest that ferromagnetism in the free-boundary condition system is caused by an intrinsic indirect exchange. On the other hand, controlled boundary conditions produce a helical spin structure due to an extrinsic indirect exchange, which arises from the interaction between iron atoms at the ends of the chain and the hydrogen in the H 2 phthalocyanine spacer. As a consequence, during magnetic reversal, ultra-short iron chains subject to boundary clamping develop a helical spin structure, leading to increased coercivity. These findings suggest unique insights and ideas for the design of atomic-scale ultra-dense magnetic storage nanodevices.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
OBSERVATIONS AND MODELS OF SLOW SOLAR WIND WITH Mg{sup 9+} IONS IN QUIESCENT STREAMERS
Quiescent streamers are characterized by a peculiar UV signature as pointed out by the results from the observations of the Ultraviolet and Coronograph Spectrometer (UVCS) on board SOHO: the intensity of heavy-ion emission lines (such as O VI) shows dimmer core relative to the edges. Previous models show that the structure of the heavy-ion streamer emission relates to the acceleration regions of the slow solar wind at streamer legs and to gravitational settling processes in the streamer core. Observations of Mg{sup 9+} ion EUV emission in coronal streamers at solar minimum were first reported by the UVCS instrument. The Mg X 625 A emission is an order of magnitude smaller than the O VI 1032 A emission, requiring longer exposures to obtain statistically significant results. Here, Mg X coronal observations are analyzed and compared, for the first time, with the solar minimum streamer structure in hydrogen and O VI emissions. We employ the 2.5D three-fluid model, developed previously to study the properties of O{sup 5+} ions in streamers, and calculate for the first time the density, temperature, and outflow structure of Mg{sup 9+} ions in the solar minimum streamer. The Mg{sup 9+} ions are heated by an empirical radial heating function constrained by observations of the kinetic ion temperature obtained from Mg X emission line profiles. The detailed structure of Mg{sup 9+} density, temperature, and outflow speed is determined by the Coulomb momentum and energy exchange as well as electromagnetic interactions with electrons and protons in the three-fluid model of the streamer. The results of the model are in good qualitative agreement with observations, and provide insights on the possible link between the magnetic structure of the streamer, slow solar wind sources, and relative abundances of heavy ions.
79 ASTRONOMY AND ASTROPHYSICS↗
Traversable wormholes in four dimensions
Here, we present a wormhole solution in four dimensions. It is a solution of an Einstein Maxwell theory plus charged massless fermions. The fermions give rise to a negative Casimir-like energy, which makes the wormhole possible. It is a long wormhole that does not lead to causality violations in the ambient space. It can be viewed as a pair of entangled near extremal black holes with an interaction term generated by the exchange of fermion fields. The solution can be embedded in the Standard Model by making its overall size small compared to the electroweak scale.
79 ASTRONOMY AND ASTROPHYSICS↗
Selective acoustic control of photon-mediated qubit-qubit interactions
Quantum technologies such as quantum sensing, quantum imaging, quantum communications, and quantum computing rely on the ability to actively manipulate the quantum state of light and matter. Quantum emitters, such as color centers trapped in solids, are a useful platform for the realization of elementary building blocks (qubits) of quantum information systems. In particular, the modular nature of such solid-state devices opens up the possibility to connect them into quantum networks and create nonclassical states of light shared among many qubits. The function of a quantum network relies on efficient and controllable interactions among individual qubits. In this context, we present a scheme where optically active qubits of differing excitation energies are mutually coupled via a dispersive interaction with a shared mode of an optical cavity. This generally off-resonant interaction is prohibitive of direct exchange of information among the qubits. However, we propose a scheme in which by acoustically modulating the qubit excitation energies it is, in fact, possible to tune to resonance a preselected pair of qubits and thus open a communication channel between them. This method potentially enables fast (approximately nanoseconds) and parallelizable on-demand control of a large number of physical qubits. In this work, we develop an analytical and a numerical theoretical model demonstrating this principle and suggest feasible experimental scenarios to test the theoretical predictions.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Phenomenological three-orbital spin-fermion model for cuprates
A spin-fermion model that captures the charge-transfer properties of Cu-based high critical temperature superconductors is proposed and investigated via Monte Carlo simulations. The strong Coulomb repulsion among d electrons in the Cu orbitals is phenomenologically replaced by an exchange coupling between the spins of the itinerant electrons and localized spins at the Cu sites, formally similar to double-exchange models for manganites. This interaction induces a charge-transfer insulator gap in the undoped case (five electrons per unit cell). Adding a small antiferromagnetic Heisenberg coupling between localized spins reinforces the global tendency towards antiferromagnetic order. To perform numerical calculations the localized spins are considered classical as in previous related efforts. Here, undoped and doped 8×8 clusters are analyzed in a wide range of temperatures. The numerical results reproduce experimental features in the one-particle spectral function and the density of states, such as: (i) the formation of a Zhang-Rice-like band with a dispersion on the order of ~0.5 eV and with rotational symmetry about the wave-vector (π/2,π/2) at the top of the band and (ii) the opening of a pseudogap at the chemical potential upon doping. We also observed incipient tendencies towards spin incommensurability. This simple model allows for an unbiased study of charge-transfer insulators and offers a formalism intermediate between standard mean-field approximations that fail at finite temperatures in regimes with short-range order and sophisticated quantum Monte Carlo techniques that suffer sign problems.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Relevance of the Heisenberg-Kitaev Model for the Honeycomb Lattice Iridates A2IrO3
Combining thermodynamic measurements with theoretical calculations we demonstrate that the iridates A{sub 2}IrO{sub 3} (A = Na, Li) are magnetically ordered Mott insulators where the magnetism of the effective spin-orbital S = 1/2 moments can be captured by a Heisenberg-Kitaev (HK) model with interactions beyond nearest-neighbor exchange. Experimentally, we observe an increase of the Curie-Weiss temperature from {Theta} {approx} -125 K for Na{sub 2}IrO{sub 3} to {Theta} {approx} -33 K for Li{sub 2}IrO{sub 3}, while the ordering temperature remains roughly the same T{sub N} {approx} 15 K. Using functional renormalization group calculations we show that this evolution of {Theta} and T{sub N} as well as the low temperature zigzag magnetic order can be captured within this extended HK model. We estimate that Na{sub 2}IrO{sub 3} is deep in a magnetically ordered regime, while Li{sub 2}IrO{sub 3} appears to be close to a spin-liquid regime.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Nonlocal Interactions in Moiré Hubbard Systems
Moiré materials formed in two-dimensional semiconductor heterobilayers are quantum simulators of Hubbard-like physics with unprecedented electron density and interaction strength tunability. Compared to atomic scale Hubbard-like systems, electrons or holes in moiré materials are less strongly attracted to their effective lattice sites because these are defined by finite-depth potential extrema. As a consequence, nonlocal interaction terms like interaction-assisted hopping and intersite exchange are more relevant. Here, we theoretically demonstrate the possibility of tuning the strength of these coupling constants to favor unusual states of matter, including spin liquids, insulating ferromagnets, and superconductors.
36 MATERIALS SCIENCE↗
Interaction models and configurational entropies of binary MoTa and the MoNbTaW high entropy alloy
We introduce a simplified method to model the interatomic interactions of high entropy alloys based on a lookup table of cluster energies. Furthermore, these interactions are employed in replica exchange Monte Carlo simulations with histogram analysis to obtain thermodynamic properties across a broad temperature range. Kikuchi's cluster variation method entropy formalism is applied to directly calculate entropy from statistics on short- and long-range chemical order, and we discuss the convergence of the entropy as clusters of differing size are included. A high temperature series expansion aids in our understanding of the convergence. Computer codes implementing these methods, and supporting data, are freely available on the internet.
36 MATERIALS SCIENCE↗
SU-E-I-43: Photoelectric Cross Section Revisited
Purpose: The importance of the precision in photoelectric cross-section value increases for recent developed technology such as dual energy computed tomography, in which some reconstruction algorithms require the energy dependence of the photo-absorption in each material composition of human being. In this study, we revisited the photoelectric cross-section calculation by self-consistent relativistic Hartree-Fock (HF) atomic model and compared with that widely distributed as “XCOM database” in National Institute of Standards and Technology, which was evaluated with localdensity approximation for electron-exchange (Fock)z potential. Methods: The photoelectric cross section can be calculated with the electron wave functions in initial atomic state (bound electron) and final continuum state (photoelectron). These electron states were constructed based on the selfconsistent HF calculation, where the repulsive Coulomb potential from the electron charge distribution (Hartree term) and the electron exchange potential with full electromagnetic interaction (Fock term) were included for the electron-electron interaction. The photoelectric cross sections were evaluated for He (Z=2), Be (Z=4), C (Z=6), O (Z=8), and Ne (Z=10) in energy range of 10keV to 1MeV. The Result was compared with XCOM database. Results: The difference of the photoelectric cross section between the present calculation and XCOM database was 8% at a maximum (in 10keV for Be). The agreement tends to be better as the atomic number increases. The contribution from each atomic shell has a considerable discrepancy with XCOM database except for K-shell. However, because the photoelectric cross section arising from K-shell is dominant, the net photoelectric cross section was almost insensitive to the different handling in Fock potential. Conclusion: The photoelectric cross-section program has been developed based on the fully self-consistent relativistic HF atomic model. Due to small effect on the Fock potential for K-shell electrons, the difference from XCOM database was limited: 1% to 8% for low-Z elements in 10keV-1MeV energy ranges. This work was partly supported by the JSPS Core-to-Core Program (No. 23003)
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Identification of the Hydrogeochemical Processes in Groundwater Using Classic Integrated Geochemical Methods and Geostatistical Techniques, in Amol-Babol Plain, Iran
Hydrogeochemical investigations had been carried out at the Amol-Babol Plain in the north of Iran. Geochemical processes and factors controlling the groundwater chemistry are identified based on the combination of classic geochemical methods with geographic information system (GIS) and geostatistical techniques. The results of the ionic ratios and Gibbs plots show that water rock interaction mechanisms, followed by cation exchange, and dissolution of carbonate and silicate minerals have influenced the groundwater chemistry in the study area. The hydrogeochemical characteristics of groundwater show a shift from low mineralized Ca-HCO 3 , Ca-Na-HCO 3 , and Ca-Cl water types to high mineralized Na-Cl water type. Three classes, namely, C 1 , C 2 , and C 3, have been classified using cluster analysis. The spatial distribution maps of Na + /Cl - , Mg 2+ /Ca 2+ , and Cl - /HCO 3 - ratios and electrical conductivity values indicate that the carbonate and weathering of silicate minerals played a significant role in the groundwater chemistry on the southern and western sides of the plain. However, salinization process had increased due to the influence of the evaporation-precipitation process towards the north-eastern side of the study area.
58 GEOSCIENCES↗