Interplay of superconductivity and magnetism in strong coupling

A model is introduced describing the interplay between superconductivity and spin-ordering. It is characterized by on-site repulsive electron-electron interactions, causing antiferromagnetism, and nearest-neighbor attractive interactions, giving rise to d-wave superconductivity. Due to a special choice for the lattice, this model has a strong-coupling limit where the superconductivity can be described by a bosonic theory, similar to the strongly coupled negative U Hubbard model. This limit is analyzed in the present paper. A rich mean-field phase diagram is found and the leading quantum corrections to the mean-field results are calculated. The first-order line between the antiferromagnetic- and the superconducting phase is found to terminate at a tricritical point, where two second-order lines originate. At these lines, the system undergoes a transition to- and from a phase exhibiting both antiferromagnetic order and superconductivity. At finite temperatures above the spin-disordering line, quantum-critical behavior is found. For specific values of the model parameters, it is possible to obtain SO(5) symmetry involving the spin- and the phase-sector at the tricritical point. Although this symmetry is explicitly broken by the projection to the lower Hubbard band, it survives on the mean-field level, and modes related to a spontaneously broken SO(5) symmetry are present on the level of the random phase approximation in the superconducting phase.Comment: 16 pages Revtex, 5 figure

Theory of site-disordered magnets

In realistic spinglasses, such as CuMn, AuFe and EuSrS, magnetic atoms are located at random positions. Their couplings are determined by their relative positions. For such systems a field theory is formulated. In certain limits it reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the Viana-Bray model. The model has a percolation transition, while for RKKY couplings the ``concentration scaling'' T_g proportional to c occurs. Within the Gaussian approximation the Ginzburg-Landau expansion is considered in the clusterglass phase, that is to say, for not too small concentrations. Near special points, the prefactor of the cubic term, or the one of the replica-symmetry- breaking quartic term, may go through zero. Around such points new spin glass phases are found.Comment: 26 pages Revtex, 6 figure

Initial Chemical Events in the Energetic Material RDX under Shock Loading: Role of Defects

We use the recently developed reactive force field ReaxFF with molecular dynamics (MD) to study the role of voids on the initial chemical events in the high-energy material RDX under shock loading. We find that for strong shocks (particles velocity of 3 km/s) very small gaps (2 nm) lead to important over-heating (~ 1000 K). This over-heating facilitates chemical reactions and leads to a larger production of small molecules (such as NO2, NO, OH) than in perfect crystals shocked with the same strength. The chemical reactions occur after the void has collapsed and the ejected material re-compressed rather than when hot molecules are ejected out of the downstream surface

Structures, Energetics, and Reaction Barriers for CH_x Bound to the Nickel (111) Surface

To provide a basis for understanding and improving such reactive processes on nickel surfaces as the catalytic steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of carbon nanotubes, we report quantum mechanics calculations (PBE flavor of density functional theory, DFT) of the structures, binding energies, and reaction barriers for all CH_x species on the Ni(111) surface using periodically infinite slabs. We find that all CH_x species prefer binding to μ3 (3-fold) sites leading to bond energies ranging from 42.7 kcal/mol for CH_3 to 148.9 kcal/mol for CH (the number of Ni-C bonds is not well-defined). We find reaction barriers of 18.3 kcal/mol for CH_(3,ad) → CH_(2,ad) + H_(ad) (with ΔE = +1.3 kcal/ mol), 8.2 kcal/mol for CH_(2,ad) → CH_(ad) + H_(ad) (with ΔE = -10.2 kcal/mol) and 32.3 kcal/mol for CH_(ad) → C_(ad) + H_(ad) (with ΔE = 11.6 kcal/mol). Thus, CH_(ad) is the stable form of CH_x on the surface. These results are in good agreement with the experimental data for the thermodynamic stability of small hydrocarbon species following dissociation of methane on Ni(111) and with the intermediates isolated during the reverse methanation process

Development and Validation of ReaxFF Reactive Force Field for Hydrocarbon Chemistry Catalyzed by Nickel

To enable the study of hydrocarbon reactions catalyzed by nickel surfaces and particles using reactive molecular dynamics on thousands of atoms as a function of temperature and pressure, we have developed the ReaxFF reactive force field to describe adsorption, decomposition, reformation and desorption of hydrocarbons as they interact with the nickel surface. The ReaxFF parameters were determined by fitting to the geometries and energy surfaces from quantum mechanics (QM) calculations for a large number of reaction pathways for hydrocarbon molecules chemisorbed onto nickel (111), (100) and (110) surfaces, supplemented with QM equations of state for nickel and nickel carbides. We demonstrate the validity and accuracy of ReaxFF by applying it to study the reaction dynamics of hydrocarbons as catalyzed by nickel particles and surfaces. For the dissociation of methyl on the (111), (100), and stepped (111) surfaces of nickel, we observe the formation of chemisorbed CH plus subsurface carbide. We observe that the (111) surface is the least reactive, the (100) surface has the fastest reaction rates, and the stepped (111) surface has an intermediate reaction rate. The importance of surface defects in accelerating reaction rates is highlighted by these results

Competing, Coverage-Dependent Decomposition Pathways for C_2H_y Species on Nickel (111)

Competing, coverage-dependent pathways for ethane (CH_3CH_3) decomposition on Ni(111) are proposed on the basis of quantum mechanics (QM) calculations, performed by using the PBE flavor of density functional theory (DFT), for all C_2H_y species adsorbed to a periodically infinite Ni(111) surface. For CH_2CH_3, CHCH_3, and CCH_3, we find that the surface C is tetrahedral in each case, with the surface C forming bonds to one, two, or three Ni atoms with bond energies scaling nearly linearly (E_(bond) = 32.5, 82.7, and 130.8 kcal/mol, respectively). In each of the remaining six C_2H_y species, both C atoms are able to form bonds to the surface. Three of these (CH_2CH_2, CHCH_2, and CCH_2) adsorb most favorably at a fcc-top site with the methylene C located at an on-top site and the other C at an adjacent fcc site. The bond energies for these species are E_(bond) = 19.7, 63.2, and 93.6 kcal/mol, respectively. The remaining species (CHCH, CCH, and C_2) all prefer binding at fcc-hcp sites, where the C atoms sit in a pair of adjacent fcc and hcp sites, with binding energies of E_(bond) = 57.7, 120.4, and 162.8 kcal/mol, respectively. We find that CHCH_(ad) is the most stable surface species (ΔH_(eth) = −18.6), and an important intermediate along the lowest-energy decomposition pathway for ethane on Ni(111). The second most stable species, CCH_3, is a close competitor (ΔH_(eth) = −18.2 kcal/mol), lying along an alternative decomposition pathway that is preferred for high-surface-coverage conditions. The existence of these competing, low- and high-coverage decomposition pathways is consistent with the experiments. The QM results reported here were used as training data in the development of the ReaxFF reactive force field describing hydrocarbon reactions on nickel surfaces [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 4939−4949]. This has enabled Reactive dynamics studying the chemisorption and decomposition of systems far too complex for quantum mechanics. Thus we reported recently, the chemisorption and decomposition of six different hydrocarbon species on a Ni_(468) nanoparticle catalysts using this ReaxFF description [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 5675−5685]

Multiparadigm modeling of dynamical crack propagation in silicon using a reactive force field

We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for several thousand atoms near the crack tip, while more than 100 000 atoms are described with a nonreactive force field. ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including changes in crack dynamics for different crack orientations

Initiation mechanisms and kinetics of pyrolysis and combustion of JP-10 hydrocarbon jet fuel

In order to investigate the initiation mechanisms and kinetics associated with the pyrolysis of JP-10 (exo-tricyclo[5.2.1.0^2,6]decane), a single-component hydrocarbon jet fuel, we carried out molecular dynamics (MD) simulations employing the ReaxFF reactive force field. We found that the primary decomposition reactions involve either (1) dissociation of ethylene from JP-10, resulting in the formation of a C8 hydrocarbon intermediate, or (2) the production of two C5 hydrocarbons. ReaxFF MD leads to good agreement with experiment for the product distribution as a function of temperature. On the basis of the rate of consumption of JP-10, we calculate an activation energy of 58.4 kcal/mol for the thermal decomposition of this material, which is consistent with a strain-facilitated C−C bond cleavage mechanism in JP-10. This compares well with the experimental value of 62.4 kcal/mol. In addition, we carried out ReaxFF MD studies of the reactive events responsible for oxidation of JP-10. Here we found overall agreement between the thermodynamic energies obtained from ReaxFF and quantum-mechanical calculations, illustrating the usefulness of ReaxFF for studying oxidation of hydrocarbons. The agreement of these results with available experimental observations demonstrates that ReaxFF can provide useful insights into the complicated thermal decomposition and oxidation processes of important hydrocarbon fuels