1,344 research outputs found
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]
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