Publications
126 results found
Horsfield AP, Lim A, Foulkes WMC, et al., 2016, Adiabatic perturbation theory of electronic stopping in insulators, Physical Review B, Vol: 93, Pages: 1-1, ISSN: 2469-9950
A model able to explain the complicated structure of electronic stopping at low velocities in insulating materials is presented. It is shown to be in good agreement with results obtained from time-dependent density-functional theory for the stopping of a channeling Si atom in a Si crystal. If we define the repeat frequency f=v/λ, where λ is the periodic repeat length of the crystal along the direction the channeling atom is traveling, and v is the velocity of the channeling atom, we find that electrons experience a perturbing force that varies in time at integer multiples l of f. This enables electronic excitations at low atom velocity, but their contributions diminish rapidly with increasing values of l. The expressions for stopping power are derived using adiabatic perturbation theory for many-electron systems, and they are then specialized to the case of independent electrons. A simple model for the nonadiabatic matrix elements is described, along with the procedure for determining its parameters.
Wearing D, Horsfield AP, Xu W, et al., 2016, Corrigendum to 'Which wets TiB2 inoculant particles: Al or Al3Ti?' [J. Alloys Compd. 664 (2016) 460-468], Journal of Alloys and Compounds, Vol: 677, Pages: 302-303, ISSN: 1873-4669
TiB2 particles are proven effective nucleants of commercial purity aluminium, resulting in smaller grains and hence greater desired mechanical properties; however, there is uncertainty as to the mechanism by which it operates. Here we clarify what happens in the initial stages by computing the total Gibbs energy change associated with four possible nucleation mechanisms, each characterised by the termination of the TiB2(0001) substrate (Ti or B) and the solid that forms on it (Al or Al3Ti). The appropriate solid//solid interfacial energies are derived from Density Functional Theory (DFT) calculations, while the bulk energies are derived from thermodynamic data, supplemented with strain energies calculated from DFT. Solid//liquid interfacial energies are estimated using simple models with parameters based on the literature and DFT calculations. The results suggest that the Ti termination of TiB2 is more stable than the B termination in the melt, and that the direct formation of Al off a Ti-terminated TiB2 substrate is the most favourable mechanism for the nucleation of Al rather than the previously proposed formation of a Al3Ti interlayer. On the B termination of TiB2, Al formation is more stable for thick solid layers, but this is much more uncertain for thin solid layers where it is possible that Al3Ti formation is more stable.
Bogatko SA, Haynes PD, Sathian J, et al., 2016, Molecular design of a room-temperature maser, The Journal of Physical Chemistry C, Vol: 120, Pages: 8251-8260, ISSN: 1932-7447
Skinner SJ, Horsfield A, Pramana S, et al., 2016, Correlation of local structure and diffusion pathways in the modulated anisotropic oxide ion conductor CeNbO4.25, Journal of the American Chemical Society, Vol: 138, Pages: 1273-1279, ISSN: 0002-7863
CeNbO4.25 is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long range commensurately modulated structural motif. Here we show that CeNbO4.25 forms with a unit cell ~12 times larger than the stoichiometric tetragonal parent phase of CeNbO4 as a result of the helical ordering of Ce3+ and Ce4+ ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species creating inter-layer “NbO6“ connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intra-layer; however when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.
Coury MEA, Dudarev SL, Foulkes WMC, et al., 2016, Hubbard-like Hamiltonians for interacting electrons in s, p, and d orbitals, Physical Review B, Vol: 93, ISSN: 1550-235X
Hubbard-like Hamiltonians are widely used to describe on-site Coulomb interactions in magnetic and strongly-correlated solids, but there is much confusion in the literature about the form these Hamiltonians should take for shells of p and d orbitals. This paper derives the most general s,p, and d orbital Hubbard-like Hamiltonians consistent with the relevant symmetries, and presents them in ways convenient for practical calculations. We use the full configuration interaction method to study p and d orbital dimers and compare results obtained using the correct Hamiltonian and the collinear and vector Stoner Hamiltonians. The Stoner Hamiltonians can fail to describe properly the nature of the ground state, the time evolution of excited states, and the electronic heat capacity.
Lim A, Foulkes WM, Horsfield AP, et al., 2016, Electron elevator: excitations across the band gap via a dynamical gap state, Physical Review Letters, Vol: 116, Pages: 1-1, ISSN: 0031-9007
We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.
Wearing D, Horsfield AP, Xu W, et al., 2015, Which wets TiB2 inoculant particles: Al or Al3Ti?, Journal of Alloys and Compounds, Vol: 664, Pages: 460-468, ISSN: 0925-8388
TiB2 particles are proven effective nucleants of commercial purity aluminium, resulting in smaller grains and hence greater desired mechanical properties; however, there is uncertainty as to the mechanism by which it operates. Here we clarify what happens in the initial stages by computing the total Gibbs energy change associated with four possible nucleation mechanisms, each characterised by the termination of the TiB2(0001) substrate (Ti or B) and the solid that forms on it (Al or Al3Ti). The appropriate solid//solid interfacial energies are derived from Density Functional Theory (DFT) calculations, while the bulk energies are derived from thermodynamic data, supplemented with strain energies calculated from DFT. Solid//liquid interfacial energies are estimated using simple models with parameters based on the literature and DFT calculations. The results suggest that the Ti termination of TiB2 is more stable than the B termination in the melt, and that the direct formation of Al off a Ti-terminated TiB2 substrate is the most favourable mechanism for the nucleation of Al rather than the previously proposed formation of a Al3Ti interlayer. On the B termination of TiB2, Al formation is more stable for thick solid layers, but this is much more uncertain for thin solid layers where it is possible that Al3Ti formation is more stable.
Horsfield AP, Wu J, skinner S, et al., 2015, Why Ni is absent from the surface of La2NiO4+delta?, Journal of Materials Chemistry A, Vol: 3, Pages: 23760-23767, ISSN: 2050-7488
La2NiO4+δ (LNO214) is a potential intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material which belongs to the Ruddlesden–Popper (RP) structure series An+1BnO3n+1. There is interest in this material as it offers a way to avoid Sr segregation and associated degradation, as LNO214 can take in oxygen interstitials and become catalytically active without A-site doping. While the bulk ionic conduction mechanisms are well studied, its surface structure and chemistry are still a matter of debate. Recent experimental studies (both with and without dopants) reveal that it has a La-terminated surface and a highly Ni deficient surface layer. These results disagree with previous computer simulations, and undermine the conventional explanation for the oxygen reduction process at the surface. In this work we evaluate the thermodynamic stability of La2NiO4+δ at IT-SOFC operation temperatures. We find that the decomposition of La2NiO4+δ to produce La2O3 and higher order RP phases is indeed thermodynamically favourable. A hypothesis for the formation mechanism of the La-terminated and Ni deficient surface based on partial decomposition and surface passivation is proposed and evaluated.
Xu W, Horsfield AP, Wearing D, et al., 2015, First-principles calculation of Mg/MgO interfacial free energies, Journal of Alloys and Compounds, Vol: 650, Pages: 228-238, ISSN: 1873-4669
Interfacial free energies strongly influence many materials properties, especially for nanomaterials that have very large interfacial areas per unit volume. Quantitative evaluation of interfacial free energy by means of computer simulation remains difficult in these cases, especially at finite temperature. Density Functional Theory (DFT) simulation offers a robust way to compute both the energies and structures of the relevant surfaces and interfaces at the atomic level at zero Kelvin, and can be extended to finite temperatures in solids by means of the harmonic approximation (HA). Here we study the Mg/MgO interface, employing DFT calculations within the HA to obtain its key physical properties. We calculate the free energies of several key surfaces/interfaces when the temperature (T) increases from 0 K to 800 K, finding that all free energies decrease almost linearly with T. We have considered two surfaces, Mg(0001) (0.520–0.486 J/m2), and MgO(100) (0.86–0.52 J/m2), and two Mg(0001)//MgO(100) interfaces with the Mg–Mg and Mg–O stacking sequences at the interface planes (1.048–0.873 J/m2 and 0.910 to 0.743 J/m2 respectively). Using these values we determine the interfacial free energy as a function of temperature and size for MgO nanoparticles in solid Mg, an important metal matrix nanocomposite material.
Puncreobutr C, Phillion AB, Fife JL, et al., 2014, In situ quantification of the nucleation and growth of Fe-rich intermetallics during Al alloy solidification, ACTA MATERIALIA, Vol: 79, Pages: 292-303, ISSN: 1359-6454
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- Citations: 82
Turin L, Skoulakis EMC, Horsfield AP, 2014, Electron spin changes during general anesthesia in <i>Drosophila</i>, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 111, Pages: E3524-E3533, ISSN: 0027-8424
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- Citations: 41
Stoneham AM, Turin L, Brookes JC, et al., 2014, Quantum vibrational effects on sense of smell, QUANTUM EFFECTS IN BIOLOGY, Editors: Mohseni, Omar, Engel, Plenio, Publisher: CAMBRIDGE UNIV PRESS, Pages: 264-276
Wiener A, Fernandez-Dominguez AI, Pendry JB, et al., 2013, Nonlocal propagation and tunnelling of surface plasmons in metallic hourglass waveguides, OPTICS EXPRESS, Vol: 21, Pages: 27509-27518, ISSN: 1094-4087
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- Citations: 12
Wiener A, Duan H, Bosman M, et al., 2013, Electron-Energy Loss Study of Nonlocal Effects in Connected Plasmonic Nanoprisms, ACS NANO, Vol: 7, Pages: 6287-6296, ISSN: 1936-0851
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- Citations: 60
Race CP, Mason DR, Foo MHF, et al., 2013, Quantum-classical simulations of the electronic stopping force and charge on slow heavy channelling ions in metals, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 25, ISSN: 0953-8984
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- Citations: 16
Brookes JC, Horsfield AP, Stoneham AM, 2012, The Swipe Card Model of Odorant Recognition, SENSORS, Vol: 12, Pages: 15709-15749
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- Citations: 33
Mason DR, Race CP, Foo MHF, et al., 2012, Resonant charging and stopping power of slow channelling atoms in a crystalline metal, NEW JOURNAL OF PHYSICS, Vol: 14, ISSN: 1367-2630
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- Citations: 15
Wiener A, Fernandez-Dominguez AI, Horsfield AP, et al., 2012, Nonlocal Effects in the Nanofocusing Performance of Plasmonic Tips, NANO LETTERS, Vol: 12, Pages: 3308-3314, ISSN: 1530-6984
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- Citations: 120
Horsfield A, 2012, Where does tight binding go from here?, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Vol: 249, Pages: 231-236, ISSN: 0370-1972
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- Citations: 7
Kena-Cohen S, Wiener A, Sivan Y, et al., 2011, Plasmonic sinks for the selective removal of long-lived states, ACS Nano
Todorov TN, Dundas D, Paxton AT, et al., 2011, Nonconservative current-induced forces: A physical interpretation, BEILSTEIN JOURNAL OF NANOTECHNOLOGY, Vol: 2, Pages: 727-733, ISSN: 2190-4286
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- Citations: 19
Miranda RP, Fisher AJ, Stella L, et al., 2011, A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. II. Coulomb interaction effects in single conjugated polymer chains, JOURNAL OF CHEMICAL PHYSICS, Vol: 134, ISSN: 0021-9606
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- Citations: 18
Miranda RP, Fisher AJ, Stella L, et al., 2011, A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. I. General formalism and application to open-shell states, JOURNAL OF CHEMICAL PHYSICS, Vol: 134, ISSN: 0021-9606
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- Citations: 45
Soin P, Horsfield AP, Nguyen-Manh D, 2011, Efficient self-consistency for magnetic tight binding, COMPUTER PHYSICS COMMUNICATIONS, Vol: 182, Pages: 1350-1360, ISSN: 0010-4655
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- Citations: 14
Stella L, Miranda RP, Horsfield AP, et al., 2011, Analog of Rabi oscillations in resonant electron-ion systems, JOURNAL OF CHEMICAL PHYSICS, Vol: 134, ISSN: 0021-9606
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- Citations: 7
Mason DR, Race CP, Foulkes WMC, et al., 2011, Quantum mechanical simulations of electronic stopping in metals, Nucl. Instrum. Meth. Phys. Res. B, Vol: 269, Pages: 1640-1645, ISSN: 0168-583X
Wang J, Horsfield A, Schwingenschloegl U, et al., 2010, Heterogeneous nucleation of solid Al from the melt by TiB<sub>2</sub> and Al<sub>3</sub>Ti: An <i>ab initio</i> molecular dynamics study, PHYSICAL REVIEW B, Vol: 82, ISSN: 2469-9950
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- Citations: 76
Race CP, Mason DR, Finnis MW, et al., 2010, The treatment of electronic excitations in atomistic models of radiation damage in metals, REPORTS ON PROGRESS IN PHYSICS, Vol: 73, ISSN: 0034-4885
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- Citations: 102
Wang J, Horsfield A, Lee PD, et al., 2010, Heterogeneous nucleation of solid Al from the melt by Al<sub>3</sub>Ti: Molecular dynamics simulations, PHYSICAL REVIEW B, Vol: 82, ISSN: 2469-9950
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- Citations: 45
McEniry EJ, Wang Y, Dundas D, et al., 2010, Modelling non-adiabatic processes using correlated electron-ion dynamics, EUROPEAN PHYSICAL JOURNAL B, Vol: 77, Pages: 305-329, ISSN: 1434-6028
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- Citations: 28
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