35 results found
Tangney P, Coiana G, Lischner J, 2024, Breakdown of phonon band theory in MgO, Physical Review B: Condensed Matter and Materials Physics, Vol: 109, ISSN: 1098-0121
We present a series of detailed images of the distribution of kinetic energy among frequencies and wave vectors in the bulk of an MgO crystal as it is heated slowly until it melts. These spectra, which are Fourier transforms of mass-weighted velocity-velocity correlation functions calculated from accurate molecular dynamics (MD) simulations, provide a valuable perspective on the growth of thermal disorder in ionic crystals. We use them to explain why the most striking and rapidly progressing departures from a band structure occur among longitudinal optical (LO) modes, which would be the least active modes at low temperature (T) if phonons did not interact. The degradation of the LO band begins, at low T, as an anomalously large broadening of modes near the center of the Brillouin zone (BZ), which gradually spreads towards the BZ boundary. The LO band all but vanishes before the crystal melts, and transverse optical (TO) modes' spectral peaks become so broad that the TO branches no longer appear band-like. Acoustic bands remain relatively well defined until melting of the crystal manifests in the spectra as their sudden disappearance. We argue that, even at high T, the long wavelength acoustic (LWA) phonons of an ionic crystal can remain partially immune to disorder generated by its LO phonons; whereas, even at low T, its LO phonons can be strongly affected by LWA phonons. This is because LO displacements average out in much less than the period of an LWA phonon; whereas during each period of an LO phonon, an LWA phonon appears as a quasistatic perturbation of the crystal, which warps the LO mode's intrinsic electric field. LO phonons are highly sensitive to acoustic warping of their intrinsic fields because their frequencies depend strongly on them: They cause the large frequency difference between LO and TO bands known as LO-TO splitting. We calculate vibrational spectra from MD trajectories using a method that we show to be classically exact and therefore applicab
Bandera S, O'Sullivan C, Tangney P, et al., 2023, Response to the discussion on ?Coarse-grained molecular dynamics of clay compression?, COMPUTERS AND GEOTECHNICS, Vol: 155, ISSN: 0266-352X
Bandera S, Angioletti-Uberti S, Tangney P, et al., 2021, Coarse-grained molecular dynamics simulations of clay compression, Computers and Geotechnics, Vol: 138, Pages: 1-18, ISSN: 0266-352X
This paper outlines a framework for using molecular dynamics to simulate the compression of kaolinite saturated at alkaline pH (=8) in a low (1 mM) concentration solution. The particles are modelled as flat (3D) ellipsoids and their interactions are described by a modified form of the Gay-Berne potential, calibrated against DLVO theory. The LAMMPS software was used to generate monodisperse and slightly polydisperse samples, and to simulate isotropic compression to 100 kPa. The influences of sample size and strain rate on the void ratio and the arrangement of particles within the samples were investigated via parametric studies. It is useful to consider the extent to which the system temperature (a measure of the average kinetic energy) is controlled when assessing whether the applied strain rate is appropriate. It is found that the number of particles that can be considered a representative element volume is orders of magnitude larger than the number simulated in earlier studies and that larger number of particles are required in polydisperse samples than in the monodisperse case. A comparison between the results obtained and those of published experimental studies show that the methodology proposed can deliver sensible results for the material considered.
Gu F, Murray E, Tangney P, 2021, Carrier-mediated control over the soft mode and ferroelectricity in BaTiO3, Physical Review Materials, Vol: 5, ISSN: 2475-9953
We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of BaTiO3 (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the A1 soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent A1 ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.
Bandera S, O'Sullivan C, Angioletti-Uberti S, et al., 2019, An evaluation of contact models for particle-scale simulation of clay, 7th International Symposium on Deformation Characteristics of Geomaterials (IS-Glasgow 2019), Publisher: EDP Sciences, Pages: 1-6
Geotechnical engineers are well aware that the particle surface chemistry and the pore fluid composition can significantly influence the mechanical behaviour of clay. Reference is often made to the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory, which enables the electrochemical interactions between charged particles to be estimated. Hitherto, the absence of an effective framework for particle-scale simulation of clay has inhibited a direct link between these electrochemical interactions and clay behaviour (e.g. load:deformation response) or fabric (i.e. the development of a disperse or flocculated fabric). Ebrahimi  demonstrated the viability of using molecular dynamics simulations where the clay grains are simulated as ellipsoidal particles whose interactions are described by an analytical expression called the Gay-Berne (GB) potential. While promising when compared to other approaches documented in the literature, Ebrahimi's work considered only a single clay mineralogy and did not explicitly account for the pore fluid composition. This paper considers the use of the Gay-Berne potential in particle-scale modelling of clay from a more general perspective. Calibration of the GB model parameters to predict kaolinite particle interactions reveals a lack of generality in Ebrahimi's approach. The Gay-Berne potential cannot simulate situations in which attractive and repulsive interactions co-exist, which lead to the classical “cardhouse” fabric, as is the case of kaolinite particles interacting via an acidic pore fluid.
Lischner J, Blumenthal L, Tangney P, et al., 2018, Band alignment at solid-liquid interfaces and nanoplasmonic photocatalysis, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Blumenthal LB, Kahk JMK, Sundararaman RS, et al., 2017, Energy level alignment at semiconductor-water interfaces from atomistic and continuum solvation models, RSC Advances: an international journal to further the chemical sciences, Vol: 7, Pages: 43660-43670, ISSN: 2046-2069
Accurate and efficient methods for predicting the alignment between a semiconductor's electronic energy levels and electrochemical redox potentials are needed to facilitate the computational discovery of photoelectrode materials. In this paper, we present an approach that combines many-body perturbation theory within the GW method with continuum solvation models. Specifically, quasiparticle levels of the bulk photoelectrode are referenced to the outer electric potential of the electrolyte by calculating the change in electric potential across the photoelectrode–electrolyte and the electrolyte–vacuum interfaces using continuum solvation models. We use this method to compute absolute energy levels for the prototypical rutile (TiO2) photoelectrode in contact with an aqueous electrolyte and find good agreement with predictions from atomistic simulations based on molecular dynamics. Our analysis reveals qualitative and quantitative differences of the description of the interfacial charge density in atomistic and continuum solvation models and highlights the need for a consistent treatment of electrode–electrolyte and electrolyte–vacuum interfaces for the determination of accurate absolute energy levels.
We propose a new deformable free energy method for generating a free-energy coarse-graining potential for C60. Potentials generated from this approach exhibit a strong temperature dependence and produce excellent agreement with benchmark fully atomistic molecular dynamics simulations. Parameter sets for analytical fits to this potential are provided at four different temperatures.
Guhl H, Lee HS, Tangney P, et al., 2015, Structural and electronic properties of sigma7 grain boundaries in alpha-Al2O3, Acta Materialia, Vol: 99, Pages: 16-28, ISSN: 1359-6454
Applying simulated annealing with a classical potential followed by screening of low-energy structures with density functional theory, we examined the atomic and electronic structures of the View the MathML source and View the MathML source symmetric tilt grain boundaries in α-Al2O3. The lowest energy View the MathML source boundary exhibits a pronounced pattern of alternating columns of exclusively four- or fivefold coordinated Al atoms, with a grain boundary energy of 1.84 Jm−2. For the View the MathML source boundary, numerous structures were found with energy just below 2.11 Jm−2. Furthermore, by analysing the full set of candidate structures generated by simulated annealing for the two grain boundaries, we find that the number of fivefold coordinated Al atoms tends to increase with grain boundary energy, which we can also correlate with the behaviour of the electronic density of states. On the other hand, we find no systematic trend with energy that might be expected for other quantities, notably the excess volume of the interface. We compare simulated high-resolution transmission electron microscope (HRTEM) images of the lowest energy calculated structures with experimental images. The disparate structural and electronic features of these two boundaries suggest reasons for their very different oxygen diffusion coefficients that have been observed experimentally.
Sarsam J, Finnis MW, Tangney P, 2013, Atomistic force field for alumina fit to density functional theory, JOURNAL OF CHEMICAL PHYSICS, Vol: 139, ISSN: 0021-9606
Avraam PW, Hine NDM, Tangney P, et al., 2012, Fermi-level pinning can determine polarity in semiconductor nanorods, PHYSICAL REVIEW B, Vol: 85, ISSN: 1098-0121
Hine NDM, Avraam PW, Tangney P, et al., 2012, Linear-Scaling Density Functional Theory Simulations of Polar Semiconductor Nanorods, 3rd Workshop on Theory, Modelling and Computational Methods for Semiconductor Materials and Nanostructures (TMCS), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Avraam PW, Hine NDM, Tangney P, et al., 2011, Factors influencing the distribution of charge in polar nanocrystals, PHYSICAL REVIEW B, Vol: 83, ISSN: 1098-0121
We perform first-principles calculations of wurtzite GaAs nanorods to explore the factors determining charge distributions in polar nanostructures. We show that both the direction and magnitude of the dipole moment d of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated and do not depend strongly on the spontaneous polarization of the underlying lattice. We identify two physical mechanisms by which d is controlled by the surface termination, and we show that the excess charge on the nanorod ends is not strongly localized. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly.
Kermode JR, Cereda S, Tangney P, et al., 2010, A first principles based polarizable O(N) interatomic force field for bulk silica, JOURNAL OF CHEMICAL PHYSICS, Vol: 133, ISSN: 0021-9606
Han XJ, Bergqvist L, Dederichs PH, et al., 2010, Polarizable interatomic force field for TiO(2) parametrized using density functional theory, PHYSICAL REVIEW B, Vol: 81, ISSN: 1098-0121
We report a classical interatomic force field for TiO(2), which has been parametrized using density functional theory forces, energies, and stresses in the rutile crystal structure. The reliability of this classical potential is tested by evaluating the structural properties, equation of state, phonon properties, thermal expansion, and some thermodynamic quantities such as entropy, free energy, and specific heat under constant volume. The good agreement of our results with ab initio calculations and with experimental data, indicates that our force field describes the atomic interactions of TiO(2) in the rutile structure very well. The force field can also describe the structures of the brookite and anatase crystals with good accuracy.
Tangney P, Scandolo S, 2009, Melting slope of MgO from molecular dynamics and density functional theory, JOURNAL OF CHEMICAL PHYSICS, Vol: 131, ISSN: 0021-9606
Choi I-H, Yu PY, Tangney P, et al., 2007, Vibrational properties of single walled carbon nanotubes under pressure from Raman scattering experiments and molecular dynamics simulations, 12th International Conference on High Pressure Semiconductor Physics (HPSP-12), Publisher: WILEY-V C H VERLAG GMBH, Pages: 121-126, ISSN: 0370-1972
Tangney P, Cohen ML, Louie SG, 2006, Giant wave-drag enhancement of friction in sliding carbon nanotubes, PHYSICAL REVIEW LETTERS, Vol: 97, ISSN: 0031-9007
Ribeiro FJ, Tangney P, Louie SG, et al., 2006, Hypothetical hard structures of carbon with cubic symmetry, PHYSICAL REVIEW B, Vol: 74, ISSN: 2469-9950
Cronin SB, Yin Y, Walsh A, et al., 2006, Temperature dependence of the optical transition energies of carbon nanotubes:: The role of electron-phonon coupling and thermal expansion, PHYSICAL REVIEW LETTERS, Vol: 96, ISSN: 0031-9007
Tangney P, 2006, On the theory underlying the Car-Parrinello method and the role of the fictitious mass parameter, JOURNAL OF CHEMICAL PHYSICS, Vol: 124, ISSN: 0021-9606
Ribeiro FJ, Tangney P, Louie SG, et al., 2005, Structural and electronic properties of carbon in hybrid diamond-graphite structures, PHYSICAL REVIEW B, Vol: 72, ISSN: 2469-9950
Tangney P, Capaz RB, Spataru CD, et al., 2005, Structural transformations of carbon nanotubes under hydrostatic pressure, NANO LETTERS, Vol: 5, Pages: 2268-2273, ISSN: 1530-6984
Capaz RB, Spataru CD, Tangney P, et al., 2005, Temperature dependence of the band gap of semiconducting carbon nanotubes, PHYSICAL REVIEW LETTERS, Vol: 94, ISSN: 0031-9007
Capaz RB, Spataru CD, Tangney P, et al., 2005, Temperature and hydrostatic pressure effects on the band gap of semiconducting carbon nanotubes, 27th International Conference on the Physics of Semiconductors (ICPS-27), Publisher: AMER INST PHYSICS, Pages: 1047-1048, ISSN: 0094-243X
Capaz RB, Spataru CD, Tangney P, et al., 2005, Family behavior of the pressure and temperature dependences of the band gap of semiconducting carbon nanotubes, 19th International Winterschool and Euroconference on Electronic Properties of Novel Materials, Publisher: AMER INST PHYSICS, Pages: 411-415, ISSN: 0094-243X
Capaz RB, Spataru CD, Tangney P, et al., 2004, Hydrostatic pressure effects on the structural and electronic properties of carbon nanotubes, 11th International Conference on High-Pressure Semiconductor Physics (HPSP-11), Publisher: WILEY-V C H VERLAG GMBH, Pages: 3352-3359, ISSN: 0370-1972
Tangney P, Louie SG, Cohen ML, 2004, Dynamic sliding friction between concentric carbon nanotubes, PHYSICAL REVIEW LETTERS, Vol: 93, ISSN: 0031-9007
Tangney P, Scandolo S, 2003, A many-body interatomic potential for ionic systems: Application to MgO, JOURNAL OF CHEMICAL PHYSICS, Vol: 119, Pages: 9673-9685, ISSN: 0021-9606
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