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  • 1
    Electronic Resource
    Electronic Resource
    Freezing of simple systems using density functional theory (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5187-5193 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional theory (DFT) has been applied to the study of the fluid–solid transition in systems with realistic potentials (soft cores and attractive forces): the purely repulsive WCA Lennard-Jones reference potential (LJT), the full Lennard-Jones potential (LJ) and the exponential-6 potential appropriate for helium and hydrogen. Three different DFT formalisms were used: the formulation of Haymet and Oxtoby (HO) and the new theories of Denton and Ashcroft (MWDA) and of Baus (MELA). The results for the melting pressure are compared with recent simulation and experimental data. The results of the HO version are always too high, the deviation increasing when going from the repulsive Lennard-Jones to the exponential-6 potential of H2. The MWDA gives too low results for the repulsive Lennard-Jones potential. At low temperatures, it fails for the full LJ potential while at high temperatures it is in good agreement. Including the attraction as a mean-field correction gives good results also for low temperatures. The MWDA results are too high for the exponential-6 potentials. The MELA fails completely for the LJT potential and the hydrogen exponential-6 potential, since it does not give a stable solid phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458657
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  • 2
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of binary soft-sphere mixtures: Jump motions of atoms in the glassy state (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3879-3886 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Three very long runs of constant temperature molecular dynamics (MD) simulations, extending over typically 2×105 time steps, have been carried out for a state just above the glass transition of equimolar soft-sphere mixtures, with 500 atoms interacting through vαβ(r) =ε(σαβ /r)12 , where σαβ =(σα +σβ )/2 , and α,β (species indices)=1, 2. The ratio of diameters σ2/σ1 was chosen to be 1.2. Jump-type atomic motions are clearly found to occur in the glassy states. A coordinated (strongly correlated) jump motion was observed, where a cluster of several atoms dynamically linked at nearest-neighbor distances jump at successive close times. These simulations demonstrate that the characteristic time scale of structural relaxation becomes very long near the glass transition, and calculated mean square displacements exhibit complicated behaviors. Thermodynamic quantities, however, such as the mean pressure, energy, and pair distribution functions, appear to behave normaly, with no significant observable relaxations for increasing time, so that their averages are well defined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453836
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  • 3
    Electronic Resource
    Electronic Resource
    Density functional theory of soft sphere freezing (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6360-6365 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The freezing transition of "soft spheres,'' which interact via inverse-power potentials v(r)∼r−n, is systematically investigated for various values of the steepness n, using density functional theory. Two "second-order'' theories are considered, which require as only input the static pair structure of the homogeneous fluid. Both theories overestimate the densities of the coexisting fluid and solid phases, and the disagreement with computer simulation data worsens as the steepness n decreases. The two theories also predict that the fcc phase is more stable than the bcc phase for all n, which contradicts the behavior expected for soft repulsions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452422
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  • 4
    Electronic Resource
    Electronic Resource
    Accurate effective pair potentials for polymer solutions (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 4296-4311 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dilute or semidilute solutions of nonintersecting self-avoiding walk (SAW) polymer chains are mapped onto a fluid of "soft" particles interacting via an effective pair potential between their centers of mass. This mapping is achieved by inverting the pair distribution function of the centers of mass of the original polymer chains, using integral equation techniques from the theory of simple fluids. The resulting effective pair potential is finite at all distances, has a range of the order of the radius of gyration, and turns out to be only moderately concentration-dependent. The dependence of the effective potential on polymer length is analyzed in an effort to extract the scaling limit. The effective potential is used to derive the osmotic equation of state, which is compared to simulation data for the full SAW segment model, and to the predictions of renormalization group calculations. A similar inversion procedure is used to derive an effective wall–polymer potential from the center of mass density profiles near the wall, obtained from simulations of the full polymer segment model. The resulting wall–polymer potential turns out to depend strongly on bulk polymer concentration when polymer–polymer correlations are taken into account, leading to a considerable enhancement of the effective repulsion with increasing concentration. The effective polymer–polymer and wall–polymer potentials are combined to calculate the depletion interaction induced by SAW polymers between two walls. The calculated depletion interaction agrees well with the "exact" results from much more computer-intensive direct simulation of the full polymer-segment model, and clearly illustrates the inadequacy—in the semidilute regime—of the standard Asakura–Oosawa approximation based on the assumption of noninteracting polymer coils. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344606
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  • 5
    Electronic Resource
    Electronic Resource
    Structure and phase behavior of a model clay dispersion: A molecular-dynamics investigation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 311-322 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reversible molecular-dynamics (MD) simulations have been carried out on simple models for dispersions of circular Laponite clay platelets to investigate the local structure on a mesoscopic scale. The platelets carry discrete charged sites interacting via a screened Coulomb potential. In model A all surface sites have identical negative charge, while model B also includes rim charges of opposite sign. These two models were used in a series of simulations in the semidilute regime, and for three values of the Debye screening length. The structure of the dispersions is characterized by translational and orientational pair distribution functions, and by the corresponding structure factors. Qualitative differences in the pair structure arising from variations in concentration and screening length lead to a tentative identification of sol, gel, and crystal phases. The rim charges have a dramatic effect on the local structure in the strong screening regime, leading to T-shaped pair configuration and clustering of the platelets at low clay concentrations, and at higher concentrations to a space-filling "house-of-cards" structure. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480582
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  • 6
    Electronic Resource
    Electronic Resource
    Structural and electrical characteristics of Ge and Se implanted InP after rapid thermal annealing (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 2249-2255 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical activation of Ge and Se implanted into 〈100(approximately-greater-than) InP at elevated temperatures and the annealing of the implantation-induced disorder have been investigated by differential Hall/resistivity measurements, Rutherford backscattering spectrometry, and transmission electron microscopy. At implantation temperatures below 170 °C, an amorphous layer is created by the implantation process. After recrystallization by rapid thermal annealing of the amorphized layer, a localized defect band was found, which gives rise to a dip (M shape) in the carrier density profile. This band is believed to be caused by a stoichiometric imbalance of In and P. For implantation temperatures above 170 °C, the samples remain crystalline during implantation and as a result, no localized defect band is observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351618
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  • 7
    Electronic Resource
    Electronic Resource
    Mass density and hydrogen content of diamondlike carbon as related to the preparation by plasma-enhanced chemical vapor deposition (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 4050-4056 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Combined neutron and x-ray reflectometry was used to determine mass density and hydrogen content for thin (less than 3000 A(ring)) diamondlike hydrocarbon films on polished Si (100) substrates. The introduction of "twin'' neutron reflectometry improved the accuracy of the measurements. Mass densities in the range of 1.60–1.84 g/cm3 and hydrogen contents of 31–36 at. % were determined together with the nucleus densities of hydrogen and carbon. A series of diamondlike carbon films prepared by plasma-enhanced chemical vapor deposition was investigated to analyze the effect of depositing films at different flow rates of methane precursor gas. The mass density was found to increase with flow in the range of 1–8 standard cm3/min (sccm). For higher flows the mass density was constant. The hydrogen content did not depend on the rate of flow in the interval of 2–30 sccm. Finally, results of the mass density measured in a simpler way, using a scanning electron microscope and a profilometer, were compared to the results obtained by neutron and x-ray reflectometry. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361833
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  • 8
    Electronic Resource
    Electronic Resource
    Role of vibrational softening in fast dynamics of an amorphous polyimide far below Tg (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6492-6497 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report quasielastic neutron scattering results on an amorphous polyimide (AURUM) in a temperature range from 10 to 300 K below the glass transition temperature Tg(=250 °C). As temperature increases from 10 K, anharmonic excess scattering intensity appears at around 200 K or about 300 K below Tg. This excess scattering is very similar to the so-called fast process in picosecond order observed for glass-forming polymers as well as organic and inorganic low molecular weight glass-forming materials. We first analyzed the spectra in a simple way assuming validity of the Bose scaling for the vibrational density of states and a Lorentzian for the additional relaxational process, and found that the excess scattering intensity appears in the frequency range below the Boson peak and the characteristic time is independent of temperature which are common features for the fast processes reported previously. The second analysis using the recent vibration-relaxation (VR) model, which is more realistic, indicated that softening of the vibrational modes may play an important role for the change of spectra from inelastic-like to quasielastic-like. Nevertheless, relaxational process is still necessary to describe the excess scattering although the temperature range examined is very far below Tg. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476055
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  • 9
    Electronic Resource
    Electronic Resource
    Smooth YBa2Cu3O7−x thin films prepared by pulsed laser deposition in O2/Ar atmosphere (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 3178-3180 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on pulsed laser deposition of YBa2Cu3O7−x in a diluted O2/Ar gas resulting in thin epitaxial films which are almost outgrowth-free. Films were deposited on SrTiO3 or MgO substrates around 800 °C at a total chamber pressure of 1.0 mbar, varying the argon partial pressure from 0 to 0.6 mbar. The density of boulders and outgrowths usual for laser deposited films varies strongly with Ar pressure: the outgrowth density is reduced from 1.4×107 to 4.5×105 cm−2 with increasing Ar partial pressure, maintaining a critical temperature Tc,zero≈90 K and a transport critical current density Jc(77 K)≥106 A/cm2 by extended oxygenation time during cool down.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111331
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  • 10
    Electronic Resource
    Electronic Resource
    Hydrogen concentration and mass density of diamondlike carbon films obtained by x-ray and neutron reflectivity (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 4636-4642 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Specular reflectivity of neutrons and x rays can be used to determine the scattering length density profile of a material perpendicular to its surface. We have applied these techniques to study amorphous, diamondlike, hydrocarbon films. By the combination of these two techniques we obtain not only the mass density, but also the concentration of hydrogen, which varies in our case between 0 and 30 at. %. This method is a new and nondestructive way to determine the concentration of hydrogen within an error of less than 2 at. % in samples with sharp interfaces. It is especially suited for diamondlike carbon films.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357300
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