Search Results - (Author, Cooperation:K. Whaley)

2014-08-30

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Amino Acid Sequence ; Animals ; Antibodies, Monoclonal/immunology/*therapeutic use ; Antibodies, Neutralizing/immunology/therapeutic use ; Antibodies, Viral/immunology/*therapeutic use ; Cross Reactions/immunology ; Ebolavirus/immunology ; Enzyme-Linked Immunosorbent Assay ; Female ; Guinea ; Guinea Pigs ; Hemorrhagic Fever, Ebola/blood/*drug therapy/immunology/virology ; *Immunization, Passive ; Macaca mulatta/immunology/virology ; Male ; Molecular Sequence Data ; Sequence Alignment ; Viral Envelope Proteins/chemistry/immunology ; Viremia/drug therapy/immunology/virology

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The dynamics of an adsorbate described by a two-band generalization of the small-polaron model [J. Chem. Phys. 95, 8599 (1991)] are investigated. Lattice induced mixing of the bands can result in the adsorbates becoming self-trapped at one end of an adsorption site or the other. Tunneling between adjacent sites can either preserve or change the end of the site in which the adsorbate resides. The associated rates have very different temperature and mass dependences, and contribute to the overall diffusion in very different ways. The observed equilibrium and transport properties of the H/W(110) system can be consistently explained within our model.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A projected continuum formulation for time-independent quantum scattering from a continuum of channels is presented and applied to atomic scattering from disordered surfaces. Projecting out the channel continua gives rise to explicitly constructed optical potentials for the coherently diffracted scattering amplitudes, while the continuum diffuse scattering contribution is obtained in terms of these wave functions. Numerical application is made to atomic scattering from weakly corrugated disordered surfaces, for which a truncated set of coupled equations can be used within which the theory is shown to be unitary. The occurrence of a new phenomenon of disorder-induced sticking on rigid surfaces is demonstrated and comparison with phonon-induced sticking is made. These new disorder-induced sticking processes are shown to have very different energy dependence than phonon-induced sticking, with strong resonant sticking and oscillatory behavior seen at low incident energies.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A two-band generalization of the Hubbard model is developed in order to examine the different roles of adsorbate interactions with one another and with phonons. Variationally optimized unitary transformations of the Hamiltonian are performed in order to obtain dressed tunneling amplitudes. These display strong dimensional dependence and demonstrate fundamental differences between the effects of coupling of adsorbate density and of adsorbate vibrational excitation to the lattice modes. The occurrence of on-site asymmetric self-trapping induced by even weak coupling of interband excitations to the lattice is demonstrated, and the effect of this on the thermally averaged, intersite tunneling is investigated. Implications for surface transport and for surface reconstructions of hydrogen/metal systems are made.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

This paper presents a mean-field solution for a one-dimensional spin Hamiltonian in the presence of spatially varying interactions and external field. In a binary alloy, such inhomogeneous interactions appear in the presence of a grain boundary. We derive the model and place it in the context of previous theories. We show how our theory is a natural extension of traditional segregation isotherm models, with the advantages that much finer detail can be observed and that no assumption is required about the grain boundary binding energy. Solving the model requires finding the global minimum of a function of several hundred variables and yields detailed concentration profiles in the presence of spatially inhomogeneous and long-range interactions. We apply the theory to the system of copper with bismuth impurities and observe on an atomic scale how the extent of segregation varies with temperature. The results predict that with lower temperature the impurity concentration in a given layer increases, the segregant peak broadens, and ordering can occur within the boundary. The results also indicate that the presence of segregation at the grain boundary can serve as a nucleus for order–disorder phase transitions in the bulk.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Theoretical methods are developed for determining the collective vibrational excitation spectra of quantum clusters, and applied to clusters of 4He. A quantum liquid drop model gives excitation energies in terms of two-point ground state density correlations, which are evaluated from microscopic calculations of the ground state wave functions. An alternative approach based on a Bijl–Feynman ansatz for the excited states also yields excitation energies in terms of ground state correlations, without the imposition of a sharp liquid surface. These methods are applied to the calculation of the excitation spectra of 4HeN , N=20, 70, and 240 clusters. The relationship between the collective spectra of these clusters and that of bulk fluid He II, and the significance of an observed roton minimum in the spectrum for N≥70 are discussed.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Self-consistent-field (SCF) methods are developed for solution of multidimensional diffusion problems. Time-dependent self-consistent-field (TDSCF) equations are derived for the Smoluchowski diffusion equation, and are applied to a two-dimensional barrier crossing problem. This is compared to both time-dependent and time-independent SCF approximations derived for the Schrödinger equation in imaginary time, which is obtained by transformation of the diffusion equation. Results for the model problem show that the TDSCF approximation for the original diffusion equation is accurate, efficient, and readily implementable in higher dimensions. Applications to diffusion problems in condensed media are noted.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

This paper describes a systematic microscopic study of solute segregation and ordering at a grain boundary. We develop for this inhomogeneous system several Monte Carlo techniques and apply these to analyze the distribution of substitutional impurities near a symmetric coincident-site-lattice tilt boundary. The calculations demonstrate the importance of ensemble and boundary condition for a Monte Carlo simulation, especially one with an inhomogeneous lattice and with ordering, as opposed to segregating, bulk interactions. The resulting concentration profiles exhibit segregation to the boundary at high temperatures and bulk ordering at low temperature. Based on our results, we propose a mechanism for a solid–solid interfacial ordering phase transition previously suggested by experiment. We also compare these simulations to our earlier one-dimensional mean-field work and find that the three-dimensional simulations confirm the essential mean-field predictions.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The ground and several l=0 breathing mode vibrational excited state wave functions of HeN clusters are determined for N=3–5, 20, 70, and 240, using the variational Monte Carlo method. These wave functions incorporate one-, two-, and three-particle correlation effects and give binding energies, density profiles, and vibrational excitation energies accurately. The larger clusters have liquid-like structure, characterized by a pair distribution function showing approximately two coordination shells. The smallest clusters (N=3, 4) have extensively delocalized structures, which on average are equilateral triangular and tetrahedral, respectively. The N=5 cluster has a totally symmetric average structure, which can only be described in terms of a quantum liquid. No molecular structure, whether rigid or floppy can be assigned in this case. The relative importance of various correlation effects in clusters of different sizes is analyzed and discussed. These wave functions are completely analytical and are convenient as importance functions in diffusion and Green's function Monte Carlo calculations.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The time dependent many-body dynamics of strongly interacting particles on a two-dimensional lattice are studied with the recently developed checkerboard propagator. Application is made to the two-dimensional infinite U Hubbard model at high concentrations, for both fermion and boson systems. The concentration, initial state, and boundary condition dependence of vacancy correlation functions are analyzed, and related to the local densities of states. Quasidissipative behavior of the correlation functions is seen beyond a short initial time scale during which single-particlelike motion is seen.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Variational Monte Carlo techniques are employed in studying 4He clusters, with and without an H2 impurity. We find that a novel, yet simple, analytic nuclear wave-function form, derived from a numerical H2He wave function, yields high accuracy in computed ground-state energies of 4HeN. For the clusters studied here, three to twenty atoms, energies range from 94% to 90% of the exact values. Density profiles and distributions of particle separation are also computed. For reasonable computational cost (e.g., 〈20 Cray/X-MP14 minutes for the largest cluster), density profiles are determined for the first time to high statistical accuracy to within 0.5 A(ring) or less of the cluster center. The density profile of He3 is found to possess a uniquely pronounced peak at the cluster center resulting from contributions of near-collinear atomic arrangements. We also study the effect of substituting an He by H2, using modified wave functions containing products of pairwise He–H2 terms. For all cluster sizes studied, we find a lowering of the total energy upon exchanging an He for an H2. The exchange energy increases in magnitude with increasing cluster size, yet is still well below bulk estimates at N=20. Size comparisons with the pure helium clusters show very little change upon He/H2 exchange, e.g., the rms radii differ by ≤2% for N〉3. Density profiles and bond distributions show noticeable differentiation between H2 and He. For N≥4, the peak in the H2 density profile is not at the cluster but does remain inside the cluster. This peak is most pronounced for H2He13 implying an enhanced resistance to H2 penetration for He13.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Quantum relaxation behavior for many-particle tunneling systems of interacting identical particles is studied for both fermions and bosons within the infinite U Hubbard model, using a numerically exact checkerboard time propagation. Dependence of relaxation times upon vacancy concentration is obtained, and conclusions for nuclear statistical effects in hydrogen surface diffusion presented.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We present a time dependent theoretical approach to calculating the electronic properties of semiconductor nanoclusters. The technique can be applied to ground and excited electronic states without using the effective mass approximation or perturbative expansions. The effects of surface properties on the electronic structure can be calculated at an atomic level. We illustrate the method with calculations of ground state densities of states for CdS and CdSe crystallites between 19 and 33 A(ring) in diameter. The size-dependence of the bandwidths and band gap is studied, and the influence of surface states and surface polarity is discussed. The calculated shift in the valence band edge with cluster size is compared with experimental results from valence-band photoemission. Good agreement is obtained.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have studied the quantum clusters, He7 and (H2)7, in their ground (L=0) and rotationally excited states (L=2–8 for He and L=2–12 for H2) by variational Monte Carlo, and selectively by diffusion Monte Carlo. The optimized trial wave functions are eigenfunctions of L2, and, as such, are orthogonal and yield upper bounds to the energy of each state. We report energies and structural results, including density profiles and moments-of-inertia distributions. Overall, the He7 cluster is more delocalized than the (H2)7 cluster, but the computed structures show that both He7 and (H2)7 are highly nonclassical and nonrigid, evolving from spherical to toroidal structures as the angular momentum increases. We find that by L=2 for He7 and by L=6 for (H2)7, the clusters become metastable with respect to dissociation, as determined by comparison of excited-state energies of the N=7 clusters with the ground-state energies for N=6. The root-mean-square bond length fluctuations indicate that both N=7 clusters are liquidlike in the L=0 ground state, according to the Lindemann criterion. These fluctuations increase with L, implying a further decrease in rigidity for the rotationally excited states.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Variational and diffusion Monte Carlo results are presented for the ground states of several SF6HeN clusters in the range N=1–499. The diffusion Monte Carlo computations are well converged, yielding an expected accuracy in the energy well under 1%. Computations are performed employing both an isotropic and an anisotropic He–SF6 interaction potential. Novel trial wave functions are used to describe both the shell structure of these clusters and the anisotropy arising from the potential. The ground state helium densities show the SF6 located at the cluster center, inducing a large degree of localization and a shell-like structure in the surrounding helium. Although the full potential causes a large degree of anisotropy in the helium density, general characteristics such as the energy and size are not greatly affected by the potential anisotropy. Finally, we compute spectral shifts for the ν3 SF6 vibration due to the instantaneous dipole–induced dipole mechanism and compare with recent experiments. We find a red shift which for N≤111 increases with N to a maximum value of 0.93 cm−1, with a width of 0.25 cm−1, at N=111.

Electronic Resource

1365-2230

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

A 47-year-old man presented with diarrhoea, acquired hypogammaglobulinaemia and a cutaneous graft-versus-host-like reaction in association with a spindle cell thymoma. Graft-versus-host reactions usually occur following allogeneic transplantation or transfusion of immunocompetent lymphoid cells but have been described rarely in the context of a thymoma.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The luminescence energies of a variety of crystalline silicon nanostructures are calculated using a time-dependent tight-binding technique. The calculated energies are compared with measured values for porous silicon samples to determine the geometries of the regions in which electron-hole recombination occurs. We observe a correlation between luminescence wavelength and nanostructure which is consistent with the porosity changes during the etching process. We conclude that porous silicon samples which emit visible light are composed of small crystallites, and that the active regions in longer wavelength emitters are wirelike structures. © 1995 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The path-integral Monte Carlo technique is applied to study the SF6He39 cluster at low temperatures. The method employs as input only pair potentials, the number of atoms, and the temperature, and is thus independent of the trial function bias which can affect calculation of structural quantities in variational and diffusion Monte Carlo. We thereby obtain an unambiguous answer to the question of the location of SF6 in small clusters (39 He atoms), as well as the temperature dependence of the cluster structure. The cluster is found to undergo a gradual transition to a superfluid between 0.625 K and 1.25 K, and to evaporate significantly at temperatures above 2 K. We also calculate spectral shifts for the ν3 vibrational mode of SF6, using the instantaneous dipole-induced dipole mechanism. The results are compared with infra-red absorption measurements and with the previous ground-state quantum Monte Carlo calculations. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Electronic Resource