14,734 research outputs found
Dynamics of Chainlike Molecules on Surfaces
We consider the diffusion and spreading of chainlike molecules on solid
surfaces. We first show that the steep spherical cap shape density profiles,
observed in some submonolayer experiments on spreading polymer films, imply
that the collective diffusion coefficient must be an increasing
function of the surface coverage for small and intermediate coverages.
Through simulations of a discrete model of interacting chainlike molecules, we
demonstrate that this is caused by an entropy-induced repulsive interaction.
Excellent agreement is found between experimental and numerically obtained
density profiles in this case, demonstrating that steep submonolayer film edges
naturally arise due to the diffusive properties of chainlike molecules. When
the entropic repulsion dominates over interchain attractions,
first increases as a function of but then eventually approaches zero
for . The maximum value of decreases for increasing
attractive interactions, leading to density profiles that are in between
spherical cap and Gaussian shapes. We also develop an analytic mean field
approach to explain the diffusive behavior of chainlike molecules. The
thermodynamic factor in is evaluated using effective free energy
arguments, and the chain mobility is calculated numerically using the recently
developed dynamic mean field theory. Good agreement is obtained between theory
and simulations.Comment: 16 pages, 13 Postscript figure
Temperature and magnetization-dependent band-gap renormalization and optical many-body effects in diluted magnetic semiconductors
We calculate the Coulomb interaction induced density, temperature and
magnetization dependent many-body band-gap renormalization in a typical diluted
magnetic semiconductor GaMnAs in the optimally-doped metallic regime as a
function of carrier density and temperature. We find a large (about 0.1 eV)
band gap renormalization which is enhanced by the ferromagnetic transition. We
also calculate the impurity scattering effect on the gap narrowing. We suggest
that the temperature, magnetization, and density dependent band gap
renormalization could be used as an experimental probe to determine the valence
band or the impurity band nature of carrier ferromagnetism.Comment: Revised versio
Euler solution of multiblade rotor flow
A numerical method for solving the Euler equations for multiblade rotors has been developed and some preliminary results reported. The numerical scheme is a combination of several recent methods and algorithm improvements, adapted to the particular requirements of rotor-body interactions. A cylindrical basic grid has been used to study conventional multiblade helicopter rotors. Test calculations have been made for two- and six-blade rotors in hover and for a two-blade rotor in forward flight, under transonic tip conditions but without lift. The results show good agreement with experimental data
Atomic Entanglement vs Photonic Visibility for Quantum Criticality of Hybrid System
To characterize the novel quantum phase transition for a hybrid system
consisting of an array of coupled cavities and two-level atoms doped in each
cavity, we study the atomic entanglement and photonic visibility in comparison
with the quantum fluctuation of total excitations. Analytical and numerical
simulation results show the happen of quantum critical phenomenon similar to
the Mott insulator to superfluid transition. Here, the contour lines
respectively representing the atomic entanglement, photonic visibility and
excitation variance in the phase diagram are consistent in the vicinity of the
non-analytic locus of atomic concurrences.Comment: 4 pages, 2 figure
Equilibrium Shape and Size of Supported Heteroepitaxial Nanoislands
We study the equilibrium shape, shape transitions and optimal size of
strained heteroepitaxial nanoislands with a two-dimensional atomistic model
using simply adjustable interatomic pair potentials. We map out the global
phase diagram as a function of substrate-adsorbate misfit and interaction. This
phase diagram reveals all the phases corresponding to different well-known
growth modes. In particular, for large enough misfits and attractive substrate
there is a Stranski-Krastanow regime, where nano-sized islands grow on top of
wetting films. We analyze the various terms contributing to the total island
energy in detail, and show how the competition between them leads to the
optimal shape and size of the islands. Finally, we also develop an analytic
interpolation formula for the various contributions to the total energy of
strained nanoislands.Comment: 9 pages, 7 figure
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