397 research outputs found
Matrix Assisted Formation of Ferrihydrite Nanoparticles in a Siloxane/Poly(Oxyethylene) Nanohybrid
Matrix-assisted formation of ferrihydrite, an iron oxide hydroxide analogue
of the protein ferritin-core, in a sol-gel derived organic-inorganic hybrid is
reported. The hybrid network (named di-ureasil) is composed of
poly(oxyethylene) chains of different average polymer molecular weights grafted
to siloxane domains by means of urea cross-linkages and accommodates
ferrihydrite nanoparticles. Magnetic measurements, Fourier transform infrared
and nuclear magnetic resonance spectroscopy reveal that the controlled
modification of the polymer molecular weight allows the fine-tuning of the
ability of the hybrid matrix to assist and promote iron coordination at the
organic-inorganic interface and subsequent nucleation and growth of the
ferrihydrite nanoparticles whose core size (2-4 nm) is tuned by the amount of
iron incorporated. The polymer chain length, its arrangement and crystallinity,
are key factors on the anchoring and formation of the ferrihydrite particles.Comment: 7 pages, 6 figures. To be published in J. Mater. Che
Langmuir wave linear evolution in inhomogeneous nonstationary anisotropic plasma
Equations describing the linear evolution of a non-dissipative Langmuir wave
in inhomogeneous nonstationary anisotropic plasma without magnetic field are
derived in the geometrical optics approximation. A continuity equation is
obtained for the wave action density, and the conditions for the action
conservation are formulated. In homogeneous plasma, the wave field E
universally scales with the electron density N as E ~ N^{3/4}, whereas the
wavevector evolution varies depending on the wave geometry
Modulational instability and nonlocality management in coupled NLS system
The modulational instability of two interacting waves in a nonlocal Kerr-type
medium is considered analytically and numerically. For a generic choice of wave
amplitudes, we give a complete description of stable/unstable regimes for zero
group-velocity mismatch. It is shown that nonlocality suppresses considerably
the growth rate and bandwidth of instability. For nonzero group-velocity
mismatch we perform a geometrical analysis of a nonlocality management which
can provide stability of waves otherwise unstable in a local medium.Comment: 15 pages, 12 figures, to be published in Physica Script
Regular spatial structures in arrays of Bose-Einstein condensates induced by modulational instability
We show that the phenomenon of modulational instability in arrays of
Bose-Einstein condensates confined to optical lattices gives rise to coherent
spatial structures of localized excitations. These excitations represent thin
disks in 1D, narrow tubes in 2D, and small hollows in 3D arrays, filled in with
condensed atoms of much greater density compared to surrounding array sites.
Aspects of the developed pattern depend on the initial distribution function of
the condensate over the optical lattice, corresponding to particular points of
the Brillouin zone. The long-time behavior of the spatial structures emerging
due to modulational instability is characterized by the periodic recurrence to
the initial low-density state in a finite optical lattice. We propose a simple
way to retain the localized spatial structures with high atomic concentration,
which may be of interest for applications. Theoretical model, based on the
multiple scale expansion, describes the basic features of the phenomenon.
Results of numerical simulations confirm the analytical predictions.Comment: 17 pages, 13 figure
Generalized Coherent States and Spin Systems
Generalized Coherent States (GCS) are constructed (and discussed) in order to
study quasiclassical behaviour of quantum spin models of the Heisenberg type.
Several such models are taken to their semiclassical limits, whose form depends
on the spin value as well as the Hamiltonian symmetry. In the continuum
approximation, SU(2)/U(1) GCS when applied give rise to the well-known
Landau-Lifshitz classical phenomenology. For arbitrary spin values one obtains
a lattice of coupled nonlinear oscillators. Corresponding classical continuum
models are described as well.Comment: 18 pages, LaTeX. Submitted to J. of Phys. A: Math. and Ge
1-(4-Methylphenyl)-1H-1,2,3,4-tetrazole
In the title compound, C8H8N4, the dihedral angle between the tetrazole and benzene rings is 21.6 (1)°. An intermolecular C—H⋯π interaction is observed
Desorption of n-alkanes from graphene: a van der Waals density functional study
A recent study of temperature programmed desorption (TPD) measurements of
small n-alkanes (CNH2N+2) from C(0001) deposited on Pt(111) shows a linear
relationship of the desorption energy with increasing n-alkane chain length. We
here present a van der Waals density functional study of the desorption barrier
energy of the ten smallest n-alkanes (N = 1 to 10) from graphene. We find
linear scaling with N, including a nonzero intercept with the energy axis,
i.e., an offset at the extrapolation to N = 0. This calculated offset is
quantitatively similar to the results of the TPD measurements. From further
calculations of the polyethylene polymer we offer a suggestion for the origin
of the offset.Comment: 3 pictures, 1 tabl
Pattern Forming Dynamical Instabilities of Bose-Einstein Condensates: A Short Review
In this short topical review, we revisit a number of works on the
pattern-forming dynamical instabilities of Bose-Einstein condensates in one-
and two-dimensional settings. In particular, we illustrate the trapping
conditions that allow the reduction of the three-dimensional, mean field
description of the condensates (through the Gross-Pitaevskii equation) to such
lower dimensional settings, as well as to lattice settings. We then go on to
study the modulational instability in one dimension and the snaking/transverse
instability in two dimensions as typical examples of long-wavelength
perturbations that can destabilize the condensates and lead to the formation of
patterns of coherent structures in them. Trains of solitons in one-dimension
and vortex arrays in two-dimensions are prototypical examples of the resulting
nonlinear waveforms, upon which we briefly touch at the end of this review.Comment: 28 pages, 9 figures, publishe
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