3,767 research outputs found
Comment on "Self-Purification in Semiconductor Nanocrystals"
In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed
that formation energies of Mn impurities in CdSe nanocrystals increase as the
size of the nanocrystal decreases, and argued that this size dependence leads
to "self-purification" of small nanocrystals. They presented
density-functional-theory (DFT) calculations showing a strong size dependence
for Mn impurity formation energies, and proposed a general explanation. In this
Comment we show that several different DFT codes, pseudopotentials, and
exchange-correlation functionals give a markedly different result: We find no
such size dependence. More generally, we argue that formation energies are not
relevant to substitutional doping in most colloidally grown nanocrystals.Comment: 1 page, 1 figur
Linear response of a grafted semiflexible polymer to a uniform force field
We use the worm-like chain model to analytically calculate the linear
response of a grafted semiflexible polymer to a uniform force field. The result
is a function of the bending stiffness, the temperature, the total contour
length, and the orientation of the field with respect to that of the grafted
end. We also study the linear response of a worm-like chain with a periodic
alternating sequence of positive and negative charges. This can be considered
as a model for a polyampholyte with intrinsic bending siffness and negligible
intramolecular interactions. We show how the finite intrinsic persistence
length affects the linear response to the external field.Comment: 6 pages, 3 figure
Diffuse Neutron Scattering Study of Relaxor Ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)
Diffuse neutron scattering is a valuable tool to obtain information about the
size and orientation of the polar nanoregions that are a characteristic feature
of relaxor ferroelectrics. In this paper, we present new diffuse scattering
results obtained on Pb(Zn1/3Nb2/3)O3 (PZN for short) and
(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)single crystals (with x=4.5 and 9%),
around various Bragg reflections and along three symmetry directions in the
[100]-[011] zone. Diffuse scattering is observed around reflections with mixed
indices, (100), (011) and (300), and along transverse and diagonal directions
only. No diffuse scattering is found in longitudinal scans. The diffuse
scattering peaks can be fitted well with a Lorentzian function, from which a
correlation length is extracted. The correlation length increases with
decreasing temperatures down to the transition at Tc, first following a
Curie-Weiss law, then departing from it and becoming flat at very low
temperatures. These results are interpreted in terms of three temperature
regions: 1) dynamic polarization fluctuations (i.e. with a finite lifetime) at
high temperatures, 2) static polarization reorientations (condensation of polar
nanoregions) that can still reorient as a unit (relaxor behavior) at
intermediate temperatures and 3) orientational freezing of the polar
nanoregions with random strain fields in pure PZN or a structural phase
transition in PZN-xPT at low temperatures. The addition of PT leads to a
broadening of the diffuse scattering along the diagonal ([111]) relative to the
transverse ([100]) direction, indicating a change in the orientation of the
polar regions. Also, with the addition of PT, the polar nanoregions condense at
a higher temperature above Tc.Comment: AIP 6x9 style files, 9 pages, 5 figures, Conference-Fundamental
Physics of Ferroelectrics 200
Overdamped Stress Relaxation in Buckled Rods
We present a comprehensive theoretical analysis of the stress relaxation in a
multiply but weakly buckled incompressible rod in a viscous solvent. In the
bulk two interesting regimes of generic self--similar intermediate asymptotics
are distinguished, which give rise to two classes of approximate and exact
power--law solutions, respectively. For the case of open boundary conditions
the corresponding non--trivial boundary--layer scenarios are derived by a
multiple--scale perturbation (``adiabatic'') method. Our results compare well
with -- and provide the theoretical explanation for -- previous results from
numerical simulations, and they suggest new directions for further fruitful
numerical and experimental investigations.Comment: 20 pages, 12 figure
Chemotaxonomic comparison of the seed ferns Odontopteris cantabrica and Odontopteris schlotheimii, Middle Pennsylvanian Sydney Coalfield, Canada
We present the first comparative spectrochemical data in the study history of Odontopteris schlotheimii Brongniart, 1828–1831 and Odontopteris cantabrica Wagner, 1969. These fossils co-occur in the shaley roof of the Lloyd Cove seam (Ro% = 0.65; Sydney Coalfield, Canada). Fourier transform-infrared spectra show that the sampled pinnules of both species are preserved as fossilized-cuticles. They are prominently aliphatic with distinct methylene and methyl peaks (at 2922 cm–1 and 2852 cm–1); with relatively high contents of carbonyl groups (peaks at 1700 cm–1) that indicate a high degree of molecular cross linking; and with smaller amounts of aromatics (peaks at 820 cm–1 and 750 cm–1) with a low condensation degree of benzene rings. No differences among respective pinnules are inferred from the 3D chemometric model and associate significance tests by the analysis of variance (α = 0.05). At the same time, this method provides new, chemical parameters that, in conjunction with morphological features, could be used to efficiently distinguish species.Fil: D`angelo, JosĂ© Alejandro. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Mendoza. Instituto Argentino de NivologĂa, GlaciologĂa y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de NivologĂa, GlaciologĂa y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de NivologĂa, GlaciologĂa y Ciencias Ambientales; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Cape Breton University; CanadáFil: Zodrow, Erwin L.. Cape Breton University; Canad
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
Conformations of confined biopolymers
Nanoscale and microscale confinement of biopolymers naturally occurs in cells
and has been recently achieved in artificial structures designed for
nanotechnological applications. Here, we present an extensive theoretical
investigation of the conformations and shape of a biopolymer with varying
stiffness confined to a narrow channel. Combining scaling arguments, analytical
calculations, and Monte Carlo simulations, we identify various scaling regimes
where master curves quantify the functional dependence of the polymer
conformations on the chain stiffness and strength of confinement.Comment: 5 pages, 4 figures, minor correction
Fluctuation-Dissipation Theorem for the Microcanonical Ensemble
A derivation of the Fluctuation-Dissipation Theorem for the microcanonical
ensemble is presented using linear response theory. The theorem is stated as a
relation between the frequency spectra of the symmetric correlation and
response functions. When the system is not in the thermodinamic limit, this
result can be viewed as an extension of the fluctuation-dissipation relations
to a situation where dynamical fluctuations determine the response. Therefore,
the relation presented here between equilibrium fluctuations and response can
have a very different physical nature from the usual one in the canonical
ensemble. These considerations imply that the Fluctuation-Dissipation Theorem
is not restricted to the context of thermal equilibrium, where it is usually
derived. Dispersion relations and sum rules are also obtained and discussed in
the present case. Although analogous to the Kramers-Kronig relations, they are
not related to the frequency spectrum but to the energy dependence of the
response function.Comment: 15 pages, v3: final version, new text added, new reference
Important role of alkali atoms in A4C60
We show that hopping via the alkali atoms plays an important role for the t1u
band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is
broadened by more than 40 % by the presence of the alkali atoms. The difference
between A4C60 and A3C60 is in particular due to the less symmetric location of
the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
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