436 research outputs found
Ordering kinetic in two-dimensional hexagonal pattern of cylinder-forming PS-b-PMMA block copolymer thin films: dependence on the segregation strength
This paper reports the experimental determination of the growth exponents and
activation enthalpies for the ordering process of standing cylinder-forming
all-organic polystyrene-block-poly (methyl methacrylate) (PS-b-PMMA) block
copolymer (BCP) thin films as a function of the BCP degree of polymerization
(N). The maximum growth exponent of 1/3 is observed for the smallest BCP at the
border of the order disorder transition. Both the growth exponents and the
activation enthalpies exponentially decrease with the BCP segregation strength
(chi N) following the same path of the diffusivity.Comment: 17 pages, 3 figures, 1 table, 7 pages (18-24) Supplemental Material
(SM
Molecular Dynamics Simulations of Solutions at Constant Chemical Potential
Molecular Dynamics studies of chemical processes in solution are of great
value in a wide spectrum of applications, which range from nano-technology to
pharmaceutical chemistry. However, these calculations are affected by severe
finite-size effects, such as the solution being depleted as the chemical
process proceeds, which influence the outcome of the simulations. To overcome
these limitations, one must allow the system to exchange molecules with a
macroscopic reservoir, thus sampling a Grand-Canonical ensemble. Despite the
fact that different remedies have been proposed, this still represents a key
challenge in molecular simulations.
In the present work we propose the Constant Chemical Potential Molecular
Dynamics (CMD) method, which introduces an external force that controls
the environment of the chemical process of interest. This external force,
drawing molecules from a finite reservoir, maintains the chemical potential
constant in the region where the process takes place. We have applied the
CMD method to the paradigmatic case of urea crystallization in aqueous
solution. As a result, we have been able to study crystal growth dynamics under
constant supersaturation conditions, and to extract growth rates and
free-energy barriers.Comment: 8 pages, 8 figures (Supplementary Information: 6 pages, 7 figures).
Typos and labelling corrected Ver. 3: Minor comments added in Sec. 3.
References 13,36,38 added. Minor text changes and typos correcte
Chemical Potential Calculations In Dense Liquids Using Metadynamics
The calculation of chemical potential has traditionally been a challenge in
atomistic simulations. One of the most used approaches is Widom's insertion
method in which the chemical potential is calculated by periodically attempting
to insert an extra particle in the system. In dense systems this method fails
since the insertion probability is very low. In this paper we show that in a
homogeneous fluid the insertion probability can be increased using
metadynamics. We test our method on a supercooled high density binary
Lennard-Jones fluid. We find that we can obtain efficiently converged results
even when Widom's method fails.Comment: 5 pages, 2 figures, Ver 2: typos corrected, Ref. 10 year correcte
Chemical Potential Calculations in Non-Homogeneous Liquids
The numerical computation of chemical potential in dense, non-homogeneous
fluids is a key problem in the study of confined fluids thermodynamics. To this
day several methods have been proposed, however there is still need for a
robust technique, capable of obtaining accurate estimates at large average
densities. A widely established technique is the Widom insertion method, that
computes the chemical potential by sampling the energy of insertion of a test
particle. Non-homogeneity is accounted for by assigning a density dependent
weight to the insertion points. However, in dense systems, the poor sampling of
the insertion energy is a source of inefficiency, hampering a reliable
convergence.
We have recently presented a new technique for the chemical potential
calculation in homogeneous fluids. This novel method enhances the sampling of
the insertion energy via Well-Tempered Metadynamics, reaching accurate
estimates at very large densities. In this paper we extend the technique to the
case of non-homogeneous fluids. The method is successfully tested on a confined
Lennard-Jones fluid. In particular we show that, thanks to the improved
sampling, our technique does not suffer from a systematic error that affects
the classic Widom method for non-homogeneous fluids, providing a precise and
accurate result.Comment: 16 pages, 4 figures Contains a Supplementary Information fil
Experimental Demonstration of Dual Polarization Nonlinear Frequency Division Multiplexed Optical Transmission System
Multi-eigenvalues transmission with information encoded simultaneously in
both orthogonal polarizations is experimentally demonstrated. Performance below
the HD-FEC limit is demonstrated for 8-bits/symbol 1-GBd signals after
transmission up to 207 km of SSMF
Dual polarization nonlinear Fourier transform-based optical communication system
New services and applications are causing an exponential increase in internet
traffic. In a few years, current fiber optic communication system
infrastructure will not be able to meet this demand because fiber nonlinearity
dramatically limits the information transmission rate. Eigenvalue communication
could potentially overcome these limitations. It relies on a mathematical
technique called "nonlinear Fourier transform (NFT)" to exploit the "hidden"
linearity of the nonlinear Schr\"odinger equation as the master model for
signal propagation in an optical fiber. We present here the theoretical tools
describing the NFT for the Manakov system and report on experimental
transmission results for dual polarization in fiber optic eigenvalue
communications. A transmission of up to 373.5 km with bit error rate less than
the hard-decision forward error correction threshold has been achieved. Our
results demonstrate that dual-polarization NFT can work in practice and enable
an increased spectral efficiency in NFT-based communication systems, which are
currently based on single polarization channels
The role of oxygen vacancies on the structure and the density of states of iron doped zirconia
In this paper we study, both with theoretical and experimental approach, the
effect of iron doping in zirconia. Combining density functional theory (DFT)
simulations with the experimental characterization of thin films, we show that
iron is in the Fe3+ oxidation state and accordingly that the films are rich in
oxygen vacancies (VO). VO favor the formation of the tetragonal phase in doped
zirconia (ZrO2:Fe) and affect the density of state at the Fermi level as well
as the local magnetization of Fe atoms. We also show that the Fe(2p) and Fe(3p)
energy levels can be used as a marker for the presence of vacancies in the
doped system. In particular the computed position of the Fe(3p) peak is
strongly sensitive to the VO to Fe atoms ratio. A comparison of the theoretical
and experimental Fe(3p) peak position suggests that in our films this ratio is
close to 0.5. Besides the interest in the material by itself, ZrO2:Fe
constitutes a test case for the application of DFT on transition metals
embedded in oxides. In ZrO2:Fe the inclusion of the Hubbard U correction
significantly changes the electronic properties of the system. However the
inclusion of this correction, at least for the value U = 3.3 eV chosen in the
present work, worsen the agreement with the measured photo-emission valence
band spectra.Comment: 24 pages, 8 figure
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