4,422 research outputs found
Growth mechanism of nanocrystals in solution: ZnO, a case study
We investigate the mechanism of growth of nanocrystals from solution using
the case of ZnO. Spanning a wide range of values of the parameters, such as the
temperature and the reactant concentration, that control the growth, our
results establish a qualitative departure from the widely accepted diffusion
controlled coarsening (Ostwald ripening) process quantified in terms of the
Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental
observations can be qualitatively and quantitatively understood within a growth
mechanism that is intermediate between the two well-defined limits of diffusion
control and kinetic control.Comment: 10 pages, 4 figure
The Individual and Collective Effects of Exact Exchange and Dispersion Interactions on the Ab Initio Structure of Liquid Water
In this work, we report the results of a series of density functional theory
(DFT) based ab initio molecular dynamics (AIMD) simulations of ambient liquid
water using a hierarchy of exchange-correlation (XC) functionals to investigate
the individual and collective effects of exact exchange (Exx), via the PBE0
hybrid functional, non-local vdW/dispersion interactions, via a fully
self-consistent density-dependent dispersion correction, and approximate
nuclear quantum effects (aNQE), via a 30 K increase in the simulation
temperature, on the microscopic structure of liquid water. Based on these AIMD
simulations, we found that the collective inclusion of Exx, vdW, and aNQE as
resulting from a large-scale AIMD simulation of (HO) at the
PBE0+vdW level of theory, significantly softens the structure of ambient liquid
water and yields an oxygen-oxygen structure factor, , and
corresponding oxygen-oxygen radial distribution function, , that
are now in quantitative agreement with the best available experimental data.
This level of agreement between simulation and experiment as demonstrated
herein originates from an increase in the relative population of water
molecules in the interstitial region between the first and second coordination
shells, a collective reorganization in the liquid phase which is facilitated by
a weakening of the hydrogen bond strength by the use of the PBE0 hybrid XC
functional, coupled with a relative stabilization of the resultant disordered
liquid water configurations by the inclusion of non-local vdW/dispersion
interactions
Future Frontiers in Organic Synthesis
The role of organic synthesis to the mankind is of paramount importance since the early nineteen century [1]. In 1828, Friedrich Wöhler discovered the synthesis of urea from ammonium cyanate, marking the starting point of modern organic synthesis. Wöhler concluded to his mentor Jöns Jacob Berzelius, "I cannot, so to say, hold my chemical water and must tell you that I can make urea without thereby needing to have kidneys, or anyhow, an animal, be it human or dog". Since then, organic synthesis has become an indispensable tool in industries such as petrochemicals, pharmaceutical, flavors, fragrances, agrochemical, and others. This is evident by the number of Nobel prizes awarded to organic chemists. The Nobel Prize in Chemistry 2001 was awarded to William S. Knowles, Ryoji Noyori, and K. Barry Sharpless for their work in asymmetric synthesis. This was followed by the award of the Nobel Prize in Chemistry 2005 to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock "for the development of the metathesis method in organic synthesis". And just recently, Richard F. Heck, Ei-ichi Negishi and Akira Suzuki won the Nobel Prize in Chemistry 2010 for “palladium-catalyzed cross couplings in organic synthesis”
Multiphysics modelling of a hybrid magnetic bearing (HMB) for calculating power loss and temperature with different loss minimization strategies
This paper represents a multiphysics modelling for calculating loss and temperature of a hybrid magnetic bearing (HMB) using finite element method (FEM). It also addresses the different loss minimization strategies for the HMB. The main sources of losses are identified as eddy current loss in permanent magnets, flywheel and copper loss in electromagnet. Due to these losses, the temperature distribution in different portions of HMB is computed using coupled field analysis. To minimize the eddy current loss, slits are fabricated in flywheel plate instead of a solid flywheel. The improvement of the control current is investigated by providing a coating of different metal, like copper, brass and stainless steel on the flywheel. A zero bias current (ZBC) scheme has been introduced where no bias current is required to levitate the rotor or to avoid singularity due to external disturbances, thus reducing the copper loss
Gas-Liquid Nucleation in Two Dimensional System
We study the nucleation of the liquid phase from a supersaturated vapor in
two dimensions (2D). Using different Monte Carlo simulation methods, we
calculate the free energy barrier for nucleation, the line tension and also
investigate the size and shape of the critical nucleus. The study is carried
out at an intermediate level of supersaturation(away from the spinodal limit).
In 2D, a large cut-off in the truncation of the Lennard-Jones (LJ) potential is
required to obtain converged results, whereas low cut-off (say, is
generally sufficient in three dimensional studies, where is the LJ
diameter) leads to a substantial error in the values of line tension,
nucleation barrier and characteristics of the critical cluster. It is found
that in 2D, the classical nucleation theory (CNT) fails to provide a reliable
estimate of the free energy barrier. It underestimates the barrier by as much
as 70% at the saturation-ratio S=1.1 (defined as S=P/PC, where PC is the
coexistence pressure at reduced temperature ). Interestingly,
CNT has been found to overestimate the nucleation free energy barrier in three
dimensional (3D)systems near the triple point. In fact, the agreement with CNT
is worse in 2D than in 3D. Moreover, the existing theoretical estimate of the
line tension overestimates the value significantly.Comment: 24 pages, 8 figure
Prospects for discovering supersymmetric long-lived particles with MoEDAL
We present a study on the possibility of searching for long-lived
supersymmetric partners with the MoEDAL experiment at the LHC. MoEDAL is
sensitive to highly ionising objects such as magnetic monopoles or massive
(meta)stable electrically charged particles. We focus on prospects of directly
detecting long-lived sleptons in a phenomenologically realistic model which
involves an intermediate neutral long-lived particle in the decay chain. This
scenario is not yet excluded by the current data from ATLAS or CMS, and is
compatible with astrophysical constraints. Using Monte Carlo simulation, we
compare the sensitivities of MoEDAL versus ATLAS in scenarios where MoEDAL
could provide discovery reach complementary to ATLAS and CMS, thanks to looser
selection criteria combined with the virtual absence of background. It is also
interesting to point out that, in such scenarios, in which charged staus are
the main long-lived candidates, the relevant mass range for MoEDAL is
compatible with a potential role of Supersymmetry in providing an explanation
for the anomalous events observed by the ANITA detector.Comment: 12 pages, 6 figures; preliminary results presented in
arXiv:1903.11022; matches published version in EPJ
Siegert pseudostates: completeness and time evolution
Within the theory of Siegert pseudostates, it is possible to accurately
calculate bound states and resonances. The energy continuum is replaced by a
discrete set of states. Many questions of interest in scattering theory can be
addressed within the framework of this formalism, thereby avoiding the need to
treat the energy continuum. For practical calculations it is important to know
whether a certain subset of Siegert pseudostates comprises a basis. This is a
nontrivial issue, because of the unusual orthogonality and overcompleteness
properties of Siegert pseudostates. Using analytical and numerical arguments,
it is shown that the subset of bound states and outgoing Siegert pseudostates
forms a basis. Time evolution in the context of Siegert pseudostates is also
investigated. From the Mittag-Leffler expansion of the outgoing-wave Green's
function, the time-dependent expansion of a wave packet in terms of Siegert
pseudostates is derived. In this expression, all Siegert pseudostates--bound,
antibound, outgoing, and incoming--are employed. Each of these evolves in time
in a nonexponential fashion. Numerical tests underline the accuracy of the
method
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