30,075 research outputs found
Diamond films from combustion of methyl acetylene and propadiene
To date diamond films grown with the combustion technique have used either acetylene or, rarely, ethylene as the fuel. However, there are barriers to large scale commercialization of the combustion technique using either fuel. For example, acetylene is relatively expensive and difficult to handle, while the use of ethylene gives relatively low growth rates. In this letter we propose replacing acetylene with MAPPTM gas, a commercial mixture of methyl acetylene and propadiene in liquefied petroleum gas (primarily propylene). MAPP gas is considerably cheaper, safer, and easier to handle than acetylene. Furthermore, the experiments described here suggest that MAPP gas flames are only slightly less efficient than acetylene flames at converting fuel carbon atoms into diamond
Facilitation of polymer looping and giant polymer diffusivity in crowded solutions of active particles
We study the dynamics of polymer chains in a bath of self-propelled particles
(SPP) by extensive Langevin dynamics simulations in a two dimensional system.
Specifically, we analyse the polymer looping properties versus the SPP activity
and investigate how the presence of the active particles alters the chain
conformational statistics. We find that SPPs tend to extend flexible polymer
chains while they rather compactify stiffer semiflexible polymers, in agreement
with previous results. Here we show that larger activities of SPPs yield a
higher effective temperature of the bath and thus facilitate looping kinetics
of a passive polymer chain. We explicitly compute the looping probability and
looping time in a wide range of the model parameters. We also analyse the
motion of a monomeric tracer particle and the polymer's centre of mass in the
presence of the active particles in terms of the time averaged mean squared
displacement, revealing a giant diffusivity enhancement for the polymer chain
via SPP pooling. Our results are applicable to rationalising the dimensions and
looping kinetics of biopolymers at constantly fluctuating and often actively
driven conditions inside biological cells or suspensions of active colloidal
particles or bacteria cells.Comment: 15 pages, 9 figures, IOPLaTe
Optimal trajectories for efficient atomic transport without final excitation
We design optimal harmonic-trap trajectories to transport cold atoms without
final excitation, combining an inverse engineering techniqe based on
Lewis-Riesenfeld invariants with optimal control theory. Since actual traps are
not really harmonic, we keep the relative displacement between the center of
mass and the trap center bounded. Under this constraint, optimal protocols are
found according to different physical criteria. The minimum time solution has a
"bang-bang" form, and the minimum displacement solution is of "bang-off-bang"
form. The optimal trajectories for minimizing the transient energy are also
discussed.Comment: 10 pages, 7 figure
Dynamical evolution of the mass function and radial profile of the Galactic globular cluster system
Evolution of the mass function (MF) and radial distribution (RD) of the
Galactic globular cluster (GC) system is calculated using an advanced and a
realistic Fokker-Planck (FP) model that considers dynamical friction,
disc/bulge shocks and eccentric cluster orbits. We perform hundreds of FP
calculations with different initial cluster conditions, and then search a
wide-parameter space for the best-fitting initial GC MF and RD that evolves
into the observed present-day Galactic GC MF and RD. By allowing both MF and RD
of the initial GC system to vary, which is attempted for the first time in the
present Letter, we find that our best-fitting models have a higher peak mass
for a lognormal initial MF and a higher cut-off mass for a power-law initial MF
than previous estimates, but our initial total masses in GCs, M_{T,i} =
1.5-1.8x10^8 Msun, are comparable to previous results. Significant findings
include that our best-fitting lognormal MF shifts downward by 0.35 dex during
the period of 13 Gyr, and that our power-law initial MF models well-fit the
observed MF and RD only when the initial MF is truncated at >~10^5 Msun. We
also find that our results are insensitive to the initial distribution of orbit
eccentricity and inclination, but are rather sensitive to the initial
concentration of the clusters and to how the initial tidal radius is defined.
If the clusters are assumed to be formed at the apocentre while filling the
tidal radius there, M_{T,i} can be as high as 6.9x10^8 Msun, which amounts to
~75 per cent of the current mass in the stellar halo.Comment: To appear in May 2008 issue of MNRAS, 386, L6
Shortcuts to adiabaticity for an ion in a rotating radially-tight trap
We engineer the fast rotation of a quantum particle confined in an
effectively one-dimensional, harmonic trap, for a predetermined rotation angle
and time, avoiding final excitation. Different schemes are proposed with
different speed limits that depend on the control capabilities. We also make
use of trap rotations to create squeezed states without manipulating the trap
frequencies.Comment: 11 pages, 6 figure
Screened hybrid functional applied to 3d^0-->3d^8 transition-metal perovskites LaMO3 (M=Sc-Cu): influence of the exchange mixing parameter on the structural, electronic and magnetic properties
We assess the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened
hybrid density functional scheme applied to the perovskite family LaMO3
(M=Sc-Cu) and discuss the role of the mixing parameter alpha (which determines
the fraction of exact Hartree-Fock exchange included in the density functional
theory (DFT) exchange-correlation functional) on the structural, electronic,
and magnetic properties. The physical complexity of this class of compounds,
manifested by the largely varying electronic characters
(band/Mott-Hubbard/charge-transfer insulators and metals), magnetic orderings,
structural distortions (cooperative Jahn-Teller like instabilities), as well as
by the strong competition between localization/delocalization effects
associated with the gradual filling of the t_2g and e_g orbitals, symbolize a
critical and challenging case for theory. Our results indicates that HSE is
able to provide a consistent picture of the complex physical scenario
encountered across the LaMO3 series and significantly improve the standard DFT
description. The only exceptions are the correlated paramagnetic metals LaNiO3
and LaCuO3, which are found to be treated better within DFT. By fitting the
ground state properties with respect to alpha we have constructed a set of
'optimum' values of alpha from LaScO3 to LaCuO3: it is found that the 'optimum'
mixing parameter decreases with increasing filling of the d manifold (LaScO3:
0.25; LaTiO3 & LaVO3: 0.10-0.15; LaCrO3, LaMnO3, and LaFeO3: 0.15; LaCoO3:
0.05; LaNiO3 & LaCuO3: 0). This trend can be nicely correlated with the
modulation of the screening and dielectric properties across the LaMO3 series,
thus providing a physical justification to the empirical fitting procedure.Comment: 32 pages, 29 figure
Holographic Meson Spectra in the Dense Medium with Chiral Condensate
We study two effects on the meson spectra by using the AdS/CFT
correspondence where the corrections from the chiral condensate and the
quark density are controlled by the gravitational backreaction of the massive
scalar field and U(1) gauge field respectively. The dual geometries with zero
and nonzero current quark masses are obtained numerically. We discuss meson
spectra and binding energy of heavy quarkonium with the subleading corrections
in the hard wall model.Comment: 16 pages, 4 figure
Non-ergodic transitions in many-body Langevin systems: a method of dynamical system reduction
We study a non-ergodic transition in a many-body Langevin system. We first
derive an equation for the two-point time correlation function of density
fluctuations, ignoring the contributions of the third- and fourth-order
cumulants. For this equation, with the average density fixed, we find that
there is a critical temperature at which the qualitative nature of the
trajectories around the trivial solution changes. Using a method of dynamical
system reduction around the critical temperature, we simplify the equation for
the time correlation function into a two-dimensional ordinary differential
equation. Analyzing this differential equation, we demonstrate that a
non-ergodic transition occurs at some temperature slightly higher than the
critical temperature.Comment: 8 pages, 1 figure; ver.3: Calculation errors have been fixe
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