24,915 research outputs found
Design of conditions for emergence of self-replicators
A self-replicator is usually understood to be an object of definite form that
promotes the conversion of materials in its environment into a nearly identical
copy of itself. The challenge of engineering novel, micro- or nano-scale
self-replicators has attracted keen interest in recent years, both because
exponential amplification is an attractive method for generating high yields of
specific products, and also because self-reproducing entities have the
potential to be optimized or adapted through rounds of iterative selection.
Substantial steps forward have been achieved both in the engineering of
particular self-replicating molecules, and also in characterizing the physical
basis for possible mechanisms of self-replication. At present, however, there
is need for a theoretical treatment of what physical conditions are most
conducive to the emergence of novel self-replicating structures from a
reservoir of building blocks on a desired time-scale. Here we report progress
in addressing this need. By analyzing the dynamics of a generic class of
heterogeneous particle mixtures whose reaction rates emerge from basic physical
interactions, we demonstrate that the spontaneous discovery of self-replication
is controlled by relatively generic features of the chemical space, namely: the
dispersion in the distribution of reaction timescales and bound-state energies.
Based on this analysis, we provide quantitative criteria that may aid
experimentalists in designing a system capable of producing self-replicators,
and in estimating the likely timescale for exponential growth to start.Comment: Supplementary Information is under the Ancillary Files ---
Application of a Reynolds stress turbulence model to the compressible shear layer
Theoretically based turbulence models have had success in predicting many features of incompressible, free shear layers. However, attempts to extend these models to the high-speed, compressible shear layer have been less effective. In the present work, the compressible shear layer was studied with a second-order turbulence closure, which initially used only variable density extensions of incompressible models for the Reynolds stress transport equation and the dissipation rate transport equation. The quasi-incompressible closure was unsuccessful; the predicted effect of the convective Mach number on the shear layer growth rate was significantly smaller than that observed in experiments. Having thus confirmed that compressibility effects have to be explicitly considered, a new model for the compressible dissipation was introduced into the closure. This model is based on a low Mach number, asymptotic analysis of the Navier-Stokes equations, and on direct numerical simulation of compressible, isotropic turbulence. The use of the new model for the compressible dissipation led to good agreement of the computed growth rates with the experimental data. Both the computations and the experiments indicate a dramatic reduction in the growth rate when the convective Mach number is increased. Experimental data on the normalized maximum turbulence intensities and shear stress also show a reduction with increasing Mach number
Cosmology of the Next-to-Minimal Supersymmetric Standard Model
We discuss the domain wall problem in the Next-to-Minimal Supersymmetric
Standard Model, with particular attention to the usual solution of explicit
breaking of the discrete symmetry by non-renormalisable operators. This
``solution'' leads to a contradiction between the requirements of cosmology and
those of avoiding the destabilisation of the hierarchy.Comment: 6 pages LaTeX, needs sprocl.sty (included at end) Talk presented by
P.L. White at Valencia 9
Primordial Non-Gaussianity in the Forest: 3D Bispectrum of Ly-alpha Flux Spectra Along Multiple Lines of Sight
We investigate the possibility of constraining primordial non-Gaussianity
using the 3D bispectrum of Ly-alpha forest. The strength of the quadratic
non-Gaussian correction to an otherwise Gaussian primordial gravitational field
is assumed to be dictated by a single parameter fnl. We present the first
prediction for bounds on fnl using Ly-alpha flux spectra along multiple lines
of sight. The 3D Ly- transmitted flux field is modeled as a biased
tracer of the underlying matter distribution sampled along 1D skewers
corresponding to quasars sight lines. The precision to which fnl can be
constrained depends on the survey volume, pixel noise and aliasing noise
(arising from discrete sampling of the density field). We consider various
combinations of these factors to predict bounds on fnl. We find that in an
idealized situation of full sky survey and negligible Poisson noise one may
constrain fnl ~ 23 in the equilateral limit. Assuming a Ly-alpha survey
covering large parts of the sky (k_{min} = 8 * 10^{-4} Mpc^{-1}) and with a
quasar density of \bar n = 5 * 10^{-3} Mpc^{-2} it is possible to constrain fnl
~ 100 for equilateral configurations. The possibility of measuring fnl at a
precision comparable to LSS studies maybe useful for joint constraining of
inflationary scenarios using different data sets.Comment: 4 pages, 1 figure, 1 table. Accepted for publication in Physical
Review Letter
On the Thermodynamic Geometry of Hot QCD
We study the nature of the covariant thermodynamic geometry arising from the
free energy of hot QCD. We systematically analyze the underlying equilibrium
thermodynamic configurations of the free energy of 2- and 3-flavor hot QCD with
or without including thermal fluctuations in the neighborhood of the QCD
transition temperature. We show that there exists a well-defined thermodynamic
geometric notion for QCD thermodynamics. The geometry thus obtained has no
singularity as an intrinsic Riemannian manifold. We further show that there is
a close connection of this geometric approach with the existing studies of
correlations and quark number susceptibilities in hot QCD.Comment: 15 pages, 12 figures, Keywords: Thermodynamic Geometry, Hot QCD,
Quasi-particles, PACS: 12.38.-t; 05.70.Fh; 02.40.Ky; 12.40.E
Electrical transport properties of nanostructured ferromagnetic perovskite oxides La_0.67Ca_0.33MnO_3 and La_0.5Sr_0.5CoO_3 at low temperatures (5 K > T >0.3 K) and high magnetic field
We report a comprehensive study of the electrical and magneto-transport
properties of nanocrystals of La_0.67Ca_0.33MnO_3 (LCMO) (with size down to 15
nm) and La_0.5Sr_0.5CoO_3 (LSCO) (with size down to 35 nm) in the temperature
range 0.3 K to 5 K and magnetic fields upto 14 T. The transport,
magnetotransport and non-linear conduction (I-V curves) were analysed using the
concept of Spin Polarized Tunnelling in the presence of Coulomb blockade. The
activation energy of transport, \Delta, was used to estimate the tunnelling
distances and the inverse decay length of the tunnelling wave function (\chi)
and the height of the tunnelling barrier (\Phi_B). The magnetotransport data
were used to find out the magnetic field dependences of these tunnelling
parameters. The data taken over a large magnetic field range allowed us to
separate out the MR contributions at low temperatures arising from tunnelling
into two distinct contributions. In LCMO, at low magnetic field, the transport
and the MR are dominated by the spin polarization, while at higher magnetic
field the MR arises from the lowering of the tunnel barrier by the magnetic
field leading to an MR that does not saturate even at 14 T. In contrast, in
LSCO, which does not have substantial spin polarization, the first contribution
at low field is absent, while the second contribution related to the barrier
height persists. The idea of inter-grain tunnelling has been validated by
direct measurements of the non-linear I-V data in this temperature range and
the I-V data was found to be strongly dependent on magnetic field. We made the
important observation that a gap like feature (with magnitude ~ E_C, the
Coulomb charging energy) shows up in the conductance g(V) at low bias for the
systems with smallest nanocrystal size at lowest temperatures (T < 0.7 K). The
gap closes as the magnetic field and the temperature are increased.Comment: 13 figure
- …