166 research outputs found
Exploring energy landscapes: from molecular to mesoscopic systems
We review a comprehensive computational framework to survey the potential energy landscape for systems composed of rigid or partially rigid molecules. Illustrative case studies relevant to a wide range of molecular clusters and soft and condensed matter systems are discussed
Exploring energy landscapes: metrics, pathways, and normal mode analysis for rigid-body molecules
We present new methodology for exploring the energy landscapes of molecular systems, using angle-axis variables for the rigid-body rotational coordinates. The key ingredient is a distance measure or metric tensor, which is invariant to global translation and rotation. The metric is used to formulate a generalized nudged elastic band method for calculating pathways, and a full prescription for normal-mode analysis is described. The methodology is tested by mapping the potential energy and free energy landscape of the water octamer, described by the TIP4P potential
Hyperbolic calorons, monopoles, and instantons
We construct families of SO(3)-symmetric charge 1 instantons and calorons on
the space H^3 x R. We show how the calorons include instantons and hyperbolic
monopoles as limiting cases. We show how Euclidean calorons are the flat space
limit of this family.Comment: 11 pages, no figures 1 reference added Published version available
at: http://www.springerlink.com/content/k0j4815u54303450
Gravitation with superposed Gauss--Bonnet terms in higher dimensions: Black hole metrics and maximal extensions
Our starting point is an iterative construction suited to combinatorics in
arbitarary dimensions d, of totally anisymmetrised p-Riemann 2p-forms (2p\le d)
generalising the (1-)Riemann curvature 2-forms. Superposition of p-Ricci
scalars obtained from the p-Riemann forms defines the maximally Gauss--Bonnet
extended gravitational Lagrangian. Metrics, spherically symmetric in the (d-1)
space dimensions are constructed for the general case. The problem is directly
reduced to solving polynomial equations. For some black hole type metrics the
horizons are obtained by solving polynomial equations. Corresponding Kruskal
type maximal extensions are obtained explicitly in complete generality, as is
also the periodicity of time for Euclidean signature. We show how to include a
cosmological constant and a point charge. Possible further developments and
applications are indicated.Comment: 13 pages, REVTEX. References and Note Adde
Near-Surface Interface Detection for Coal Mining Applications Using Bispectral Features and GPR
The use of ground penetrating radar (GPR) for detecting the presence of near-surface interfaces is a scenario of special interest to the underground coal mining industry. The problem is difficult to solve in practice because the radar echo from the near-surface interface is often dominated by unwanted components such as antenna crosstalk and ringing, ground-bounce effects, clutter, and severe attenuation. These nuisance components are also highly sensitive to subtle variations in ground conditions, rendering the application of standard signal pre-processing techniques such as background subtraction largely ineffective in the unsupervised case. As a solution to this detection problem, we develop a novel pattern recognition-based algorithm which utilizes a neural network to classify features derived from the bispectrum of 1D early time radar data. The binary classifier is used to decide between two key cases, namely whether an interface is within, for example, 5 cm of the surface or not. This go/no-go detection capability is highly valuable for underground coal mining operations, such as longwall mining, where the need to leave a remnant coal section is essential for geological stability. The classifier was trained and tested using real GPR data with ground truth measurements. The real data was acquired from a testbed with coal-clay, coal-shale and shale-clay interfaces, which represents a test mine site. We show that, unlike traditional second order correlation based methods such as matched filtering which can fail even in known conditions, the new method reliably allows the detection of interfaces using GPR to be applied in the near-surface region. In this work, we are not addressing the problem of depth estimation, rather confining ourselves to detecting an interface within a particular depth range
Yang-Mills Solutions on Euclidean Schwarzschild Space
We show that the apparently periodic Charap-Duff Yang-Mills `instantons' in
time-compactified Euclidean Schwarzschild space are actually time independent.
For these solutions, the Yang-Mills potential is constant along the time
direction (no barrier) and therefore, there is no tunneling. We also
demonstrate that the solutions found to date are three dimensional monopoles
and dyons. We conjecture that there are no time-dependent solutions in the
Euclidean Schwarzschild background.Comment: 12 pages, references added, version to appear in PR
The politics of innovation: why innovations need a godfather
Innovation is closely linked to the development of technology. Hence it is often assumed that when an innovation fails it is the technology that is at fault. While this may be true in many instances, there are occasions when it is not the technology that is at fault, rather, it is managerial and organisational aspects that cause problems and lead to failure. Studies have shown that individuals who take on specific roles can play an important part in avoiding these problems. These roles include the technological gatekeeper, the product champion and the sponsor/coach. In addition to these roles, this paper argues that there is another, namely that of godfather. With this role a highly respected, senior figure within an organisation provides support that is critical in ensuring the project overcomes the hurdles that lie in the path of any major new development. The nature of the godfather role is explored through three case studies. These provide examples of the role and show how it can facilitate the innovation process
Glassy Vortex State in a Two-Dimensional Disordered XY-Model
The two-dimensional XY-model with random phase-shifts on bonds is studied.
The analysis is based on a renormalization group for the replicated system. The
model is shown to have an ordered phase with quasi long-range order. This
ordered phase consists of a glass-like region at lower temperatures and of a
non-glassy region at higher temperatures. The transition from the disordered
phase into the ordered phase is not reentrant and is of a new universality
class at zero temperature. In contrast to previous approaches the disorder
strength is found to be renormalized to larger values. Several correlation
functions are calculated for the ordered phase. They allow to identify not only
the transition into the glassy phase but also an additional crossover line,
where the disconnected vortex correlation changes its behavior on large scales
non-analytically. The renormalization group approach yields the glassy features
without a breaking of replica symmetry.Comment: latex 12 pages with 3 figures, using epsf.sty and multicol.st
Shortest paths on systems with power-law distributed long-range connections
We discuss shortest-path lengths on periodic rings of size L
supplemented with an average of pL randomly located long-range links whose
lengths are distributed according to P_l \sim l^{-\xpn}. Using rescaling
arguments and numerical simulation on systems of up to sites, we show
that a characteristic length exists such that for
. For small p we find
that the shortest-path length satisfies the scaling relation
\ell(r,\xpn,p)/\xi = f(\xpn,r/\xi). Three regions with different asymptotic
behaviors are found, respectively: a) \xpn>2 where , b)
1<\xpn<2 where 0<\theta_s(\xpn)<1/2 and, c) \xpn<1 where
behaves logarithmically, i.e. . The characteristic length is
of the form with \nu=1/(2-\xpn) in region b), but depends
on L as well in region c). A directed model of shortest-paths is solved and
compared with numerical results.Comment: 10 pages, 10 figures, revtex4. Submitted to PR
Commensurate and Incommensurate Vortex Lattice Melting in Periodic Pinning Arrays
We examine the melting of commensurate and incommensurate vortex lattices
interacting with square pinning arrays through the use of numerical
simulations. For weak pinning strength in the commensurate case we observe an
order-order transition from a commensurate square vortex lattice to a
triangular floating solid phase as a function of temperature. This floating
solid phase melts into a liquid at still higher temperature. For strong pinning
there is only a single transition from the square pinned lattice to the liquid
state. For strong pinning in the incommensurate case, we observe a multi-stage
melting in which the interstitial vortices become mobile first, followed by the
melting of the entire lattice, consistent with recent imaging experiments. The
initial motion of vortices in the incommensurate phase occurs by an exchange
process of interstitial vortices with vortices located at the pinning sites. We
have also examined the vortex melting behavior for higher matching fields and
find that a coexistence of a commensurate pinned vortex lattice with an
interstitial vortex liquid occurs while at higher temperatures the entire
vortex lattice melts. For triangular arrays at incommensurate fields higher
than the first matching field we observe that the initial vortex motion can
occur through a novel correlated ring excitation where a number of vortices can
rotate around a pinned vortex. We also discuss the relevance of our results to
recent experiments of colloidal particles interacting with periodic trap
arrays.Comment: 8 figure
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