2,826 research outputs found
Inelastic fingerprints of hydrogen contamination in atomic gold wire systems
We present series of first-principles calculations for both pure and hydrogen
contaminated gold wire systems in order to investigate how such impurities can
be detected. We show how a single H atom or a single H2 molecule in an atomic
gold wire will affect forces and Au-Au atom distances under elongation. We
further determine the corresponding evolution of the low-bias conductance as
well as the inelastic contributions from vibrations. Our results indicate that
the conductance of gold wires is only slightly reduced from the conductance
quantum G0=2e^2/h by the presence of a single hydrogen impurity, hence making
it difficult to use the conductance itself to distinguish between various
configurations. On the other hand, our calculations of the inelastic signals
predict significant differences between pure and hydrogen contaminated wires,
and, importantly, between atomic and molecular forms of the impurity. A
detailed characterization of gold wires with a hydrogen impurity should
therefore be possible from the strain dependence of the inelastic signals in
the conductance.Comment: 5 pages, 3 figures, Contribution to ICN+T2006, Basel, Switzerland,
July-August 200
Local polynomial Whittle estimation of perturbed fractional processes
We propose a semiparametric local polynomial Whittle with noise estimator of the memory parameter in long memory time series perturbed by a noise term which may be serially correlated. The estimator approximates the log-spectrum of the short-memory component of the signal as well as that of the perturbation by two separate polynomials. Including these polynomials we obtain a reduction in the order of magnitude of the bias, but also inflate the asymptotic variance of the long memory estimator by a multiplicative constant. We show that the estimator is consistent for d in (0,1), asymptotically normal for d in (0,3/4), and if the spectral density is sufficiently smooth near frequency zero, the rate of convergence can become arbitrarily close to the parametric rate, sqrt(n). A Monte Carlo study reveals that the proposed estimator performs well in the presence of a serially correlated perturbation term. Furthermore, an empirical investigation of the 30 DJIA stocks shows that this estimator indicates stronger persistence in volatility than the standard local Whittle (with noise) estimator.Bias reduction, local Whittle, long memory, perturbed fractional process, semiparametric estimation, stochastic volatility
Covariant Lyapunov Vectors and Finite-Time Normal Modes for Geophysical Fluid Dynamical Systems
Dynamical vectors characterizing instability and applicable as ensemble
perturbations for prediction with geophysical fluid dynamical models are
analysed. The relationships between covariant Lyapunov vectors (CLVs),
orthonormal Lyapunov vectors (OLVs), singular vectors (SVs), Floquet vectors
and finite-time normal modes (FTNMs) are examined for periodic and aperiodic
systems. In the phase-space of FTNM coefficients, SVs are found to equate with
unit norm FTNMs at certain times. In the long-time limit, when SVs approach
OLVs, the Oseledec theorem and the relationships between OLVs and CLVs are used
to connect CLVs to FTNMs in this phase-space. The covariant properties of both
the CLVs, and the FTNMs, together with their phase-space independence, and the
norm independence of global Lyapunov exponents and FTNM growth rates,
establishes their asymptotic convergence. Conditions on the dynamical systems
for the validity of these results, particularly ergodicity, boundedness and
non-singular FTNM characteristic matrix and propagator, are documented. Systems
with nondegenerate OLVs, and with degenerate Lyapunov spectrum as is the rule
in the presence of waves such as Rossby waves, are examined, and efficient
numerical methods for the calculation of leading CLVs proposed. Norm
independent finite-time versions of the Kolmogorov-Sinai entropy production and
Kaplan-Yorke dimension are presented.Comment: 38 pages, no figure
Induction of reversions in Neurospora crassa by nitrous acid
Induction of reversions in Neurospora crassa by nitrous aci
Error growth and dynamical vectors during southern hemisphere blocking
International audienceThe structural organization of initially random perturbations or "errors" evolving in a barotropic tangent linear model with time-dependent basic states taken from observations, is examined for cases of block development, maturation and decay in the Southern Hemisphere atmosphere during April, November and December 1989. We determine statistical results relating the structures of evolved errors to singular vectors (SVs), Lyapunov vectors (LVs) and finite-time normal modes (FTNMs). The statistics of 100 evolved error fields are studied for six day periods or longer and compared with the growth and structures of leading fast growing SVs, LVs and FTNMs. The SVs are studied in the kinetic energy (KE), enstrophy (EN) and streamfunction (SF) norms, while all FTNMs and the first LV are norm independent. The mean of the largest pattern correlations between the 100 error fields and dynamical vectors, taken over the five fastest growing SVs, in any of the three norms, or over the five fastest growing FTNMs, increases with increasing time interval to a value close to 0.6 after six days. Corresponding pattern correlations with the five fastest growing LVs are slightly lower. The leading dynamical vectors (SVs 1, FTNM1 or LV 1) generally, but not always, give the largest pattern correlations with the error fields. It is found that viscosity slightly increases the average correlations between the evolved errors and LV 1 and evolved SVs 1. Mean pattern correlations with fast growing dynamical vectors increase further for time intervals longer than six days. The properties of the dynamical vectors during Southern Hemisphere blocking are briefly outlined. After a few days integration, the structures of the leading evolved SVs in the KE, EN and SF norms, are in general quite similar and also similar to some of the dominant FTNMs that are norm independent. For optimization times of six days or less, the evolved SVs and FTNMs are, in general, different from the dominant LVs on the same day. Nevertheless, amplification factors of the first FTNMs and first LVs are very similar, and also similar to, but slightly larger than, the mean amplification factor of 100 initially random perturbations in the SF norm, while the amplification factors in the SF norm of KE SVs 1 and SF SV 1 are much higher. For longer optimization times, the first SVs and the first FTNM increasingly turn towards the leading LV with convergence achieved within a month
Iterative methods for Lyapunov vectors and singular vectors and atmospheric predictability
Iterative methods are used to generate Lyapunov vectors (LVs) and singular vectors (SVs). Their roles in describing atmospheric error growth and predictability are studied. LVs are produced by evolving a set of initially random perturbations and using a modified Gram-Schmidt re-orthogonalisation to ensure their orthogonality. The structures of LVs and SVs, and finite-time normal modes (FTNMs), are compared with patterns of 100 errors evolving in a barotropic tangent linear model
Detecting an invisible Higgs boson at Fermilab Tevatron and CERN LHC
In this paper, we study the observability of an invisible Higgs boson at
Fermilab Tevatron and CERN LHC through the production channel q \bar{q} \to Z
H \to \ell^+\ell^-+ \ptmiss , where \ptmiss is reconstructed from the
with or . A new strategy is proposed to eliminate
the largest irreducible background, namely . This strategy utilizes the precise measurements of . For GeV and
with luminosity at Tevatron, a observation of the
invisible Higgs boson is possible. For GeV with only luminosity at LHC, a discovery signal over can be achieved.Comment: 4 Revtex pages including 2 figure
Twentieth Century Winter Changes in Southern Hemisphere Synoptic Weather Modes
During the last sixty years, there have been large changes in the southern hemisphere winter circulation and reductions in rainfall particularly in the southern Australian region. Here we examine the corresponding changes in dynamical modes of variability ranging from storm tracks, onset-of-blocking modes, northwest cloud-band disturbances, Antarctic low-frequency modes, intraseasonal oscillations, and African easterly waves. Our study is performed using a global two-level primitive equation instability-model with reanalyzed observed July three-dimensional basic states for the periods 1949–1968, 1975–1994, and 1997–2006. We relate the reduction in the winter rainfall in the southwest of Western Australia since the mid-1970s and in south-eastern Australia since the mid-1990s to changes in growth rate and structures of leading storm track and blocking modes. We find that cyclogenesis and onset-of-blocking modes growing on the subtropical jet have significantly reduced growth rates in the latter periods. On the other hand there is a significant increase in the growth rate of northwest cloud-band modes and intraseasonal oscillation disturbances that cross Australia and are shown to be related to recent positive trends in winter rainfall over northwest Western Australia and central Australia, in general. The implications of our findings are discussed
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