34,552 research outputs found
Constraints on the Brans-Dicke gravity theory with the Planck data
Based on the new cosmic CMB temperature data from the Planck satellite, the 9
year polarization data from the WMAP, the BAO distance ratio data from the SDSS
and 6dF surveys, we place a new constraint on the Brans-Dicke theory. We adopt
a parametrization \zeta=\ln(1+1/\omega}), where the general relativity (GR)
limit corresponds to . We find no evidence of deviation from general
relativity. At 95% probability, , correspondingly,
the region is excluded. If we restrict ourselves to
the (i.e. ) case, then the 95% probability interval is
. We can also translate this
result to a constraint on the variation of gravitational constant, and find the
variation rate today as yr ( error bar), the integrated change since the epoch of
recombination is ( error
bar). These limits on the variation of gravitational constant are comparable
with the precision of solar system experiments.Comment: 7 pages, 5 figures, 2 table
A single-level random-effects cross-lagged panel model for longitudinal mediation analysis
Cross-lagged panel models (CLPMs) are widely used to test mediation with longitudinal panel data. One major limitation of the CLPMs is that the model effects are assumed to be fixed across individuals. This assumption is likely to be violated (i.e., the model effects are random across individuals) in practice. When this happens, the CLPMs can potentially yield biased parameter estimates and misleading statistical inferences. This article proposes a model named a random-effects cross-lagged panel model (RE-CLPM) to account for random effects in CLPMs. Simulation studies show that the RE-CLPM outperforms the CLPM in recovering the mean indirect and direct effects in a longitudinal mediation analysis when random effects exist in the population. The performance of the RE-CLPM is robust to a certain degree, even when the random effects are not normally distributed. In addition, the RE-CLPM does not produce harmful results when the model effects are in fact fixed in the population. Implications of the simulation studies and potential directions for future research are discussed
Efficient Estimation of Copula-based Semiparametric Markov Models
This paper considers efficient estimation of copula-based semiparametric strictly stationary Markov models. These models are characterized by nonparametric invariant (one-dimensional marginal) distributions and parametric bivariate copula functions; where the copulas capture temporal dependence and tail dependence of the processes. The Markov processes generated via tail dependent copulas may look highly persistent and are useful for financial and economic applications. We first show that Markov processes generated via Clayton, Gumbel and Student's copulas and their survival copulas are all geometrically ergodic. We then propose a sieve maximum likelihood estimation (MLE) for the copula parameter, the invariant distribution and the conditional quantiles. We show that the sieve MLEs of any smooth functionals are root- consistent, asymptotically normal and efficient; and that their sieve likelihood ratio statistics are asymptotically chi-square distributed. We present Monte Carlo studies to compare the finite sample performance of the sieve MLE, the two-step estimator of Chen and Fan (2006), the correctly specified parametric MLE and the incorrectly specified parametric MLE. The simulation results indicate that our sieve MLEs perform very well; having much smaller biases and smaller variances than the two-step estimator for Markov models generated via Clayton, Gumbel and other tail dependent copulas.Copula, Tail dependence, Nonlinear Markov models, Geometric ergodicity, Sieve MLE, Semiparametric efficiency, Sieve likelihood ratio statistics, Value-at-Risk
Symmetry Reduction and Boundary Modes for Fe-Chains on an s-wave Superconductor
We investigate the superconducting phase diagram and boundary modes for a
quasi-1D system formed by three Fe-Chains on an s-wave superconductor,
motivated by the recent Princeton experiment. The onsite
spin-orbit term, inter-chain diagonal hopping couplings, and magnetic disorders
in the Fe-chains are shown to be crucial for the superconducting phases, which
can be topologically trivial or nontrivial in different parameter regimes. For
the topological regime a single Majorana and multiple Andreew bound modes are
obtained in the ends of the chain, while for the trivial phase only low-energy
Andreev bound states survive. Nontrivial symmetry reduction mechanism induced
by the term, diagonal hopping couplings, and magnetic
disorder is uncovered to interpret the present results. Our study also implies
that the zero-bias peak observed in the recent experiment may or may not
reflect the Majorana zero modes in the end of the Fe-chains.Comment: 5 pages, 4 figures; some minor errors are correcte
Towards a practice-as-research model of Performise: taking scenography for Antigone the Unbrave (2019) as a case study
This work was published in a book funded by national funds through FCT - FundaĆ§Ć£o para a CiĆŖncia e a Tecnologia, I.P., within the project UIDB/04041/2020 (Centro de Estudos Arnaldo AraĆŗjo).info:eu-repo/semantics/publishedVersio
Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures
Hydrogen-rich compounds are important for understanding the dissociation of
dense molecular hydrogen, as well as searching for room temperature
Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure
experiment reported the successful synthesis of novel insulating lithium
polyhydrides when above 130 GPa. However, the results are in sharp contrast to
previous theoretical prediction by PBE functional that around this pressure
range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. In order to
address this discrepancy, we perform unbiased structure search with first
principles calculation by including the van der Waals interaction that was
ignored in previous prediction to predict the high pressure stable structures
of LiHn (n = 2-11, 13) up to 200 GPa. We reproduce the previously predicted
structures, and further find novel compositions that adopt more stable
structures. The van der Waals functional (vdW-DF) significantly alters the
relative stability of lithium polyhydrides, and predicts that the stable
stoichiometries for the ground-state should be LiH2 and LiH9 at 130-170 GPa,
and LiH2, LiH8 and LiH10 at 180-200 GPa. Accurate electronic structure
calculation with GW approximation indicates that LiH, LiH2, LiH7, and LiH9 are
insulative up to at least 208 GPa, and all other lithium polyhydrides are
metallic. The calculated vibron frequencies of these insulating phases are also
in accordance with the experimental infrared (IR) data. This reconciliation
with the experimental observation suggests that LiH2, LiH7, and LiH9 are the
possible candidates for lithium polyhydrides synthesized in that experiment.
Our results reinstate the credibility of density functional theory in
description H-rich compounds, and demonstrate the importance of considering van
der Waals interaction in this class of materials.Comment: 34 pages, 15 figure
- ā¦