125,565 research outputs found
Wigner Gaussian dynamics. Simulating the anharmonic and quantum ionic motion
No full textThe atomic motion controls important properties of materials, such as thermal transport, phase transitions, and vibrational spectra. However, simulating the ionic dynamics is exceptionally challenging when quantum fluctuations are relevant (e.g., at low temperatures or with light atoms) and the energy landscape is anharmonic. In this paper, we present the time-dependent self-consistent harmonic approximation (TDSCHA) [L. Monacelli and F. Mauri, Phys. Rev. B 103, 104305 (2021)] in the Wigner framework, paving the way for the efficient computation of the nuclear motion in systems with sizable quantum and thermal anharmonic fluctuations. Besides the improved numerical efficiency, the Wigner formalism unveils the classical limit of TDSCHA and provides a link with the many-body perturbation theory of Feynman diagrams. We further extend the method to account for the nonlinear couplings between phonons and photons, responsible, e.g., for a nonvanishing Raman signal in high-symmetry Raman inactive crystals, firstly discussed by Rasetti and Fermi. We benchmark the method in phase III of high-pressure hydrogen ab initio. The nonlinear photon-phonon coupling reshapes the IR spectra and explains the high-frequency shoulder of the H2 vibron observed in experiments. The Wigner TDSCHA is computationally cheap and derived from first principles. It is unbiased by assumptions on the phonon-phonon and phonon-photon scattering and does not depend on empirical parameters. Therefore, the method can be adopted in unsupervised high-throughput calculations
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Putting Up a Good Fight: The Galí-Monacelli Model versus “The Six Major Puzzles in International Macroeconomics”
No full textIn this paper, the following question is posed: Can the New Keynesian Open Economy Model by Galí and Monacelli (2005b) explain “Six Major Puzzles in International Macroeconomics”, as documented in Obstfeld and Rogoff (2000b)? The model features a small open economy with complete markets, Calvo sticky prices and monopolistic competition. As extensions, I explore the effects of an estimated Taylor rule and additional trade costs. After translating the six puzzles into moment conditions for the model, I estimate the five most effective parameters using simulated method of moments (SMM) to fit the moment conditions implied by the data. Given the simplicity of the model, its fit is surprisingly good: among other things, the home bias puzzles can easily be replicated, the exchange rate volatility is formidably increased and the exchange rate correlation pattern is relatively close to realistic values. Trade costs are one important ingredient for this finding.International Macroeconomics, New Keynesian open economy model, trade costs, simulated method of moments (SMM)
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Influence of two different growth regulators on the adventitious root origin in vitro of Vismia guianensis regenerated plantlets
No full textEffect of light on epidermic and subepidermic tissues in the in vitro induction of floral and vegetative buds.
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Comparison between in vivo and in vitro metabolite production of Morus nigra
No full textNine flavonoids and seven Diels-Alder type adducts were isolated from the acetonic extract of Morus nigra L. root bark, and three flavonoids and six Diels-Alder type adducts were isolated from the acetonic extract of M. nigra L. callus. However, none of the metabolites isolated from the root bark was found in the cell culture. Moreover, adducts isolated from the plant and cell culture showed not only a different cyclohexene ring substitution but also a different stereochemistry, that was found cis-trans in the metabolites isolated from callus, and trans-trans in those isolated from the root bark
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