170,027 research outputs found
First principles study of the normal state electronic properties of the Bi-2212 cuprate superconductor
No full textWe present detailed first-principles calculations of the normal state electronic properties of the Bi2Sr2CaCu2O8 (Bi-2212) high T-C superconductor by means of the linearized augmented plane wave (LAPW) method within the framework of density functional theory (DFT). We describe the crystal structure by the body-centered tetragonal (BCT) cell with space group I4/mmm and optimize its volume, c/a ratio and atomic positions by total energy and atomic-force minimization. The optimization of the cell leads to small but visible changes in the topology of the Fermi surface, rounding the shape of the CuO2 barrels, and moving both the BiO bands, responsible for the pockets near the (M) over bar point, below the Fermi level. We have then studied the influence of the large distortions in the BiO plane observed in nature by means of an root2 x root2 orthorhombic cell (with Bbmb space group). Differently to what has been observed in the literature for the Bi-2201 compound, we find that for Bi-2212 the orthorhombic distortion does not induce relevant changes to the binding energies of the BiO bands, conserving their metallic character. (C) 2003 Elsevier B.V. All rights reserved
Role of electron-electron correlation in the valence states of YBa2Cu3O7: Low-energy excitations and Fermi surface
No full textWe have studied the role of on-site correlation in the low-energy excitations of YBa2Cu3O7 in the normal state, calculating self-energies and spectral functions according to the three-body-scattering approach. The method allows to include all the details of realistic ab initio band-structure calculations and to augment them with the inclusion of electron-electron correlations, getting quasiparticle energies for one-particle removal. It is found that correlation effects modify the energy dispersion of hole quasiparticle states, both in the high and low binding energy region, and strongly modify the Fermi surface topology
Structure optimization effects on the electronic and vibrational properties of Bi2Sr2CaCu2O8
No full textWe have studied the effect of structure optimization on the topology of the Fermi surface and on the Raman-active phonons of Bi2Sr2CaCu2O8. By total energy and force minimizations within the density-functional theory we have identified the most stable atomic geometry considering both an idealized body-centered tetragonal structure, inclusive of surface truncation, and a root2xroot2 orthorhombic cell simulating the observed distortions in the BiO planes. The optimization of the tetragonal cell leads to small but visible changes in the topology of the Fermi surface, rounding the shape of the CuO2 barrels, while the orthorhombic distortion is responsible for the umklapp bands that have been observed by angle-resolved photoemission spectroscopy. The latter also gives rise to Raman-active vibrations not permitted in the tetragonal cell and strongly influences the attribution of the phonon peaks measured by experiments
XMCD Spectra Based on DensityFunctional Theory
No full textIn this Chapter, we discuss how to treat XMCD spectra for solidsfrom first-principles by employingdensity functional theory(DFT) . To this aim we will firstintroduce the reader to the basics of DFT as well as to thelinearized augmented planewave (LAPW) method , which is our method of choice tosolve the corresponding single-particle equations. Thereafter, wewill describe how to obtain the scattering cross section withinFermi's golden rule, and how thisapproach has been implemented into the LAPW method. Finally, we willgive results for the transition metals iron, cobalt, and nickel. Wewill compare the XMCD spectra with experiments and show the impact ofmany-body effects as obtained within the three-body scattering theory
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
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach
No full textThe crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Delta, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Delta, from below to above the thermal energy k(B)T at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices. (C) 2002 American Institute of Physics.The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Π, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Π, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices. © 2002 American Institute of Physics
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
First-principles study of second-order Resonance Raman scattering of silicon
No full textIn this work, we want to investigate the evolution of the second-order Raman scattering process when the laser frequency is changed. The second-order intensity can be computed from the generalization of the approach used for first-order intensity [Y. Gillet et al, Phys. Rev. B 88, 094305 (2013) and C. Ambrosch-Draxl et al, Phys. Rev. B 65, 064501 (2002)], combining multiple finite difference calculations with supercells generated on different points in the Brillouin Zone. We present the general methodology and the results obtained in the silicon case
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