1,450 research outputs found
Crystal structure and electronic states of tripotassium picene
The crystal structure of potassium doped picene with an exact stoichiometry
(K3C22H14, K3picene from here onwards) has been theoretically determined within
Density Functional Theory allowing complete variational freedom of the crystal
structure parameters and the molecular atomic positions. A modified herringbone
lattice is obtained in which potassium atoms are intercalated between two
paired picene molecules displaying the two possible orientations in the
crystal.Along the c-axis, organic molecules alternate with chains formed by
three potassium atoms. The electronic structureof the doped material resembles
pristine picene, except that now the bottom of the conduction band is occupied
by six electrons coming from the ionized K atoms (six per unit cell).
Wavefunctions remain based mainly on picene molecular orbitals getting their
dispersion from intralayer edge to face CH/pi bonding, while eigenenergies have
been modified by the change in the electrostatic potential. The small
dispersion along the c-axis is assigned to small H-H overlap. From the
calculated electronic density of states we expect metallic behavior for
potassium doped picene.Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural
.cif files include
The role of potassium orbitals in the metallic behavior of K3picene
Detailed electronic structure calculations of picene clusters doped by
potassium modeling the crystalline K3picene structure show that while two
electrons are completely transferred from potassium atoms to the LUMO of
pristine picene, the third one remains closely attached to both material
components. Multiconfigurational analysis is necessary to show that many
structures of almost degenerate total energies compete to define the cluster
ground state. Our results prove that the 4s orbital of potassium should be
included in any interaction model describing the material. We propose a quarter
filled two orbital model as the most simple model capable of describing the
electronic structure of K-intercalated picene. Precise solutions obtained by a
development of Lanczos method show low energy electronic excitations involving
orbitals located at different positions. Consequently, metallic transport is
possible in spite of the clear dominance of interaction over hopping.Comment: 12 pages, 10 figures (6 of them really heavy
Time variability of cyclonic geostrophic circulation in the Mediterranean
Interannual variability and trends of the surface geostrophic cyclonic circulation and cyclone frequency in Western and Eastern Mediterranean areas are analyzed, based on a cyclone data base derived from the ERA-40 ECWMF reanalysis (within the MEDEX project tasks), spanning from September/1957 to August/2002. In this 45 years, the cyclonic circulation show a significant decrease in the Western Mediterranean, mostly in winter and spring, and an increase in the Eastern, mainly due to the summer and autumn increase in the frequency of thermal lows
Acquisition of Demonstratives in English and Spanish
The present work re-evaluates the long-standing claim that demonstratives are among infants’ earliest and most common words. Although demonstratives are deictic words important for joint attention, deictic gestures and non-word vocalizations could serve this function in early language development; the role of demonstratives may have been overestimated. Using extensive data from the CHILDES corpora (Study 1, N = 66, 265 transcripts) and McArthur-Bates CDI database (Study 2, N = 950), the language production of 18- to 24-month-old Spanish- and English-speaking children was analyzed to determine the age and order of acquisition, and frequency of demonstratives. Results indicate that demonstratives do not typically appear before the 50th word and only become frequent from the two-word utterance stage. Corpus data show few differences between Spanish and English, whereas parental report data suggest much later acquisition for demonstratives in English. These findings expand our knowledge of the foundations of deictic communication, and of the methodological challenges of assessing early production of function words
Extrusión del componente plástico de la copa acetabular sin luxación protésica: a propósito de un caso
Se present a un caso de extrusión del component e plástic o (polietileno)
ocurrido en el transcurso del primer me s del postoperatorio en un pacient e que había
sufrido un recambio del component e acetabular. Se destaca el hecho de que cursase de
forma asintomática y de que no se acompañase de luxación de la prótesis.A case of disassembly of the polyethylene component of the acetabular
cup acurred in a patient one mouth after revision surgery for loosening of a screwed
acetabular cup is reported. Special emphasis is made in the asyntomati c clinic and on
the no association with dislocation of the femoral component
Libanon : Heiße Spuren im „Mordfall Hariri“?
The teleosts Chelidonichthys cuculus, Trigloporus lastoviza, Serranus cabrilla and Trachinus draco, are important by-catch species, in terms of landed biomass and commercial value, from the continental shelf bottom trawl fishery off the Balearic Islands (western Mediterranean). The main biological parameters of these species were calculated from monthly biological samplings, and were used along with three years pseudo-cohorts (2008-2010) obtained from monitoring on-board of the bottom trawl fleet, to assess their exploitation level through Virtual Population Analysis (VPA) and Yield per Recruit (Y/R) analysis. Time series of fishery independent indicators based on MEDITS surveys data such as the evolution of the abundance and biomass, the distribution range, and the Conservation Status of Fish b (CSFb), were also analyzed. All four species analyzed showed growth overfishing. The reductions of the fishing effort required to reach the Y/RF0,1 reference point were lower, 66-73% depending on the species, to those reported for the target species M. merluccius (87%), but higher than that reported for M. surmuletus (53%), in the study area in the period 2000-2010. The abundance and biomass, and the distribution range did not show any trend for the period 2001-2011, whereas the CSFb showed signs of recovery that may be due to the displacement of the trawl fishing effort from the shelf to the slope during the last decade. Our results underline the necessary incorporation of monitoring and assessment of by-catch species to the management of fisheries as their populations can show a more pronounced overexploitation than target ones
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