217 research outputs found
Rotational dynamics in the plastic-crystal phase of ethanol: Relevance for understanding the dynamics during the structural glass transition
The reorientational dynamics within the rotationally disordered cubic plastic phase of solid ethanol is investigated by means of the concurrent use of computer molecular dynamics and quasielastic neutron scattering. Motions involving widely different time scales are shown to take place above the calorimetric "glass transition" which is centered at Tg≈97 K. These correspond to well-defined reorientations belonging to the cubic point group. The dynamics of this solid exhibits features remarkably close to those of the supercooled liquid that can exist at the same temperature. Such similitude of dynamic behavior serves to provide some clues for the understanding of the nature of molecular motions at temperatures close to the canonical liquid→glass transitio
Collective excitations in liquid D2 confined within the mesoscopic pores of a MCM-41 molecular sieve
We present a comparative study of the excitations in bulk and liquid D2
confined within the pores of MCM-41. The material (Mobile Crystalline
Material-41) is a silicate obtained by means of a template that yields a
partially crystalline structure composed by arrays of nonintersecting hexagonal
channels of controlled width having walls made of amorphous SiO2. Its porosity
was characterized by means of adsorption isotherms and found to be composed by
a regular array of pores having a narrow distribution of sizes with a most
probable value of 2.45 nm. The assessment of the precise location of the sample
within the pores is carried out by means of pressure isotherms. The study was
conducted at two pressures which correspond to pore fillings above the
capillary condensation regime. Within the range of wave vectors where
collective excitations can be followed up (0.3<Q<3.0 −1), we found
confinement brings forward a large shortening of the excitation lifetimes that
shifts the characteristic frequencies to higher energies. In addition, the
coherent quasielastic scattering shows signatures of reduced diffusivity.Comment: 6 page
Purely dynamical signature of the orientational glass transition
The dynamics of the freezing transition of the rotator phase crystal of ethanol into its orientational glass phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. We demonstrate that phenomena observed at pico- and nanosecond scales can be mapped onto those shown by a model of infinitely thin hard needles rotating around body-centered-cubic lattice positions. As the model glass transition is of purely dynamical origin, our findings support the idea that the glass transition is purely dynamical and not associated with any thermodynamic phase transition.Dirección General de Investigación Científica y Técnica PB95-0075-C03-0
Rotational dynamics in the plastic-crystal phase of ethanol: Relevance for understanding the dynamics during the structural glass transition
12 págs.; 14 figs.; 1 tab.; 1 apéndiceThe reorientational dynamics within the rotationally disordered cubic plastic phase of solid ethanol is investigated by means of the concurrent use of computer molecular dynamics and quasielastic neutron scattering. Motions involving widely different time scales are shown to take place above the calorimetric >glass transition> which is centered at Tg≈97 K. These correspond to well-defined reorientations belonging to the cubic point group. The dynamics of this solid exhibits features remarkably close to those of the supercooled liquid that can exist at the same temperature. Such similitude of dynamic behavior serves to provide some clues for the understanding of the nature of molecular motions at temperatures close to the canonical liquid→glass transition. ©2000 The American Physical Society.This work was supported in part by Grant No. DGICYTPB95-
0072-C03 (Spain). Work at ANL was supported by the
U.S. Department of Energy, BES–Materials Sciences, under
Contract No. W-31-109-ENG-38.Peer Reviewe
Thermally Induced Diffusion and Restructuring of Iron Triade (Fe, Co, Ni) Nanoparticles Passivated by Several Layers of Gold
9 pags., 5 figs., 3 tabs.The temperature-induced structural changes of Fe−,
Co−, and Ni−Au core−shell nanoparticles with diameters around
5 nm are studied via atomically resolved transmission electron
microscopy. We observe structural transitions from local toward
global energy minima induced by elevated temperatures. The
experimental observations are accompanied by a computational
modeling of all core−shell particles with either centralized or
decentralized core positions. The embedded atom model is
employed and further supported by density functional theory
calculations. We provide a detailed comparison of vacancy formation
energies obtained for all materials involved in order to explain the
variations in the restructuring processes which we observe in
temperature-programmed TEM studies of the particles.This research has been supported by the Austrian Science
Fund (FWF) under Grant No. P 29893-N36, the FWF and the
Christian Doppler Research Association (CDG) under Grant
No. PIR8-N34, the Horizon 2020 research program of the
European Union under Grant No. 823717-ESTEEM3, and the
Spanish Agencia Estatal de Investigacion (AEI) and the Fondo ́
Europeo de Desarrollo Regional (FEDER, UE) under Grant
No. MAT2016-75354-P. The authors acknowledge the use of
HPC resources provided by the ZID of Graz University of
Technology and by the Vienna Scientific Cluster (VSC).
Further support by NAWI Graz is gratefully acknowledged.
The CESGA supercomputer center (Spain) is also acknowledged for having provided computational resources.Peer reviewe
Anharmonic dynamics in crystalline, glassy, and supercooled-liquid glycerol: A case study on the onset of relaxational behavior
10 págs.; 8 figs.The temperature dependence of the spectral distributions of the glass, crystalline, and supercooled-liquid phases of glycerol is investigated by means of neutron inelastic scattering. The importance of anharmonic effects is quantified by the temperature dependence of reduced spectral frequency moments. The onset of relaxational (i.e., stochastic, zero-frequency) motions in the supercooled liquid state is monitored by neutron quasielastic scattering. A substantial deviation of the observed linewidths from the hydrodynaimc prescription is observed and is interpreted at a microscopic level, by comparison with the crystalline phase. ©1998 American Physical SocietyWork supported in part by DGICYT ~Spain! Grant No.
PB95-0075-c03-01. Dr. O. Randl of the Institute Laue
Langevin is acknowledged for the help given during the
measurements at the IN10 spectrometer. The work at ANL
was supported by the U.S. Department of Energy, BESMaterials
Sciences, under Contract No. W-31-109-ENG-38.Peer Reviewe
The ALHAMBRA Project: A large area multi medium-band optical and NIR photometric survey
(ABRIDGED) We describe the first results of the ALHAMBRA survey which
provides cosmic tomography of the evolution of the contents of the Universe
over most of Cosmic history. Our approach employs 20 contiguous, equal-width,
medium-band filters covering from 3500 to 9700 A, plus the JHKs bands, to
observe an area of 4 sqdeg on the sky. The optical photometric system has been
designed to maximize the number of objects with accurate classification by SED
and redshift, and to be sensitive to relatively faint emission lines. The
observations are being carried out with the Calar Alto 3.5m telescope using the
cameras LAICA and O-2000. The first data confirm that we are reaching the
expected magnitude limits of AB<~25 mag in the optical filters from the blue to
8300 A, and from AB=24.7 to 23.4 for the redder ones. The limit in the NIR is
(Vega) K_s~20, H~21, J~22. We expect to obtain accurate redshift values, Delta
z/(1+z) <~ 0.03 for about 5x10^5 galaxies with I<~25 (60% complete), and
z_med=0.74. This accuracy, together with the homogeneity of the selection
function, will allow for the study of the redshift evolution of the large scale
structure, the galaxy population and its evolution with redshift, the
identification of clusters of galaxies, and many other studies, without the
need for any further follow-up. It will also provide targets for detailed
studies with 10m-class telescopes. Given its area, spectral coverage and its
depth, apart from those main goals, the ALHAMBRA-Survey will also produce
valuable data for galactic studies.Comment: Accepted to the Astronomical Journal. 43 pages, 18 figures. The
images have been reduced in resolution to adapt to standard file sizes.
Readers can find the full-resolution version of the paper at the ALHAMBRA web
site (http://www.iaa.es/alhambra) under the "Publications" lin
The North Maladeta fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: recent activity revealed by geomorphological and geophysical research.
The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E-W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is ~ 730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of Mw=6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1-8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip slip rate for the NMF to be between 0.06-0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust
The ALHAMBRA photometric system
This paper presents the characterization of the optical range of the ALHAMBRA
photometric system, a 20 contiguous, equal-width, medium-band CCD system with
wavelength coverage from 3500A to 9700A. The photometric description of the
system is done by presenting the full response curve as a product of the
filters, CCD and atmospheric transmission curves, and using some first and
second order moments of this response function. We also introduce the set of
standard stars that defines the system, formed by 31 classic spectrophotometric
standard stars which have been used in the calibration of other known
photometric systems, and 288 stars, flux calibrated homogeneously, from the
Next Generation Spectral Library (NGSL). Based on the NGSL, we determine the
transformation equations between Sloan Digital Sky Survey (SDSS) ugriz
photometry and the ALHAMBRA photometric system, in order to establish some
relations between both systems. Finally we develop and discuss a strategy to
calculate the photometric zero points of the different pointings in the
ALHAMBRA project.Comment: Astronomical Journal on the 14th of January 201
Stellar physics with the ALHAMBRA photometric system
The ALHAMBRA photometric system was specifically designed to perform a tomography of the Universe in some selected areas. Although mainly designed for extragalactic purposes, its 20 contiguous, equal-width, medium-band photometric system in the optical wavelength range, shows a great capacity for stellar classification. In this contribution we propose a methodology for stellar classification and physical parameter estimation (Teff, log g, [Fe/H], and color excess E(B - V)) based on 18 independent reddening-free Q-values from the ALHAMBRA photometry. Based on the theoretical Spectral library BaSeL 2.2, and applied to 288 stars from the Next Generation spectral Library (NGSL), we discuss the reliability of the method and its dependence on the extinction law used
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