190 research outputs found
Ultrafast Surface Plasmonic Switch in Non-Plasmonic Metals
We demonstrate that ultrafast carrier excitation can drastically affect
electronic structures and induce brief surface plasmonic response in
non-plasmonic metals, potentially creating a plasmonic switch. Using
first-principles molecular dynamics and Kubo-Greenwood formalism for
laser-excited tungsten we show that carrier heating mobilizes d electrons into
collective inter and intraband transitions leading to a sign flip in the
imaginary optical conductivity, activating plasmonic properties for the initial
non-plasmonic phase. The drive for the optical evolution can be visualized as
an increasingly damped quasi-resonance at visible frequencies for pumping
carriers across a chemical potential located in a d-band pseudo-gap with
energy-dependent degree of occupation. The subsequent evolution of optical
indices for the excited material is confirmed by time-resolved ultrafast
ellipsometry. The large optical tunability extends the existence spectral
domain of surface plasmons in ranges typically claimed in laser self-organized
nanostructuring. Non-equilibrium heating is thus a strong factor for
engineering optical control of evanescent excitation waves, particularly
important in laser nanostructuring strategies
Combined effects of pressure and Ru substitution on BaFe2As2
The ab-plane resistivity of Ba(Fe1-xRux)2As2 (x = 0.00, 0.09, 0.16, 0.21, and
0.28) was studied under nearly hydrostatic pressures, up to 7.4 GPa, in order
to explore the T-P phase diagram and to compare the combined effects of
iso-electronic Ru substitution and pressure. The parent compound BaFe2As2
exhibits a structural/magnetic phase transition near 134 K. At ambient
pressure, progressively increasing Ru concentration suppresses this phase
transition to lower temperatures at the approximate rate of ~5 K/% Ru and is
correlated with the emergence of superconductivity. By applying pressure to
this system, a similar behavior is seen for each concentration: the
structural/magnetic phase transition is further suppressed and
superconductivity induced and ultimately, for larger x Ru and P, suppressed. A
detailed comparison of the T-P phase diagrams for all Ru concentrations shows
that 3 GPa of pressure is roughly equivalent to 10% Ru substitution.
Furthermore, due to the sensitivity of Ba(Fe1-xRux)2As2 to pressure conditions,
the melting of the liquid media, 4 : 6 light mineral oil : n-pentane and 1 : 1
iso-pentane : n-pentane, used in this study could be readily seen in the
resistivity measurements. This feature was used to determine the freezing
curves for these media and infer their room temperature, hydrostatic limits:
3.5 and 6.5 GPa, respectively.Comment: 27 pages, 19 figure
Complete pressure dependent phase diagrams for SrFe2As2 and BaFe2As2
The temperature dependent electrical resistivity of single crystalline
SrFe2As2 and BaFe2As2 has been measured in a liquid medium, modified Bridgman
anvil cell for pressures in excess of 75 kbar. These data allow for the
determination of the pressure dependence of the higher temperature, structural
/ antiferromagnetic phase transitions as well as the lower temperature
superconducting phase transition. For both compounds the ambient pressure,
higher temperature structural / antiferromagnetic phase transition can be fully
suppressed with a dome-like region of zero resistivity found to be centered
about its critical pressure. Indeed, qualitatively, the temperature dependence
of the resistivity curves closest to the critical pressures are the closest to
linear, consistent with possible quantum criticality. For pressures
significantly higher than the critical pressure the zero resistivity state is
suppressed and the low temperature resistivity curves asymptotically approach a
universal, low temperature manifold. These results are consistent with the
hypothesis that correlations / fluctuations associated with the
ambient-pressure, high-temperature, tetragonal phase have to be brought to low
enough temperature to allow superconductivity, but if too fully suppressed can
lead to the loss of the superconducting state
Ageing in the musculoskeletal system
The extent of ageing in the musculoskeletal system during the life course affects the quality and length of life. Loss of bone, degraded articular cartilage, and degenerate, narrowed intervertebral discs are primary features of an ageing skeleton, and together they contribute to pain and loss of mobility. This review covers the cellular constituents that make up some key components of the musculoskeletal system and summarizes discussion from the 2015 Aarhus Regenerative Orthopaedic Symposium (AROS) (Regeneration in the Ageing Population) about how each particular cell type alters within the ageing skeletal microenvironment
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Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses
We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes
Evaluación del comportamiento a macro-fisuración por fatiga de mezclas bituminosas modificadas con polvo de neumático
The use of crumb rubber modified bitumen (CRMB) in asphalt mixes is a road engineering technology
that has become increasingly important in recent years. Given the many economic and environmental
benefits of this type of binder, the goal is to give CRMB the same level of performance as conventional polymermodified
bitumen. The appearance and propagation of cracks due to fatigue phenomena is one of the most
common distresses affecting road pavements. Since crumb rubber enhances the mechanical properties of asphalt
mixes, it can provide a viable solution for fatigue cracking. This paper presents the results of a comparative
analysis of the fatigue-cracking behavior of asphalt mixtures manufactured with crumb rubber modified bitumen
and polymer-modified bitumen.El empleo de betunes modificados con polvo de neumático usado en la fabricación
de mezclas bituminosas es una de las técnicas que mayor auge está teniendo en los últimos años en la
ingeniería de carreteras. Dadas sus grandes ventajas económicas y ambientales, este tipo de ligantes pretende
conseguir prestaciones similares a la de los betunes modificados con polímeros utilizados habitualmente.
La aparición de fisuras debido a fenómenos de fatiga es una de las patologías más comunes en firmes de
carretera. Debido a las mejoras de las propiedades mecánicas del betún aportadas tras la incorporación
de polvo de neumático, las mezclas fabricadas con estos ligantes se postulan como una posible solución a
dicho problema. En este artículo se lleva a cabo un análisis comparativo del comportamiento a fisuración
por fatiga realizado sobre mezclas bituminosas fabricadas con betún modificado con polvo de neumático y
con polímeros.This research was carried out within the framework
of the R + D + i project entitled Proyecto Integrado
de Investigación, Desarrollo y Demostración de
Tecnologías para la aplicación de neumáticos fuera
de uso en firmes de carretera resistentes a la propagación
de grietas (ref. IDI-20091076), funded by the
Center for Industrial Technological Development
(CDTI) of the Ministry of Science and Innovation
in Spain
Vesiculation and Quenching During Surtseyan Eruptions at Hunga Tonga-Hunga Ha'apai Volcano, Tonga
Surtseyan eruptions are shallow to emergent subaqueous explosive eruptions that owe much of their characteristic behavior to the interaction of magma with water. The difference in thermal properties between water and air affects the cooling and postfragmentation vesiculation processes in magma erupted into the water column. Here we study the vesiculation and cooling processes during the 2009 and 2014–2015 Surtseyan eruptions of Hunga Tonga‐Hunga Ha'apai volcano by combining 2‐D and 3‐D vesicle‐scale analyses of lapilli and bombs and numerical thermal modeling. Most of the lapilli and bombs show gradual textural variations from rim to core. The vesicle connectivity in the lapilli and bombs increases with vesicularity from fully isolated to completely connected and also increases from rim to core in transitional clasts. We interpret the gradual textural variations and the connectivity‐vesicularity relationships as the result of postfragmentation bubble growth and coalescence interrupted at different stages by quenching in water. The measured vesicle size distributions are bimodal with a population of small and large vesicles. We interpret this bimodality as the result of two nucleation events, one prefragmentation with the nucleation and growth of large bubbles and one postfragmentation with nucleation of small vesicles. We link the thermal model with the textural variations in the clasts—showing a dependence on particle size, Leidenfrost effect, and initial melt temperature. In particular, the cooling profiles in the bombs are consistent with the gradual textural variations from rim to core in the clasts, likely caused by variations in time available for vesiculation before quenching
Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2
The discovery of a new family of high Tc materials, the iron arsenides
(FeAs), has led to a resurgence of interest in superconductivity. Several
important traits of these materials are now apparent, for example, layers of
iron tetrahedrally coordinated by arsenic are crucial structural ingredients.
It is also now well established that the parent non-superconducting phases are
itinerant magnets, and that superconductivity can be induced by either chemical
substitution or application of pressure, in sharp contrast to the cuprate
family of materials. The structure and properties of chemically substituted
samples are known to be intimately linked, however, remarkably little is known
about this relationship when high pressure is used to induce superconductivity
in undoped compounds. Here we show that the key structural features in
BaFe2As2, namely suppression of the tetragonal to orthorhombic phase transition
and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same
behavior under pressure as found in chemically substituted samples. Using
experimentally derived structural data, we show that the electronic structure
evolves similarly in both cases. These results suggest that modification of the
Fermi surface by structural distortions is more important than charge doping
for inducing superconductivity in BaFe2As2
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