151 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
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
High shock release in ultrafast laser irradiated metals: Scenario for material ejection
We present one-dimensional numerical simulations describing the behavior of
solid matter exposed to subpicosecond near infrared pulsed laser radiation. We
point out to the role of strong isochoric heating as a mechanism for producing
highly non-equilibrium thermodynamic states. In the case of metals, the
conditions of material ejection from the surface are discussed in a
hydrodynamic context, allowing correlation of the thermodynamic features with
ablation mechanisms. A convenient synthetic representation of the thermodynamic
processes is presented, emphasizing different competitive pathways of material
ejection. Based on the study of the relaxation and cooling processes which
constrain the system to follow original thermodynamic paths, we establish that
the metal surface can exhibit several kinds of phase evolution which can result
in phase explosion or fragmentation. An estimation of the amount of material
exceeding the specific energy required for melting is reported for copper and
aluminum and a theoretical value of the limit-size of the recast material after
ultrashort laser irradiation is determined. Ablation by mechanical
fragmentation is also analysed and compared to experimental data for aluminum
subjected to high tensile pressures and ultrafast loading rates. Spallation is
expected to occur at the rear surface of the aluminum foils and a comparison
with simulation results can determine a spall strength value related to high
strain rates
Pre‐Eruptive Outgassing and Pressurization, and Post‐Fragmentation Bubble Nucleation, Recorded by Vesicles in Breadcrust Bombs From Vulcanian Activity at Guagua Pichincha Volcano, Ecuador
Breadcrust bombs formed during Vulcanian eruptions are assumed to originate from the shallow plug or dome. Their rim to core texture reflects the competition between cooling and degassing timescales, which results in a dense crust with isolated vesicles contrasting with a highly vesicular vesicle network in the interior. Due to relatively fast quenching, the crust can shed light on pre- and syn-eruptive conditions prior to or during fragmentation, whereas the interior allows us to explore post-fragmentation vesiculation. Investigation of pre- to post-fragmentation processes in breadcrust bombs from the 1999 Vulcanian activity at Guagua Pichincha, Ecuador, via 2D and 3D textural analysis reveals a complex vesiculation history, with multiple, spatially localized nucleation and growth events. Large vesicles (Type 1), present in low number density in the crust, are interpreted as pre-eruptive bubbles formed by outgassing and collapse of a permeable bubble network during ascent or stalling in the plug. Haloes of small, syn-fragmentation vesicles (Type 2), distributed about large vesicles, are formed by pressurization and enrichment of volatiles in these haloes. The nature of the pressurization process in the plug is discussed in light of seismicity and ground deformation signals, and previous textural and chemical studies. A third population (Type 3) of post-fragmentation small vesicles appears in the interior of the bomb, and growth and coalescence of Type 2 and 3 vesicles causes the transition from isolated to interconnected bubble network in the interior. We model the evolution of viscosity, bubble growth rate, diffusion timescales, bubble radius and porosity during fragmentation and cooling. These models reveal that thermal quenching dominates in the crust whereas the interior undergoes a viscosity quench caused by degassing, and that the transition from crust to interior corresponds to the onset of percolation and development of permeability in the bubble network
Pressure effects on superconducting properties of single-crystalline Co doped NaFeAs
Resistivity and magnetic susceptibility measurements under external pressure
were performed on single-crystals NaFe1-xCoxAs (x=0, 0.01, 0.028, 0.075,
0.109). The maximum Tc enhanced by pressure in both underdoped and optimally
doped NaFe1-xCoxAs is the same, as high as 31 K. The overdoped sample with x =
0.075 also shows a positive pressure effect on Tc, and an enhancement of Tc by
13 K is achieved under pressure of 2.3 GPa. All the superconducting samples
show large positive pressure coefficient on superconductivity, being different
from Ba(Fe1-xCox)2As2. However, the superconductivity cannot be induced by
pressure in heavily overdoped non-superconducting NaFe0.891Co0.109As. These
results provide evidence for that the electronic structure is much different
between superconducting and heavily overdoped non-superconducting NaFe1-xCoxAs,
being consistent with the observation by angle-resolved photoemission
spectroscopy.Comment: 6 pages, 6 figure
Pressure versus concentration tuning of the superconductivity in Ba(Fe(1-x)Cox)2As2
In the iron arsenide compound BaFe2As2, superconductivity can be induced
either by a variation of its chemical composition, e.g., by replacing Fe with
Co, or by a reduction of the unit-cell volume through the application of
hydrostatic pressure p. In contrast to chemical substitutions, pressure is
expected to introduce no additional disorder into the lattice. We compare the
two routes to superconductivity by measuring the p dependence of the
superconducting transition temperature Tc of Ba(Fe(1-x)Cox)2As2 single crystals
with different Co content x. We find that Tc(p) of underdoped and overdoped
samples increases and decreases, respectively, tracking quantitatively the
Tc(x) dependence. To clarify to which extent the superconductivity relies on
distinct structural features we analyze the crystal structure as a function of
x and compare the results with that of BaFe2As2 under pressure.Comment: 14 pages, 4 figures, to be published in JPSJ Vol. 79 No. 12. The
copyright is held by The Physical Society of Japa
Chemical Pressure and Physical Pressure in BaFe_2(As_{1-x}P_{x})_2
Measurements of the superconducting transition temperature, T_c, under
hydrostatic pressure via bulk AC susceptibility were carried out on several
concentrations of phosphorous substitution in BaFe_2(As_{1-x}P_x)_2. The
pressure dependence of unsubstituted BaFe_2As_2, phosphorous concentration
dependence of BaFe_2(As_{1-x}P_x)_2, as well as the pressure dependence of
BaFe_2(As_{1-x}P_x)_2 all point towards an identical maximum T_c of 31 K. This
demonstrates that phosphorous substitution and physical pressure result in
similar superconducting phase diagrams, and that phosphorous substitution does
not induce substantial impurity scattering.Comment: 5 pages, 4 figures, to be published in Journal of the Physical
Society of Japa
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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