197 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
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
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
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
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