168 research outputs found
Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors
In the transient state of d-wave superconductors, we investigate the temporal
variation of photoinduced changes in the superfluid weight. We derive the
formula that relates the nonlinear response function to the nonequilibrium
distribution function. The latter qunatity is obtained by solving the kinetic
equation with the electron-electron and the electron-phonon interaction
included. By numerical calculations, a nonexponential decay is found at low
temperatures in contrast to the usual exponential decay at high temperatures.
The nonexponential decay originates from the nonmonotonous temporal variation
of the nonequilibrium distribution function at low energies. The main physical
process that causes this behavior is not the recombination of quasiparticles as
previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80,
No.
A superconducting-nanowire 3-terminal electronic device
In existing superconducting electronic systems, Josephson junctions play a
central role in processing and transmitting small-amplitude electrical signals.
However, Josephson-junction-based devices have a number of limitations
including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability
to drive large impedances, and (4) difficulty in controlling the junction
critical current (which depends sensitively on sub-Angstrom-scale thickness
variation of the tunneling barrier). Here we present a nanowire-based
superconducting electronic device, which we call the nanocryotron (nTron), that
does not rely on Josephson junctions and can be patterned from a single thin
film of superconducting material with conventional electron-beam lithography.
The nTron is a 3-terminal, T-shaped planar device with a gain of ~20 that is
capable of driving impedances of more than 100 k{\Omega}, and operates in
typical ambient magnetic fields at temperatures of 4.2K. The device uses a
localized, Joule-heated hotspot formed in the gate to modulate current flow in
a perpendicular superconducting channel. We have characterized the nTron,
matched it to a theoretical framework, and applied it both as a digital logic
element in a half-adder circuit, and as a digital amplifier for superconducting
nanowire single-photon detectors pulses. The nTron has immediate applications
in classical and quantum communications, photon sensing and astronomy, and its
performance characteristics make it compatible with existing superconducting
technologies. Furthermore, because the hotspot effect occurs in all known
superconductors, we expect the design to be extensible to other materials,
providing a path to digital logic, switching, and amplification in
high-temperature superconductors
Non-equilibrium Superconductivity and Quasiparticle Dynamics studied by Photo Induced Activation of Mm-Wave Absorption (PIAMA)
We present a study of non-equilibrium superconductivity in DyBa2Cu3O7-d using
photo induced activation of mm-wave absorption (PIAMA). We monitor the time
evolution of the thin film transmissivity at 5 cm-1 subject to pulsed infrared
radiation. In addition to a positive bolometric signal we observe a second,
faster, decay with a sign opposite to the bolometric signal for T>40 K. We
attribute this to the unusual properties of quasi-particles residing near the
nodes of an unconventional superconductor, resulting in a strong enhancement of
the recombination time.Comment: 4 pages, REVTeX, Submitted to Phys. Rev. Letter
Calculation of magnetic anisotropy energy in SmCo5
SmCo5 is an important hard magnetic material, due to its large magnetic
anisotropy energy (MAE). We have studied the magnetic properties of SmCo5 using
density functional theory (DFT) calculations where the Sm f-bands, which are
difficult to include in DFT calculations, have been treated within the LDA+U
formalism. The large MAE comes mostly from the Sm f-shell anisotropy, stemming
from an interplay between the crystal field and the spin-orbit coupling. We
found that both are of similar strengths, unlike some other Sm compounds,
leading to a partial quenching of the orbital moment (f-states cannot be
described as either pure lattice harmonics or pure complex harmonics), an
optimal situation for enhanced MAE. A smaller portion of the MAE can be
associated with the Co-d band anisotropy, related to the peak in the density of
states at the Fermi energy. Our result for the MAE of SmCo5, 21.6 meV/f.u.,
agrees reasonably with the experimental value of 13-16 meV/f.u., and the
calculated magnetic moment (including the orbital component) of 9.4 mu_B agrees
with the experimental value of 8.9 mu_B.Comment: Submitted to Phys. Rev.
Vortex avalanches and magnetic flux fragmentation in superconductors
We report results of numerical simulations of non isothermal dendritic flux
penetration in type-II superconductors. We propose a generic mechanism of
dynamic branching of a propagating hotspot of a flux flow/normal state
triggered by a local heat pulse. The branching occurs when the flux hotspot
reflects from inhomogeneities or the boundary on which magnetization currents
either vanish, or change direction. Then the hotspot undergoes a cascade of
successive splittings, giving rise to a dissipative dendritic-type flux
structure. This dynamic state eventually cools down, turning into a frozen
multi-filamentary pattern of magnetization currents.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Role of deep levels and interface states in the capacitance characteristics of all‐sputtered CuInSe2/CdS solar cell heterojunctions
All‐sputtered CuInSe2/CdS solar cellheterojunctions have been analyzed by means of capacitance‐frequency (C‐F) and capacitance‐bias voltage (C‐V) measurements. Depending on the CuInSe2 layer composition, two kinds of heterojunctions were analyzed: type 1 heterojunctions (based on stoichiometric or slightly In‐rich CuInSe2 layers) and type 2 heterojunctions (based on Cu‐rich CuInSe2 layers). In type 1 heterojunctions, a 80‐meV donor level has been found. Densities of interface states in the range 101 0–101 1 cm2 eV− 1 (type 1) and in the range 101 2–101 3 cm− 2 eV− 1 (type 2) have been deduced. On the other hand, doping concentrations of 1.6×101 6 cm− 3 for stoichiometric CuInSe2 (type 1 heterojunction) and 8×101 7 cm− 3 for the CdS (type 2 heterojunction) have been deduced from C‐Vmeasurements
IKKβ regulates essential functions of the vascular endothelium through kinase-dependent and -independent pathways
Vascular endothelium provides a selective barrier between the blood and tissues, participates in wound healing and angiogenesis, and regulates tissue recruitment of inflammatory cells. Nuclear factor (NF)-κB transcription factors are pivotal regulators of survival and inflammation, and have been suggested as potential therapeutic targets in cancer and inflammatory diseases. Here we show that mice lacking IKKβ, the primary kinase mediating NF-κB activation, are smaller than littermates and born at less than the expected Mendelian frequency in association with hypotrophic and hypovascular placentae. IKKβ-deleted endothelium manifests increased vascular permeability and reduced migration. Surprisingly, we find that these defects result from loss of kinase-independent effects of IKKβ on activation of the serine-threonine kinase, Akt. Together, these data demonstrate essential roles for IKKβ in regulating endothelial permeability and migration, as well as an unanticipated connection between IKKβ and Akt signalling
Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition
We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons
in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density
wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165
K using femtosecond time-resolved pump-probe spectroscopy. The contributions in
transient differential reflectivity arising from exponentially decaying QP
relaxation and oscillatory coherent acoustic phonon mode show large variations
in the vicinity of T_SDW. From the temperature-dependence of the QP
recombination dynamics in the SDW phase, we evaluate a BCS-like temperature
dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW,
whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has
been estimated from the linear temperature-dependence of the QP relaxation
time. The long-wavelength coherent acoustic phonons with typical time-period of
~100 ps have been analyzed in the light of propagating strain pulse model
providing important results for the optical constants, sounds velocity and the
elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.Comment: Revised version (to appear as Full Paper in Journal of Physical
Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715
Particulate Matter-Induced Lung Inflammation Increases Systemic Levels of PAI-1 and Activates Coagulation Through Distinct Mechanisms
Exposure of human populations to ambient particulate matter (PM) air pollution significantly contributes to the mortality attributable to ischemic cardiovascular events. We reported that mice treated with intratracheally instilled PM develop a prothrombotic state that requires the release of IL-6 by alveolar macrophages. We sought to determine whether exposure of mice to PM increases the levels of PAI-1, a major regulator of thrombolysis, via a similar or distinct mechanism. mice but was absent in mice treated with etanercept, a TNF-α inhibitor. Treatment with etanercept did not prevent the PM-induced tendency toward thrombus formation.Mice exposed to inhaled PM exhibited a TNF-α-dependent increase in PAI-1 and an IL-6-dependent activation of coagulation. These results suggest that multiple mechanisms link PM-induced lung inflammation with the development of a prothrombotic state
Role of deep levels and interface states in the capacitance characteristics of all‐sputtered CuInSe 2
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