12,255 research outputs found
Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps
Density-functional calculations of the phonon spectrum and electron-phonon
coupling in MgB are presented. The phonons, which involve in-plane
B displacements, couple strongly to the electronic bands. The
isotropic electron-phonon coupling constant is calculated to be about 0.8.
Allowing for different order parameters in different bands, the superconducting
in the clean limit is calculated to be significantly larger. The
phonons are strongly anharmonic, and the non-linear contribution to
the coupling between the modes and the p bands is significant.Comment: 4 pages, 3 figure
Nonmagnetic impurity effects in MgB
We study nonmagnetic impurity effects in MgB using the quasiclassical
equations of superconductivity for a weak-coupling two-band model. Parameters
in the model are fixed so as to reproduce experiments on MgB as closely
as possible. The quasiparticle density of states and the specific heat are
calculated for various values of the interband impurity scattering. The density
of states changes gradually from a two-gap structure into the conventional
single-gap structure as the interband scattering increases. It is found that
the excitation threshold is not a monotonic function of the interband
scattering. Calculated results for the specific heat are in good agreements
with experiments on samples after irradiation
Correlation effects during liquid infiltration into hydrophobic nanoporous mediums
Correlation effects arising during liquid infiltration into hydrophobic
porous medium are considered. On the basis of these effects a mechanism of
energy absorption at filling porous medium by nonwetting liquid is suggested.
In accordance with this mechanism, the absorption of mechanical energy is a
result expenditure of energy for the formation of menisci in the pores on the
shell of the infinite cluster and expenditure of energy for the formation of
liquid-porous medium interface in the pores belonging to the infinite cluster
of filled pores. It was found that in dependences on the porosity and,
consequently, in dependences on the number of filled pores neighbors, the
thermal effect of filling can be either positive or negative and the cycle of
infiltration-defiltration can be closed with full outflow of liquid. It can
occur under certain relation between percolation properties of porous medium
and the energy characteristics of the liquid-porous medium interface and the
liquid-gas interface. It is shown that a consecutive account of these
correlation effects and percolation properties of the pores space during
infiltration allow to describe all experimental data under discussion
Polyvinylpyrrolidone-Coordinated Single-Site Platinum Catalyst Exhibits High Activity for Hydrogen Evolution Reaction
The essence of developing a Ptâbased singleâatom catalyst (SAC) for hydrogen evolution reaction (HER) is the preparation of wellâdefined and stable single Pt sites with desired electrocatalytic efficacy. Herein, we report a facile approach to generate uniformly dispersed Pt sites with outstanding HER performance via a photochemical reduction method using polyvinylpyrrolidone (PVP) molecules as the key additive to significantly simplify the synthesis and enhance the catalytic performance. The asâprepared catalyst displays remarkable kinetic activities (20 times higher current density than the commercially available Pt/C) with excellent stability (76.3â% of its initial activity after 5000â
cycles) for HER. EXAFS measurements and DFT calculations demonstrate a synergetic effect, where the PVP ligands and the support together modulate the electronic structure of the Pt atoms, which optimize the hydrogen adsorption energy, resulting in a considerably improved HER activity
Capacitive Spring Softening in Single-Walled Carbon Nanotube Nanoelectromechanical Resonators
We report the capacitive spring softening effect observed in single-walled
carbon nanotube (SWNT) nanoelectromechanical (NEM) resonators. The nanotube
resonators adopt dual-gate configuration with both bottom-gate and side-gate
capable of tuning the resonance frequency through capacitive coupling.
Interestingly, downward resonance frequency shifting is observed with
increasing side-gate voltage, which can be attributed to the capacitive
softening of spring constant. Furthermore, in-plane vibrational modes exhibit
much stronger spring softening effect than out-of-plan modes. Our dual-gate
design should enable the differentiation between these two types of vibrational
modes, and open up new possibility for nonlinear operation of nanotube
resonators.Comment: 12 pages/ 3 figure
Effect of crude canine pituitary extract (cCPE) on the in vitro production of progesterone and nuclear maturation of canine oocytes
We examined the effect of crude canine pituitary extract (cCPE) on the in vitro nuclear maturation of canine oocytes and production of progesterone by cumulus cells. cCPE was extracted from canine pituitaries and the concentrations of canine follicle stimulating hormone (FSH) and luteinizing hormone (LH) were determined. Cumulus oocyte complexes (COCs) were harvested from anestrus cycle ovaries and matured in NCSU-37 supplemented with 10% estrus bitch serum, 50 Όg/ml gentamycin and 0, 40 or 400 Όg/ml cCPE at 38°C in a humidified atmosphere of 5% CO2 for 72 h. The nuclear maturation of the oocytes and the level of progesterone in the culture medium were evaluated. Development to metaphase I (MI) - metaphase II (MII) of canine oocytes in 400 Όg/ml cCPE (15.4%) was significantly higher than in 0 and 40 Όg/ml cCPE (4.3 and 8.7%), respectively. Treatment with 40 and 400 Όg/ml cCPE also generated 0.33 and 0.65 ng/ml progesterone in the culture medium, respectively. Thus, the addition of cCPE to the culture medium promotes the nuclear maturation of canine oocytes and elevates the production of progesterone by cumulus cells.Key words: In vitro maturation, pituitary extract, canine oocyte
Ultrafast Hole Trapping and Relaxation Dynamics in p-Type CuS Nanodisks
CuS nanocrystals are potential materials for developing low-cost solar energy conversion devices. Understanding the underlying dynamics of photoinduced carriers in CuS nanocrystals is essential to improve their performance in these devices. In this work, we investigated the photoinduced hole dynamics in CuS nanodisks (NDs) using the combination of transient optical (OTA) and X-ray (XTA) absorption spectroscopy. OTA results show that the broad transient absorption in the visible region is attributed to the photoinduced hot and trapped holes. The hole trapping process occurs on a subpicosecond time scale, followed by carrier recombination (~100 ps). The nature of the hole trapping sites, revealed by XTA, is characteristic of S or organic ligands on the surface of CuS NDs. These results not only suggest the possibility to control the hole dynamics by tuning the surface chemistry of CuS but also represent the first time observation of hole dynamics in semiconductor nanocrystals using XTA
Phonon structure in I-V characteristic of MgB point-contacts
The search of the phonon structure at the above-gap energies was carried out
for spectra of MgB point contacts with a normal metal.
The two-band model is assumed not only for the gap structure in
-characteristics, but also for phonons in
point-contact spectra, with up to the maximum lattice vibration energy. Since
the current is carried mostly by charges of 3D-band, whereas the strong
electron-phonon interaction occurs in 2D-band, we observe the phonon
peculiarities due to ''proximity'' effect in {\it k}-space, which depends on
the variation of interband coupling through the elastic scattering.Comment: 6 pages, 4 figures, revtex4, reported in International Conference
"Modern Problems in Superconductivity", 9-13 September, Yalta, Ukrain
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