79 research outputs found
Kinetics of air-hydrate nucleation in polar ice sheets
Nucleation of air clathrate hydrates in air bubbles and diffusive air-mass exchange between coexisting ensembles of bubbles and hydrate crystals are the major interrelated processes that determine the phase change in the air-ice system in polar ice. In continuation of Salamatin et al. where the post-nucleation conversion of single air bubbles to hydrates was considered, we present here a statistical description for transformation of air bubbles to air clathrate hydrates based on the general theory of evolution of these two ensembles, including the gas fractionation effects. The model is fit to data on ice cores from central Antarctica, and then compared to other ice-core data. The focus is on the rate of clathrate-hydrate nucleation, which is determined to be the product of the inverse relative bubble size raised to the power λ≈5.8 with the relative supersaturation to the power β≈2. The clathration-rate constant is k0≈3.2-4.5×10-6 yr-1 at 220 K. The N2- and O2-permeation coefficients in ice, at 220 K, are inferred to be DN(2) 0≈1.8-2.5×10-8 mm2 yr-1 and DO(2) 0≈5.4-7.5×10-8 mm2 yr-1, respectively. Comparison of observations to simulations of bubble-to-hydrate transformation in Greenland ice sheet gave estimates for activation energies of hydrate formation and air diffusion of QJ≈70 kJ mol-1 and Qd≈50 kJ mol-1, respectively
Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
We report the 75As nuclear quadrupole resonance (NQR) and specific heat
measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The
spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite
gradual temperature dependence with no coherence peak below Tc. The
quasi-particle specific heat C_QP/T shows small specific heat jump which is
about 30% of electronic specific heat coefficient just below Tc. In addition,
it suggests the existence of low-energy quasi-particle excitation at the lowest
measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of
1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario
rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. No.8 issue
(2009
Evidence for superconducting gap nodes in the zone-centered hole bands of KFe2As2 from magnetic penetration-depth measurements
Among the iron-based pnictide superconductors the material KFeAs is
unusual in that its Fermi surface does not consist of quasi-nested electron and
hole pockets. Here we report measurements of the temperature dependent London
penetration depth of very clean crystals of this compound with residual
resistivity ratio . We show that the superfluid density at low
temperatures exhibits a strong linear-in-temperature dependence which implies
that there are line nodes in the energy gap on the large zone-centered hole
sheets. The results indicate that KFeAs is an unconventional
superconductor with strong electron correlations.Comment: 6 pages, 6 figures, 1 table. Extended version to be published in
Phys. Rev.
Maintenance of glucose-sensitive insulin secretion of cryopreserved human islets with University of Wisconsin solution and ascorbic acid-2 glucoside
Normal human islet cells are an ideal source for pancreas-targeted cell therapies, but the availability of human donor pancreata for islet isolation is severely limited. To effectively utilize such scarce donor organs for cell therapies, it is crucial to develop an excellent isolation, effective cryopreservation, and efficient gene transfer techniques for the transportation of isolated cells. In the present study, we investigate the effect of University of Wisconsin (UW) solution and ascorbic acid-2 glucoside (AA2G) on the cryopreservation of human islets. We also evaluate the gene transfer efficiency of a lentiviral vector expressing the E. coli LacZ gene, Lt-NLS/LacZ, in human islets. Human islets were isolated with a standard digestion method at the University of Alberta. Isolated islets were transported to Japan for 40 h and then subjected to cryopreservation experiments. The following preservation solutions were tested: UW solution with 100 mug/mL of AA2G, UW solution, 100% fetal bovine serum (FBS), and CMRL supplemented with 10% FBS. Following three months of cryopreservation, the islets were thawed and analyzed for viability, glucose-sensitive insulin secretion, proinsulin gene expression profile, and in vivo engraftment. The islets were also subjected to monolayer formation with 804G-cell-line-derived extracellular matrix (ECM), followed by Lt-NLS/LacZ transduction. The viability, morphology, glucose-sensitive insulin secretion, proinsulin gene expression, and monolayer formation efficiency of the thawed cryopreserved islets are significantly better maintained by the use of UW solution. When AA2G (100 mug/mL) is combined with UW, such parameters are further improved. The adequate engraftment of UW + AA2G-cryopreserved human islets is achieved in the liver of nude mice. Efficient Lt-NLS/LacZ transduction is identified in monolayered islets cryopreserved with UW solution with AA2G. The present work demonstrates that the combination of UW solution with AA2G (100 mug/mL) would be a useful cryopreservation means for human islets. Human islets monolayer-cultured with 804G-derived ECM are efficiently transduced with a lentiviral vector Lt-NLS/LacZ
Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors
We propose a simple way to parameterize the gap function in iron pnictides.
The key idea is to use orbital representation, not band representation, and to
assume real-space short-range pairing. Our parameterization reproduces fairly
well the structure of gap function obtained in microscopic calculation. At the
same time the present parameterization is simple enough to obtain an intuitive
picture and to develop a phenomenological theory. We also discuss
simplification of the treatment of the superconducting state.Comment: 4 page
Filling Control of the Mott Insulator Ca2RuO4
We have grown single crystals of electron doping system Ca2-xLaxRuO4 (0.00 <=
x <= 0.20) by a floating zone method. The first order metal/non-metal
transition and canted antiferromagnetic ordering occur for 0.00 < x < 0.15,
similar to those in the bandwidth controlled system Ca2-xSrxRuO4 (CSRO).
However, comparing with CSRO, we found a rather different metallic ground state
adjacent to the non-metallic ground state with canted antiferromagnetic order.
Instead of short-range antiferromagnetic correlation found in CSRO (0.20 <= x <
0.50), the metallic ground state of the present system is characterized by
strong ferromagnetic correlation.Comment: 8 pages, 8 figures (eps), submitted to J. Phys. Soc. Jp
Strong-coupling Spin-singlet Superconductivity with Multiple Full Gaps in Hole-doped BaKFeAs Probed by Fe-NMR
We present Fe-NMR measurements of the novel normal and
superconducting-state characteristics of the iron-arsenide superconductor
BaKFeAs ( = 38 K). In the normal state, the
measured Knight shift and nuclear spin-lattice relaxation rate
demonstrate the development of wave-number ()-dependent spin fluctuations,
except at = 0, which may originate from the nesting across the disconnected
Fermi surfaces. In the superconducting state, the spin component in the
Fe-Knight shift decreases to almost zero at low temperatures, evidencing
a spin-singlet superconducting state. The Fe- results are totally
consistent with a -wave model with multiple full gaps, regardless of
doping with either electrons or holes.Comment: 4 pages, 4 figures, 1 tabl
Chandra High Energy Grating Observations of the Fe Ka Line Core in Type 2 Seyfert Galaxies: A Comparison with Type 1 Nuclei
We present a study of the core of the Fe Ka emission line at ~6.4 keV in a
sample of type II Seyfert galaxies observed by the Chandra High Energy Grating
(HEG). The sample consists of 29 observations of 10 unique sources. We present
measurements of the Fe Ka line parameters with the highest spectral resolution
currently available. In particular, we derive the most robust intrinsic line
widths for some of the sources in the sample to date. We obtained a weighted
mean FWHM of 2000 \pm 160 km/s for 8 out of 10 sources (the remaining sources
had insufficient signal-to-noise). From a comparison with the optical
emission-line widths obtained from spectropolarimetric observations, we found
that the location of Fe Ka line-emitting material is a factor of ~0.7-11 times
the size of the optical BLR. Furthermore, compared to 13 type I AGNs for which
the best Fe Ka line FWHM constraints were obtained, we found no difference in
the FWHM distribution or the mean FWHM, and this conclusion is independent of
the central black hole mass. This result suggests that the bulk of the Fe Ka
line emission may originate from a universal region at the same radius with
respect to the gravitational radius, ~30,000 Rg on average. By examining the
correlation between the Fe Ka luminosity and the [O IV] line luminosity, we
found a marginal difference in the Fe K line flux between type I and type II
AGNs, but the spread in the ratio of L(Fe) to L([O IV]) is about two orders of
magnitude. Our results confirm the theoretical expectation that the Fe Ka
emission-line luminosity cannot trivially be used as a proxy of the intrinsic
AGN luminosity, unless a detailed comparison of the data with proper models is
applied.Comment: 16 pages, 6 figures, and 2 tables, accepted for publication in Ap
Possible Pairing Symmetry of Three-dimensional Superconductor UPt -- Analysis Based on a Microscopic Calculation --
Stimulated by the anomalous superconducting properties of UPt, we
investigate the pairing symmetry and the transition temperature in the
two-dimensional(2D) and three-dimensional(3D) hexagonal Hubbard model. We solve
the Eliashberg equation using the third order perturbation theory with respect
to the on-site repulsion . As results of the 2D calculation, we obtain
distinct two types of stable spin-triplet pairing states. One is the
-wave(B) pairing around and in a small region, which is
caused by the ferromagnetic fluctuation. Then, the other is the (or
)-wave(E) pairing in large region far from the half-filling () which is caused by the vertex corrections only. However, we find that the
former -wave pairing is destroyed by introduced 3D dispersion. This is
because the 3D dispersion breaks the favorable structures for the -wave
pairing such as the van Hove singularities and the small pocket structures.
Thus, we conclude that the ferromagnetic fluctuation mediated spin-triplet
state can not explain the superconductivity of UPt. We also study the case
of the pairing symmetry with a polar gap. This -wave(A) is stabilized
by the large hopping integral along c-axis . It is nearly degenerate with
the suppressed (or )-wave(E) in the best fitting parameter region
to UPt (). These two p-wave pairing states exist in
the region far from the half-filling, in which the vertex correction terms play
crucial roles like the case in SrRuO.Comment: 15 pages, 12 figure
Antiferromagnetic Order and Superconductivity in Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with Electron Doping: 75As-NMR Study
We report an 75As-NMR study on iron (Fe)-based superconductors with thick
perovskitetype blocking layers Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with x=0 and 0.2.
We have found that antiferromagnetic (AFM) order takes place when x=0, and
superconductivity (SC) emerges below Tc=36 K when x=0.2. These results reveal
that the Fe-pnictides with thick perovskitetype blocks also undergo an
evolution from the AFM order to the SC by doping electron carriers into FeAs
planes through the chemical substitution of Ti+4 ions for Mg+2 ions, analogous
to the F-substitution in LaFeAsO compound. The reason why the Tc=36 K when
x=0.2 being higher than the optimally electron-doped LaFeAsO with Tc=27 K
relates to the fact that the local tetrahedron structure of FeAs4 is optimized
for the onset of SC.Comment: 4 pages, 3 figures, 1 tabl
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