22,216 research outputs found
Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects
The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are
discussed based on structural, transport, and high pressure experiments. The
positive Seebeck coefficient and its linear temperature dependence for Tc<T<160
K provide evidence that the low-temperature transport in MgB2 is due to
hole-like metallic carriers. Structural and transport data show the important
role of defects as indicated by the correlation of Tc, the residual resistance
ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc
with hydrostatic pressure is well explained by the strong-coupling BCS theory.
The large scatter of the pressure coefficients of Tc for different MgB2
samples, however, cannot be explained within this theory. We speculate that
pressure may increase the defect density, particularly in samples with large
initial defect concentration.Comment: Presented at NATO Advanced Research Workshop "New Trends in
Superconductivity", Yalta (Ukraine) 16-20 September, 200
Water Content and Superconductivity in Na0.3CoO2*yH2O
We report here the correlation between the water content and
superconductivity in Na0.3CoO2*yH2O under the influences of elevated
temperature and cold compression. The x-ray diffraction of the sample annealed
at elevated temperatures indicates that intergrowths exist in the compound at
equilibrium when 0.6 < y < 1.4. Its low-temperature diamagnetization varies
linearly with y, but is insensitive to the intergrowth, indicative of quasi-2D
superconductivity. The Tc-onset, especially, shifts only slightly with y. Our
data from cold compressed samples, on the other hand, show that the water-loss
non-proportionally suppresses the diamagnetization, which is suggestive of weak
links.Comment: 10 pages, 10 figures; submitted to Physica C (August 13, 2003
Increase in soil organic carbon by agricultural intensification in northern China
Acknowledgements. This research was supported by National Natural Science Foundation of China (no. 31370527 and 31261140367) and the National Science and Technology Support Program of China (no. 2012BAD14B01-2). The authors gratefully thank the Huantai Agricultural Station for providing of the Soil Fertility Survey data. We also thank Zheng Liang from China Agricultural University for the soil sampling and analysis in 2011. Thanks are extended to Jessica Bellarby for helpful discussion and suggestions.Peer reviewedPublisher PD
Self-consistent relativistic quasiparticle random-phase approximation and its applications to charge-exchange excitations and -decay half-lives
The self-consistent quasiparticle random-phase approximation (QRPA) approach
is formulated in the canonical single-nucleon basis of the relativistic
Hatree-Fock-Bogoliubov (RHFB) theory. This approach is applied to study the
isobaric analog states (IAS) and Gamov-Teller resonances (GTR) by taking Sn
isotopes as examples. It is found that self-consistent treatment of the
particle-particle residual interaction is essential to concentrate the IAS in a
single peak for open-shell nuclei and the Coulomb exchange term is very
important to predict the IAS energies. For the GTR, the isovector pairing can
increase the calculated GTR energy, while the isoscalar pairing has an
important influence on the low-lying tail of the GT transition. Furthermore,
the QRPA approach is employed to predict nuclear -decay half-lives. With
an isospin-dependent pairing interaction in the isoscalar channel, the
RHFB+QRPA approach almost completely reproduces the experimental -decay
half-lives for nuclei up to the Sn isotopes with half-lives smaller than one
second. Large discrepancies are found for the Ni, Zn, and Ge isotopes with
neutron number smaller than , as well as the Sn isotopes with neutron
number smaller than . The potential reasons for these discrepancies are
discussed in detail.Comment: 34 pages, 14 figure
Pseudospin symmetry and its approximation in real nuclei
The origin of pseudospin symmetry and its broken in real nuclei are discussed
in the relativistic mean field theory. In the exact pseudospin symmetry, even
the usual intruder orbits have degenerate partners. In real nuclei, pseudospin
symmetry is approximate, and the partners of the usual intruder orbits will
disappear. The difference is mainly due to the pseudo spin-orbit potential and
the transition between them is discussed in details. The contribution of
pseudospin-orbit potential for intruder orbits is quite large, compared with
that for pseudospin doublets. The disappearance of the pseudospin partner for
the intruder orbit can be understood from the properties of its wave function.Comment: 10 pages, 3 figure
- …