1,156 research outputs found
LaCo2B2: A Co-based layered superconductor with a ThCr2Si2-type structure
LaCo2B2 with a ThCr2Si2-type structure composed of alternately stacked La and
CoB layers exhibits metallic electrical conductivity and Pauli paramagnetic
behavior down to 2K. Bulk superconductivity with a Tc of ~4K emerges upon
substitution with dopant elements; i.e., isovalent substitution to form
(La1-xYx)Co2B2, or aliovalent substitution to form La(Co1-xFex)2B2. Highly
covalent bonding between Co 3d and B 2p levels in the CoB layers, which is
caused by the B 2p level being shallower than the Fermi level, removes magnetic
ordering from Co 3d electrons even in the undoped samples.Comment: 3 figure
Hall coefficient and Hc2 in underdoped LaFeAsO0.95F0.05
The electrical resistivity and Hall coefficient of LaFeAsO0.95F0.05
polycrystalline samples were measured in pulsed magnetic fields up to m0H = 60
T from room temperature to 1.5 K. The resistance of the normal state shows a
negative temperature coefficient (dr/dT < 0) below 70 K for this composition,
indicating insulating ground state in underdoped LaFeAsO system in contrast to
heavily doped compound. The charge carrier density obtained from Hall effect
can be described as constant plus a thermally activated term with an energy gap
DE = 630 K. Upper critical field, Hc2, estimated from resistivity measurements,
exceeds 75 T with zero-field Tc = 26.3 K, suggesting an unconventional nature
for superconductivity.Comment: 12 pages and 4 figure
Crossover between magnetism and superconductivity in low H-doped LaFeAsO
By a systematic study of the hydrogen-doped LaFeAsO system by means of dc
resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the
question of universality of the phase diagram of rare-earth-1111 pnictides. In
many respects, the behaviour of LaFeAsO_(1-x)H_(x) resembles that of its widely
studied F-doped counterpart, with H^- realizing a similar (or better)
electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range
SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K
and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk
superconductivity. Unlike the abrupt M-SC transition found in the parent
La-1111 compound, the presence a crossover region makes the H-doped system
qualitatively similar to other Sm-, Ce-, or Nd-1111 families.Comment: to appear in Journal of Physics: Condensed Matte
Comparison of crystal structures and effects of Co substitution in a new member of Fe-1111 superconductor family AeFeAsF(Ae = Ca and Sr): a possible candidate for higher Tc superconductor
We refined crystal structures of newly found members of the Fe-1111
superconductor family, CaFe\_{1-x}Co\_{x}AsF and SrFe\_{1-x}Co\_{x}AsF (x = 0,
0.06, 0.12) by powder synchrotron X-ray diffraction analysis. The tetragonal to
orthorhombic phase transitions were observed at ~120 K for unsubstituted
CaFeAsF and at ~180 K for unsubstituted SrFeAsF, the transition temperatures
agreeing with kinks observed in temperature-dependent resistivity curves.
Although the transition temperature decreases, the structural phase transitions
were observed below 100 K in both samples of x = 0.06, and finally they were
suppressed in the doping level of x = 0.12. The refined structures reveal that
distortions of the FeAs4 tetrahedron from the regular tetrahedron likely
originate from mismatches in atomic radii among the constituent elements. In
this system, the enlarged FeAs4 tetrahedron resulting from larger radius of Sr
than that of Ca is flattened along a-b plane, whereas the smaller radius of Ca
makes the tetrahedron closer to regular one, and their characteristic shapes
are further enhanced by Co substitution. These results suggest that the CaFeAsF
compound is a promising candidate for higher-Tc superconductor.Comment: 17 pages, 8 figures, 2 tables, Supplementary information is included
at the end of the documen
Psuedo-isotropic upper critical field in cobalt-doped SrFe2As2 epitaxial films
The temperature and angular dependence of the upper critical field (Hc2) is
reported for cobalt-doped SrFe2As2 epitaxial films between Tc and 0.5 K in
pulsed magnetic fields up to 50 T. For H parallel c, Hc2 is close to a linear
function of temperature, while in the perpendicular direction there is
significant downward curvature that results in an Hc2 ratio (gamma =
Hc2(perpendicular)/Hc2(parallel) that decreases nearly linearly with
temperature, approaching gamma = 1 at low temperature with Hc2(0) = 47 T. We
measure the complete upper-critical field phase diagram including angular
dependence and model the data using a two band theory allowing us to determine
the anisotropy of both bands, their relative diffusivities, and the
relationship between BCS coupling constant matrix elements. We find an unusual
relationship between the diffusivities of the two bands, with two anisotropic
and opposite bands. This relationship is supported by the observation of a
local maximum for Hc2(parallel) at low temperature
Helicobacter pylori infection appears essential for stomach carcinogenesis: Observations in Semarang, Indonesia
The gastric cancer incidence in Semarang, Indonesia, is exceedingly low: only approximately 1/100th of the level in Japan. To elucidate the reason, we carried out an ecological study recruiting 69 male and 102 female participants from the general populace in January 2005. Positive urea breath tests were 0% for both men and women, and Helicobacter pylori (H. pylori) IgG antibodies were found in 2% (0–5, 95% confidence interval) of men and 2% (0–4) of women, significantly lower than the 62% (58–65) and 57% (53–60), respectively, in Japan. Furthermore, there were no positive findings with the pepsinogen tests in Semarang, again significant in comparison with the 23% (22–25) and 22% (20–23) in Japan. Variation in smoking levels and consumption of NaCl, vegetables and fruit were found, but not to an extent that would allow explanation of the major differences in gastric cancer incidence. We may conclude that the very low prevalence of H. pylori infection and thus chronic atrophic gastritis account for the rarity of stomach cancer in Semarang, Indonesia. (Cancer Sci 2005; 96: 873–875
Enhanced Symmetries in Multiparameter Flux Vacua
We give a construction of type IIB flux vacua with discrete R-symmetries and
vanishing superpotential for hypersurfaces in weighted projective space with
any number of moduli. We find that the existence of such vacua for a given
space depends on properties of the modular group, and for Fermat models can be
determined solely by the weights of the projective space. The periods of the
geometry do not in general have arithmetic properties, but live in a vector
space whose properties are vital to the construction.Comment: 32 pages, LaTeX. v2: references adde
Electronic structure and light-induced conductivity in a transparent refractory oxide
Combined first-principles and experimental investigations reveal the
underlying mechanism responsible for a drastic change of the conductivity (by
10 orders of magnitude) following hydrogen annealing and UV-irradiation in a
transparent oxide, 12CaO.7Al2O3, found by Hayashi et al. The charge transport
associated with photo-excitation of an electron from H, occurs by electron
hopping. We identify the atoms participating in the hops, determine the exact
paths for the carrier migration, estimate the temperature behavior of the
hopping transport and predict a way to enhance the conductivity by specific
doping.Comment: 4 pages including 4 figure
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