72 research outputs found
Microwave Properties of Ba(0.6)K(0.4)BiO(3) Crystals
We report on field-induced variations of the microwave surface resistance at
9.6 GHz of Ba(0.6)K(0.4)BiO(3) crystals. Energy losses have been investigated
as a function of the static magnetic field in the range of temperatures 4.2 K -
Tc. By analyzing the experimental results in the framework of the Coffey and
Clem model we determine the temperature dependence of the first-penetration
field, upper critical field and depinning frequency. The results show that the
pinning energy of this bismuthate superconductor is weaker than those of
cuprates.Comment: 6 pages, 8 embedded figure
Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_{1-x}K_xBiO_3 Single Crystals
A study of temperature dependences of the upper critical field B_{c2}(T) and
surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have
transition temperatures in the range 6 x>0.4) reveals
a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves
corresponding to the crystals that have T_c>20 K have positive curvature (like
in some HTSC), and those of the crystals with T_c<15 K fall on the usual
Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals
with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and
exponential (BCS) in the temperature range T << T_c. The experimental results
are discussed in connection with the extended saddle point model by Abrikosov.Comment: 5 pages, 5 figure
Non-cubic layered structure of Ba(1-x)K(x)BiO3 superconductor
Bismuthate superconductor Ba(1-x)K(x)BiO3 (x=0.27-0.49, Tc=25-32K) grown by
an electrolysis technique was studied by electron diffraction and
high-resolution electron microscopy. The crystalline structure thereof has been
found to be non-cubic, of the layered nature, and non-centrosymmetric, with the
lattice parameters a ~ ap, c ~ 2ap (ap is a simple cubic perovskite cell
parameter) containing an ordered arrangement of barium and potassium. The
evidence for the layered nature of the bismuthate superconductor removes the
principal crystallographic contradiction between bismuthate and cuprate high-Tc
superconductors.Comment: 4 pages, 3 figures, to be published in Physical Review B as a Rapid
Communicatio
Modern methods for identification and quantification of cardiac glycosides
Scientific relevance. Cardiac glycosides have been used in medicine for over two centuries. Current studies suggest that biologically active substances from this group can be used to treat not only heart conditions but also viral infections, cancers, and other diseases. Therefore, quality control methods for cardiac glycosides are becoming increasingly relevant.Aim. Based on a review of Russian and international quality standards, as well as up-to-date scientific data, this study aimed to identify promising methods for the identification and quantification of cardiac glycosides in herbal drugs and herbal medicinal products, as well as to evaluate the possibility of substituting physicochemical methods for biological methods.Discussion. The methods that are currently used to standardise cardiac glycosides are either not selective or require laboratory animals (biological test systems). According to a study of pharmacopoeial methods for the identification of cardiac glycosides in herbal drugs and herbal medicinal products, chemical identification tests and thin-layer chromatography continue to be relevant. Quantitative testing of herbal drugs and extracts uses biological and non-selective (spectrophotometry) methods, whereas chromatography is described only in general and individual monographs for herbal drug preparations containing individual cardiac glycosides and medicinal products containing these preparations. Upon analysing quality standards and scientific publications, the authors identified potentially promising methods for the quantification of cardiac glycosides in herbal drugs, herbal drug preparations, and herbal medicinal products, namely chromatographic methods.Conclusions. Reverse-phase high-performance liquid cjromatography (HPLC) with spectrophotometric detection is the most suitable method for pharmacopoeial analysis. The development of an HPLC-based analytical procedure to determine the cardiac glycoside content will provide an opportunity to advance from biological or non-selective methods to more ethical and selective up-to-date techniques
Electronic Collective Modes and Superconductivity in Layered Conductors
A distinctive feature of layered conductors is the presence of low-energy
electronic collective modes of the conduction electrons. This affects the
dynamic screening properties of the Coulomb interaction in a layered material.
We study the consequences of the existence of these collective modes for
superconductivity. General equations for the superconducting order parameter
are derived within the strong-coupling phonon-plasmon scheme that account for
the screened Coulomb interaction. Specifically, we calculate the
superconducting critical temperature Tc taking into account the full
temperature, frequency and wave-vector dependence of the dielectric function.
We show that low-energy plasmons may contribute constructively to
superconductivity. Three classes of layered superconductors are discussed
within our model: metal-intercalated halide nitrides, layered organic materials
and high-Tc oxides. In particular, we demonstrate that the plasmon contribution
(electronic mechanism) is dominant in the first class of layered materials. The
theory shows that the description of so-called ``quasi-two-dimensional
superconductors'' cannot be reduced to a purely 2D model, as commonly assumed.
While the transport properties are strongly anisotropic, it remains essential
to take into account the screened interlayer Coulomb interaction to describe
the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure
Trading or coercion? Variation in male mating strategies between two communities of East African chimpanzees
Across taxa, males employ a variety of mating strategies, including sexual coercion and the provision, or trading, of resources. Biological Market theory (BMT) predicts that trading of commodities for mating opportunities should exist only when males cannot monopolise access to females and/or obtain mating by force, in situations where power differentials between males are low; both coercion and trading have been reported for chimpanzees (Pan troglodytes). Here, we investigate whether the choice of strategy depends on the variation in male power differentials, using data from two wild communities of East African chimpanzees (P.t. schweinfurthii): the structurally despotic Sonso community (Budongo, Uganda) and the structurally egalitarian M-group (Mahale, Tanzania). We found evidence of sexual coercion by male Sonso chimpanzees, and of tradingâof grooming for matingâby M-group males; females traded sex for neither meat nor protection from male aggression. Our results suggest that the despotismâegalitarian axis influences strategy choice: male chimpanzees appear to pursue sexual coercion when power differentials are large and trading when power differentials are small and coercion consequently ineffective. Our findings demonstrate that trading and coercive strategies are not restricted to particular chimpanzee subspecies; instead, their occurrence is consistent with BMT predictions. Our study raises interesting, and as yet unanswered, questions regarding female chimpanzeesâ willingness to trade sex for grooming, if doing so represents a compromise to their fundamentally promiscuous mating strategy. It highlights the importance of within-species cross-group comparisons and the need for further study of the relationship between mating strategy and dominance steepness
Primary crystallization field of Y n Ba m Cu m + n O y oxides with YBa2Cu3O6 tetragonal structure
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