11 research outputs found
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
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
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
Primary crystallization field of Y n Ba m Cu m + n O y oxides with YBa2Cu3O6 tetragonal structure
Spatial resolution of transmission electron spectroscopy for the study of ordered materials
Positive curvature in the temperature dependence of H-c2 in KxBa1-xBiO3
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