We report an upward curvature in the temperature dependence of the upper critical field H c2 (T) in K x Ba 1Ϫx BiO 3 single crystals using ac-susceptibility measurements and magnetic fields up to 25 T. A possible role of Landau quantization in such uncommon behavior is discussed. ͓S0163-1829͑96͒03034-2͔

We report on field-induced variations of the microwave surface resistance at 9.6 GHz of Ba0.6K0.4BiO3 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. .Nf Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.) -74.25.Op Mixed state, critical fields, and surface sheaths PACS

Bismuthate superconductor 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, non-centrosymmetric and of the layered nature, 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.Superconducting bismuthates including the firstly dis-[2] with the highest transition temperature (T c =32-35K for x≈0.4) have the basic characteristics similar to those of high-T c copper-oxide superconductors [3]. This might favor for a common pairing mechanism. On the other hand, bismuthates have been generally considered to be fundamentally different from cuprates due to two reasons. Bismuthates are non-magnetic, and they have three-dimensional structures rather than layered twodimensional ones characteristic of cuprates. Of these two contradictions, the most important one seems to be the latter. The absence of magnetic fluctuations in bismuthates may point to a non-magnetic nature of the pairing mechanism, whereas it is unlikely that a common superconducting scenario in cuprates and bismuthates does not depend on lattice dimensionality.The study of BKBO is of a particular interest also for understanding the relation between charge-density-waves (CDWs) and superconductivity in high-T c oxides. The parent compound for BKBO is the perovskite BaBiO 3 containing a CDW formed of an ordered arrangement of non-equivalent bismuth ions referred to as Bi 3+ and Bi 5+[4]. This CDW is assumed to be responsible for the semiconducting behavior of BaBiO 3 and Ba 1−x K x BiO 3 materials with low potassium content (x < 0.25). The widely known notion that BKBO superconductors have a simple cubic ABO 3 solid-solution structure of a non-layered nature, with barium and potassium randomly occupying the A-position, was inferred from long-range structural studies of ceramic samples by X-ray [5] and neutron diffraction [6]. A simple cubic structure excludes the existence of a CDW that leads to the conclusion of the total incompatibility of CDWs and superconductivity. However, studies of BKBO by methods sensitive to short-range symmetry, in particular, by Raman scattering spectroscopy [7], a paired-distribution function analysis of neutron diffraction data [8], extended X-ray fine structure analysis (EXAFS) [9,10] evidence that the local structure of BKBO superconductors is not cubic. An X-ray diffraction study [11] [15]. Ba n Bi n+m O y are assumed to be matrices transformed, when intercalated with potassium, into superconducting oxides retaining the layered nature of the matrices structures. Besides, Ba-rich oxides with a solid-solution structure that attributed to BKBO superconductors were discovered [16]. Such oxides are formed in two-phase regions of the Ba-Bi-O ...

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