BiO3 Single Crystals Research Articles

The Stratified Superconductivity in Ba0.6K0.4BiO3Single Crystal: Direct Measurement of Energy Gap between Homo-, and Inhomogeneous States

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TCsystemBa0.6K0.4BiO3(TC≈30 K) is confirmed onBa0.6K0.4BiO3single crystal. AtT*(T*<TC,T*≈17 K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gaps on the Fermi surface, semi- and superconducting, which are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. NonlinearI-Vcurves are observed onBa0.6K0.4BiO3single crystal at temperatures belowT*. Dependence ofI-Vcurves on temperature and magnetic field, typical to a Josephson system, was found out. Besides, a step-like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier than critical current. The new data firstly correlate with the model of SISIS and secondly permit for the first time to determining directly the energy gap between homogeneous and stratified superconductor states.

Ba0.6K0.4BiO3 single crystal as a multiple Josephson system: new coherent effect?

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TC system Ba0.6K0.4BiO3 (TC≍30 K) is confirmed on Ba0.6K0.4BiO3 single crystal. At T* (T*<TC, T*≍17 K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gapes on the Fermi surface: semi- and superconducting, that are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. Nonlinear I-V curves, depended on temperature and magnetic field, that are typical to a Josephson system, are observed on Ba0.6K0.4BiO3 single crystal at temperatures below T*. Besides, a step like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier then critical current. Perhaps the last phenomenon is the consequence of “coherent” state of several successive Josephson junctions, appeared in the exfoliation state.

Universal scaling for both isotropic and anisotropic irradiated superconductors

Abstract Current–voltage characteristics are investigated by transport measurements on heavy-ion irradiated Bi2Sr2CaCu2O8+δ (γ=380), Bi1.8Pb0.2Sr2CaCu2O8+δ (γ=60) and isotropic (K,Ba)BiO3 single crystals. In the range of magnetic fields from Bϕ/10 to Bϕ (where Bϕ is the matching field), the same scaling functions can be used to describe the transition in these systems. We obtain ν⊥=1.1±0.1, z=5.3±0.2 using α (≡ν∥/ν⊥)=2, in good agreement with numerical simulations for the case of strongly screened vortex line interactions. The most important finding is that the scaling laws of the I–V characteristics in these systems could be the same despite the difference in anisotropy.

Vortex Melting and Superconducting Transition in High-Tc Oxides

The existence of an order-disorder phase transition which can be induced in the vortex matter of type II superconductors by increasing T or H is now well established. At high temperatures the location of the so-called vortex melting line can then be shifted either downwards or upwards by introducing point or columnar defects. We show that the introduction of the latter in (K,Ba)BiO3 single crystals has also a strong influence on the transition from superconducting to normal state.

Anisotropic enhancement of superconductivity in heavy-ion irradiated (K, Ba)BiO3.

We have measured the specific heat, resistivity, and ac susceptibility of (K,Ba)BiO3 single crystals before and after introduction of either point or columnar defects by electron (EI) or heavy-ion irradiation (HII). While the magnetic field dependence of these properties remains mainly unaffected by EI, the irreversibility line and the location of the specific heat anomaly are both shifted up in temperature after HII. The shift is apparent only if the magnetic field is applied parallel to the ion tracks. For perpendicularly applied fields, both lines lie at the same field as in the pristine sample. These experiments call the nature of the vortex liquid state into question.

Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)BiO3.

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K,Ba)BiO3 single crystals. The temperature dependence of the magnetic field H(C(p)) corresponding to the onset of the specific heat anomaly presents a clear positive curvature. H(C(p)) is significantly smaller than the field H(Delta) for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization presents a strong deviation from the Ginzburg-Landau theory emphasizing the peculiar nature of the superconducting transition in this material.

Nonlinear ac response of the vortex system in the cubic superconductor

We present ac susceptibility measurements performed on (K, Ba)BiO3single crystals with different geometries: thick films, bars and hollow cylinders. We show that the hac dependence of the real (χ′) and imaginary (χ′) parts of the ac susceptibility is in very good agreement with Brandt’s numerical calculations (Phys. Rev. B 58, 6523 (1998)) in the modified Bean critical state. Creep effects (at Hdc = 0.3T) are investigated by studying the frequency dependence of the current density deduced from the temperature scans of the ac susceptibility over a large frequency range (0.02 Hz < ω < 2000 HZ). The relaxation rate S = dln(J)/dln(ω) ∼ 4% is temperature independent and very similar to the one usually obtained in high Tc cuprates.

Growth-induced anisotropy in Ba 0.6 K 0.4 BiO 3 superconducting single crystal

K0.4BiO3 single crystal have been performed to study the nature of the growth-induced anisotropy revealed using chemical etching technique. It was found that this anisotropy is responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic field orientations.

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part R s and the imaginary part X s of the surface impedance Z s =R s +iX s of the superconductor Ba0.6K0.4BiO3 (T c ≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence Z s (T) and that of the complex conductivity σ s (T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T 5), with decreasing temperature T<T c . This model also describes well the curves Z s (T) and σs (T) recently measured for YBa2Cu3O6.95 and Bi2Sr2CaCu2O8 single crystals.

Temperature, current, and magnetic-field dependence of the activation energy in the vortex-glass phase of (K,Ba)BiO3 single crystals.

Temperature, current, and magnetic-field dependence of the activation energy in the vortex-glass phase of (K,Ba)BiO3 single crystals.

Optical conductivity of nonmetallic Ba0.69K0.31BiO3 single crystals: Evidence for bipolaron formation.

Optical conductivity of nonmetallic Ba0.69K0.31BiO3 single crystals: Evidence for bipolaron formation.