C. Escribe-Filippini

Electrochemical synthesis and characterization of superconducting Ba1−xKxBiO3 single crystals

Abstract We succeeded in the elaboration of Ba1−xKxBiO3 single crystals using the electrocrystallization method. All the characterizations (X-Ray diffraction, microanalysis, susceptibility) have been made on the same single crystal. By this way we have shown that single crystals, even superconducting, present a gradient of concentration and that the superconducting transition can be strongly lowered by less than 1% of Na.

Superconducting properties of the low dimensional purple bronze Li0.9Mo6O17

Abstract The purple bronze Li0.9Mo6O17 has recently been found to be a quasi two dimensional metal showing an instability towards a semiconducting type behaviour near 25 K and a superconducting transition at Tc = 1.9 K. We report results of ac-susceptibility obtained on single crystals grown by the electrocrystallization technique. The transition at T ≅ 25 K is attributed to a charge density wave instability, as established for the neighbouring compound K0.9Mo6O17. The superconducting transition temperature is found to be sample dependent (Tc = 1.1. to 1.6 K probably in relation with the crystal stoichiometry. The critical field Hc is anisotropic with Hc2|;/Hc2⊥ ≈- and temperature independent (Hc2| corresponds to H| layers), Hc2|;) ≈- 20 kOe at 0.3 K. Specific heat data corroborate the existence of both transitions. These results are discussed in relation with the anisotropy of the Fermi surface and with the Ginzburg-Landau theory with anisotropic effective masses.

Upper critical field of Ba1−χKχBiO3 single crystal

We measured the upper critical field Hc2 on Ba1−χKχBiO3 (0.35<χ<0.40) single crystal down to 2 K in pulsed magnetic fields up to 35 T. Resistive transitions in the magnetic field systematically shift as the temperature decreases and the ϱ-H curves saturate for high enough magnetic field strength. The initial slope of the upper critical field (dHc2/dT)Tc is -0.5 T/K. As temperature decreases, the Hc2−T curve shows an upward curvature. At 2 K, we find Hc2=30 T, which gives a coherence length ξ=33 A. Such an Hc2 value is almost twice as large as what one can deduce by using the standard expression Hc2(0)=−0.693Tc(dHc2/ dT)Tc. From the comparison with several models, we conclude that this unusual temperature dependence of Hc2 can be due either to structural instability or to microscopic sample inhomogeneities.

Magnetic and transport properties of some Ln-Ba-Cu-O (Ln=Yb, Tm, Sm) superconducting oxides

Abstract Powders of Yb-Ba-Cu-O, Tm-Ba-Cu-O and Sm-Ba-Cu-O have been synthetized. Electrical resistivity and magnetic susceptibility measurements show that they are superconducting with onset transition temperatures in the range 91 K – 85 K. The thermopower, in these oxides as well as in Sr x La 2−x CuO 4−δ and Ba 2 YCu 3 O 7−δ , is found to be positive between T c and 300 K. Hysteresis of the diamagnetic magnetization is also reported.

Upper critical field in Ba1 − xKxBiO3: Magnetotransport vs. magnetotunneling

Elastic tunneling is used as a powerful direct tool to determine the upper critical field Hc2(T) in the high-Tc oxide Ba1 − xKxBiO3. The temperature dependence of Hc2 inferred from the tunneling follows the Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A comparison will be made with resistively determined critical-field data.

Short time study of the fishtail effect in Ba0.6K0.4BiO3 single crystals

Abstract A magnetization study using three techniques with different time scales down to 10 −4 s has been performed on single crystals of the three-dimensional high- T c superconductor Ba 0.6 K 0.4 BiO 3 . A fishtail effect (second peak in M ( B )) is observed in a large temperature range down to the shortest time scale. At high temperature ( T = 0.74 T c ) the peak is still visible for the shortest time scale but disappears with increasing waiting time showing that it is not related to flux creep phenomena. The origin of the fishtail effect in high- T c superconductors is discussed.

Transport properties of Ba1−xKxBiO3 single crystals

Abstract We measured resistivity and Hall effect on Ba1−xKxBiO3 single crystals (0.35 R H (270 K ) = -1.22×10 -9 m 3 C , a value that is quite close to that predicted by Hamada et al. by electronic band calculations.

Charge Density Waves: Pinning and Dynamics

The properties of charge density wave (CDW) low dimensional conductors are reviewed, with special reference to the molybdenum bronzes. For quasi-one-dimensional materials showing non linear transport due to the depinning of the CDW, the origin of metastability in the low electric field state as well as some aspects of CDW dynamics, are discussed. Results obtained by local techniques such as EPR and proton channeling may be consistent with the creation, annihilation and motion of CDW structural defects, such as phase dislocations, under applied electrical fields or through thermal treatments. We also discuss the properties of quasi 2D CDW materials such as the purple Mo bronze, which may show SDW and magnetic field induced transitions.

Cross-sectional TEM investigations of the microstructure and film-substrate substrate interface in Y-Ba-Cu-O thin films

Abstract Superconducting YBaCuO films deposited on yttria-stabilized zirconia (YSZ) and SrTiO 3 substrates have been studied by transmission electron microscopy. The films were elaborated by RF diode sputtering, the annealing procedure being performed ex-situ, at temperatures close to 900°C. Characterization by X-ray diffraction and resistivity measurements shows that the films mainly consist of the desired 123 phase. The c -axis perpendicular orientation has been obtained whatever the substrate orientation with YSZ and with (100) oriented SrTiO 3 . We report here the results of the cross-sectional TEM investigations of the microstructures and film/substrate interfaces. The presence of a 300 nm thick intermediate layer in films deposited on YSZ is reported while HREM studies clearly show the epitaxial growth of the 123 phase on SrTiO 3 (100).

Andreev reflection on the Ag−BaPb1−xBi x O3 microconstriction: Temperature and magnetic field dependence

The point-contact spectroscopy has been performed on the normal metal-superconductor microconstriction Ag−BaPb1−xBixO3. The conductance curves of our junctions reveal almost ideal Andreev reflection behavior described by theBlonder-Tinkham-Klapwijk theory, with the in-gap conductance twice higher than the conductance at higher voltages. We studied the effect of the temperature and magnetic field on the Andreev reflection. The temperature dependence of the energy gap (determined from the BTK theory) follows the BCS prediction. The value of the reduced gap yields a weak coupling strength in BaPb1−xBixO3. ¿From magnetic measurements up to 10 Tesla the temperature dependence of the upper critical field Hc2 has been determined and found to follow the Werthamer-Helfand-Hohenberg prediction.