J.-Y. Genoud

Reversible suppression of the so-called fishtail effect in ultra pure single crystals of YBa2Cu3O7−δ achieved by proper oxygenation

We report on the absence of the so-called fishtail or peak effect in the magnetisation curves of properly oxygenated high purity (5N) single crystals of YBa2Cu3O7−δ. As we recently reported [1] high pressure oxygenation procedures are suitable to produce a monotonous dependence of the critical current density with the magnetic field. In this article we report furthermore how to reversibly produce or remove this effect on the same crystal by proper oxygenation methods. Along with the high purity of the samples, — due to crystal growth in non reactive BaZrO3 crucibles —, this proves that at least in pure samples the fishtail effect is due to different oxygenation states and/or oxygen distributions. In less pure samples, however, other sources for this anomaly like metallic impurities can act as an additional source for the fishtail anomaly.

D-WAVE SCALING RELATIONS IN THE MIXED-STATE SPECIFIC HEAT OF YBA2CU3O7

The low temperature specific heat C(B,T) of an YBa2Cu3O7.00 single crystal is measured from 1.2 to 10 K in magnetic fields up to 14 T. The anisotropic component Caniso(T,B)=C(T,B//c)-C(T,B//ab) is a pure vortex quantity obtained directly from experiment. It follows a scaling relation predicted recently for line nodes characteristic of d-wave vortices. Our experimental field and temperature range corresponds to a crossover region where the limit Caniso(T,B)is proportional to T*sqrt(B) does not strictly apply. The variation of the entropy caused by the magnetic field at low T is thermodynamically compatible with measurements near Tc.

{110}⟨110⟩ textured Ag ribbons for biaxially aligned YBa2Cu3O7-x coated conductor tapes

Texture development in pure Ag was investigated in order to achieve pure {110}011 textured Ag ribbons that can be used as a substrate for YBa2Cu3O7-x superconducting coated tapes without a buffer layer. Silver ingots were rolled by different percentages and to different thicknesses. They were then annealed at different temperatures to get a stable recrystallization texture. The textures were measured using x-ray pole figures and orientation distribution function (ODF) analysis. The rolled texture is found to be {110}112, with a {110}011 component for high deformation rates. As the recrystallization temperature is increased from 100 to 800 °C, the broad {110}112 as-rolled texture is reduced and the {110}110 texture is enhanced. A small amount of a secondary texture originating from twinning is evident in fully recrystallized samples. No change in the {110}110 recrystallization texture was observed after several weeks at room temperature and after 1 h at 900 °C, which is promising for coated conductor processing.

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi2Sr2CaCu2O8+δ single crystals for different oxygen concentrations, from optimally doped (Tc=92.2 K) to highly overdoped (Tc=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (Δp) is reduced and the dip structure beyond Δp at negative sample bias1 shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with Tc, and 2Δp/kBTc stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of Tc

Monotonic dependence of jc on magnetic field is twinned crystals of YBa2Cu3O7−δ and ErBa2Cu3O7−δ

Abstract We have investigated the magnetic hysteresis and in particular the “fishtail” anomaly in crystals of YBa 2 Cu 3 O 7−δ and ErBa 2 Cu 3 O 7−δ , oxygenated in various fashions. For a high-pressure (100 bar) oxygenation this anomaly disappears, but only at high annealing temperature (700°C). It is concluded that for proper oxygenation the diffusion distance for oxygen must be larger than the size of the crystals, leading to a more homogeneous oxygen distribution throughout the crystal.

Specific heat of high temperature superconductors in high magnetic fields

Abstract We review investigations of the mixed-state specific heat (C) of extreme type-II superconductors in magnetic fields B≤16 T, from 1 K to Tc. In YBa2Cu3Ox (Y123) at low temperature T, a scaling law Δ C ∞( TB 1 2 )ƒ( T/B 1 2 ) , which is a signature of d-wave symmetry, is observed for the background-free anisotropic component ΔCC(B//c)-C(B//ab). At intermediate T, a C(B)C(0)∞TnBlnB behaviour, which may be explained by the London model, smoothly sets in. Near Tc, superconducting fluctuations are investigated using the background-free derivative ∂(C/T)/∂B. 3D-XY scaling properties are verified up to the overdoped regime. Fluctuations obey different laws in Bi2Sr2CaCu2Ox (Bi2212), which is strongly affected by its 2D character. Finally, the melting line of the vortex solid, which is now believed to be the only true phase transition above Hc1 in the presence of fluctuations, is observed in Y123 either as a 1st- or 2nd-order transition, depending on pinning.

New elaboration technique, structure and physical properties of infinite-layer Sr1−xLnxCuO2(Ln = Nd, Pr)

Abstract The infinite-layer compounds of the (Sr, Ln)CuO 2 family have been prepared by a new synthesis method from the low-pressure phase of SrCuO 2 and NdCuO 2 as starting materials. The crystal structure, microstructure and superconductivity of these samples have been studied with use of neutron powder diffraction, optical micrographs, energy-dispersive X-ray analysis. magnetic-susceptibility and electrical-resistivity measurements. The ratio of the lattice constants a/c as an empirical parameter for the existence of superconductivity in the electron-doped infinite-layer compounds is discussed.

Reinforced {110} textured Ag/Ni composite ribbons for biaxially oriented YBCO coated tapes

New, mechanically reinforced, {110} textured Ag composite ribbons were fabricated as possible inexpensive substrates for coated conductors without any buffer layer. The key part of this work was the use of a thin but textured Ag outlayer on an AgMg, Ni, or Ni alloys core that ensures the mechanical reinforcement while decreasing the amount of Ag. A thin Cu (50 µm) foil was intercalated between rectangular Ag and AgMg, Ni, or Ni alloys pieces to obtain a tough bond. After sintering, the ingots sustained a cold plastic deformation up to thicknesses of 300, 100 and 50 µm without any intermediate annealing. The prepared composite ribbons were then recrystallized to obtain the {110} texture in the top Ag layer. For the Ag/AgMg ribbons, a further annealing in flowing oxygen was performed to reinforce the ribbons by an internal oxidation process in the AgMg layer without changing the {110}011 texture in the Ag outerlayer. A unique and stable {110}011 texture was obtained in Ag/Ni composite ribbons after annealing at 800 °C. The highest degree of texture was observed in 300 µm thick Ag/Ni ribbon, x-ray orientation distribution function and electro backscattered diffraction (EBSD) analysis of the top Ag layer in this ribbon revealed a sharp {110}011 texture with an in-plane misorientation angle of 10-15°. Meanwhile, a promising {110}011 texture was also found in 50 µm thick ribbons with 25° in-plane misorientation angle. In the last case, a substrate with no more than 16 µm of the textured Ag layer was produced by this new technique, which decreased the total amount of Ag by 40% compared with pure Ag ribbons of the same thickness.

Growth of high purity YBa2Cu4O8 and Y2Ba4Cu7O15-δ single crystals in BaZrO3 crucibles under high oxygen pressure, and absence of magnetic “fishtail” effect

Abstract We report on the first HTS crystal growth in BaZrO 3 crucibles under very high gas pressure. YBa 2 Cu 4 O 8 (Y124) crystals were grown in an autoclave under 1400 bar of pure oxygen. The eutectic temperature was determined to be 1113°C. X-ray analysis and electronic (EDX) micrographs showed that the crystals were contamination-free. The high (T c =81.5K) and sharp superconducting transition and the small magnetic hysteresis further established the purity of these Y124 crystals. In particular, no “fishtail” anomaly, but a monotonous decrease of the hysteresis was observed in magnetic fields up to 5.5T and down to 10K. The use of BaZrO 3 crucibles shows that this stoechiometric and untwinned compound exhibits an intrinsically low pinning.

Material developments in Ag/Bi,Pb(2223), Ag/RE(123) and Ag/Tl(1223) tapes: texture in HTS

Abstract High energy X-ray diffraction was used to compare Bi,Pb(2223) tapes prepared by the standard rolling technique and from a periodic pressing deformation process. Periodic pressing increases the critical current, although low energy X-rays on the tapes surface show a poorer texture. From transmission measurements, no significant effect of the different deformation process is detected on the texture of the whole ceramic core. The key toward high critical currents in Bi,Pb(2223) tapes has to be found in the thermodynamics of the phase formation process rather than in further improvement of texture. To get an enhanced critical current density in the Y123 and Tl2223 systems, a biaxial texture is mandatory. In Ag/RE(123) systems, there is a commensurability between the Ag surface and the superconducting cell as shown form sputtering experiments on oriented Ag single crystals. To be of industrial interest, long lengths of textured Ag ribbons have to be produced and fast deposition techniques are needed. We have shown that it is possible to control the texture formation in Ag ribbons. X-rays and EBSD analysis of highly textured {1 1 0}〈0 1 1〉 is reported. Electrodeposition of Tl(1223) and spray pyrolysis of RE(123) are presented as two fast and cheap techniques for biaxially aligned coated tapes preparation on textured Ag ribbons. Pure Tl(1223) is formed from the simultaneous deposition of all metals dissolved in DMSO followed by a phase formation process under high pressure (50 bar He/1 bar O 2 ). Pure and biaxially aligned Y123 is produced from an aqueous nitrate solution in a one step process at 900°C.