C. Opagiste

Reversible magnetization below Tc in high-quality superconducting ceramics

We have investigated the reversible magnetization below Tc in high-quality YBa2Cu307_d (Y-123), YBa2Cu4Os (Y-124), Y2Ba4Cu7O15+x (Y-247) and Bi2Sr2CaCu2O8+x (Bi-2212), Tl2Ba2Cu106+d (Tl-2201) and Tl2Ba2CalCu2O8+d (T1-2212) ceramics. Except for the stoichiometric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the normal-state susceptibility becomes temperature independent. Using the simple London model, we are able to fit the reversible magnetization M(T, H) outside the region near Tc with good accuracy for the nearly three-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into account the fluctuations of vortices. An important result is that Y-123 exhibits a critical field clearly higher than those of the BiSrCaCuO or TlBaCaCuO phases. We obtain for the Y-123 phase a slope at Tc μodHC2,C/dT = -4.3 T/K and an extrapolated μoHC2,C (0) = 280 T.

Phase diagram of the Tl2Ba2CuO6 compounds in the T, p(O2) plane

We present a detailed investigation of the structural and superconducting properties of Tl2Ba2CuOx compounds as a function of oxygen pressure and annealing temperature prior to quenching. The crystal structure depends both on the oxygen concentration and on thallium deficiency. At the stoichiometric cation composition Tl:Ba:Cu=2:2:1, the parameters p(O2) and Tquench uniquely define the structure and the superconducting Tc. With adequate heat treatment sequences, reversible transitions from the tetragonal to the orthorhombic phase have been observed.

Preparation of pure Tl2Ba2CuO6±x: The contribution of phase equilibrium studies

The formation of the pure thallium-based cuprate Tl2Ba2CuO6+x (2201) using a convenient process in the ternary TIO1.5-BaO-CuO phase diagram is described. This process is based on the congruent formation of the oxide Tl2Ba2O5 to which CuO is mixed, constituting a quasi-binary system. In the resulting 2201 compound no trace of the main poisoning magnetic impurity BaCuO2 could be detected by AC susceptibility measurements. The orthorhombic to tetragonal transformation is asserted to be due to thallium deficiency which occurs during the heat treatments at a temperature depending on the oxygen partial pressure.

Thermodynamic and kinetic studies of the phase transitions in Tl2Ba2CuO6±x

We present a kinetic and thermodynamic analysis of the phase transitions in stoichiometric Tl2Ba2CuO6 samples prepared under a pressure of 100 bar. We have found a first-order transformation which maintains the orthorhombic structure from high pressure to 1 bar. The as-prepared tetragonal structure in an inert gas converts to orthorhombic symmetry when annealed in an oxygen flux. The activation energies have been deduced from a dynamic analysis of weight losses during the phase changes. The enthalpy and entropy for the formation of oxygen vacancies have been determined.

Equilibrium diagram Tc(T;p(O2)) of Tl2Ba2CuO6

Abstract Homogeneous Tl 2 Ba 2 CuO x ceramics were synthesized by a novel route allowing tight control of thallium losses, starting from Tl 2 Ba 2 O 5 precursors and using high gas pressures. Annealing followed by fast quenching into gallium allowed us to map the critical temperature (0 to 92 K) and the crystal symmetry (orthorhombic or tetragonal) in the (T;p(O 2 )) plane. The orthorhombic phase is found to be stoichiometric, whereas the tetragonal phase has a homogeneity domain in the thallium concentration. The critical temperature, determined by the oxygen concentration, is not sensitive to the crystal structure. Selected physical properties of Tl 2 Ba 2 CuO x compounds are presented.

Reversibility of the mixed state of the 107K Tl2Ba2CaCu2O8 superconductor investigated by different susceptibility measurements

Abstract The reversible mixed state of the Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) high T c superconductor with T c of 107K has been investigated by direct measurement of the AC differential susceptibility ρ H which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux flow viscous contribution to ρ H could be neglected. The following results have been obtained: a strong (field enhanced) contribution from diamagnetic fluctuation above T c , while below T c one observes deviation from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vortex density and distortions in the layered structure. This behaviour appears to be common to all the high T c superconductors as we have shown in an earlier work for the YBa 2 Cu 3 O 7−x , Tl-2201, Tl-2223 and Bi-2212 compounds. Moreover, these deviations prevent any reliable determination of the H c2 critical field line by the standard method of linear extrapolation of M(H,T) curves by the step anomaly on ρ H expected to occur at T c (H) if the mean field theory was to be followed in the mixed state.

The London penetration depth λab evolution with Tc in the TI-2201 overdoped superconductor

The Cu-O monolayer Tl2Ba2CuO6±δ is known to be an overdoped cuprate and Tc is controlled by the oxygen off-stoechiometry parameter δ. Samples with Tc ranging from ∼ 40K to 92K for orthorhombic and tetragonal phases have been prepared and systematically investigated by magnetization and differential susceptibility measurements in the reversible regime. This allows the deduction of the in-plane London penetration depth λab. We show the evolution of λab with Tc which does not fall on the Uemura plot established for the underdoped high Tc superconductors.

Evolution of the (IR) reversibility domain extension with Tc in the overdoped Tl-2201 high Tc superconductor

Abstract A serie of the Tl-2201 cuprate with T c ranging from ∼ 40 K to 92 K with tetragonal as well as orthorhombic symmetry has been prepared and their differential magnetic susceptibility χ H measured under static field up to 2 Tesla. Independently of the criteria defining the irreversibility onset, we show that the extension of the reversible regime in the (H,T/T c ) diagram increases with T c . Thus, contrary to what has been observed for YBa 2 Cu 3 O 6+x where the reversibility domain was enlarged with the T c decreasing, here for low T c the reversibility domain is more limited and shifted toward H c2 (T) line. These results are discussed taking into consideration the evolution of λ and the possible evolution of ξ with T c in this Cu-O monolayer overdoped HTS.

Characterization of the 105 K superconductor Tl2Ba2CaCu2O8 (“2212”)

Homogeneous Tl1.8Ba2CaCu2O8 ceramics were synthesized by a novel route, starting from Tl2Ba2O5 precursors and using high gas pressures. This method allows tight control of thallium losses, resulting in dense, large-grained samples with sharp superconducting transitions above 105 K. Results of a characterization by X-ray diffraction, optical micrographs and micro-probe analysis are presented, together with selected physical properties.

Calcium substitution in the Y2Ba4Cu7O15 + δ superconducting phase

We have attempted to substitute calcium on the yttrium and barium sites in the “247” superconducting phase, i.e. (Y1 - x Cax)2Ba4 Cu7O15 + δ and Y2(Ba1 - xCax)4Cu7O15 + δ. X-Ray diffraction and microprobe analyses indicate that calcium enters the structure but preferentially occupies the barium site. The solubility of calcium in the “247” phase is clearly lower than 10% (x < 0.1). The samples are not single phase even for small substitutions (x = 0.05). Higher values of x result in a higher amount of impurity phases. The (Y1 - xCax)2Ba4Cu7O15 + δ samples contain BaCuO2 as the major impurity, whereas in Y2 (Ba1 - xCax)4Cu7O15 + δ a new phase is found. This new phase was identified by microprobe analysis to have the composition YCaCu2O4.5 ± δ. No increase is observed in the critical temperature Tc as a function of calcium substitution. This is in contrast with the calcium-substituted “124” superconductor (Y1 - xCax)Ba2Cu4O8 where Tc increases from 80 K (x = 0) to about 90 K (x = 0.1).