CHARACTERIZATION OF THE 105 K SUPERCONDUCTOR Tl2Ba2CaCu208 ("2212")

International audienceHomogeneous Tl1.8Ba2CaCu208 ceramics were synthesized by a novel route, starting from Tl2Ba205 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

Preparation of pure Tl2Ba2CuO6+x: the contribution of phase equilibrium studies

International audienceThe 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

Reversible magnetization below Tc in high-quality superconducting ceramics

International audienceWe 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

ULTRASONIC ATTENUATION AND OCCURENCE OF THE MARTENSITIC TRANSFORMATION IN THE HIGH Tc A15 COMPOUND V3Si

The [100] shear modulus C44 is known to be nearly temperature independent in the case of V3Si, an extensively studied compound for wich many authors tried to associate the lattice instability and the high superconducting transition temperature. We report new sound velocity and attenuation measurements of this [100] shear mode. The temperature change of the sound velocity displays a 6 % relative decrease while a large increase in the attenuation of this mode occurs for T decreasing from 80 K to Tc = 17 K. This new observation gives a strong support for a very recently proposed approach to the occurence of the structural transformation in V3Si

Back to the A15 instability problems

Recent experimental results obtained on A15 compounds allow a better understanding of the correlation between superconductivity and lattice instability in this system. The effect of the lattice distortion is shown to reduce the superconducting transition T c. An enhancement of Tc in the case of V3Si is obtained when the temperature of the martensitic transition is lowered (by pressure or defects). Theoretically, these effects are well explained and predictable within the frame work of the band Jahn-Teller « models »; then also give more arguments in favour of these models.De récents résultats expérimentaux sur les composés A15 permettent une meilleure compréhension de la relation entre la supraconductivité et l'instabilité de réseau dans ce système. L'effet de la déformation structurale est de réduire la température de transition supraconductrice Tc. Une augmentation de Tc, dans le cas de V 3 Si, est obtenue lorsque la température de la transition structurale est abaissée (par effet de pression ou de défauts). 11 est possible, d'un point de vue théorique, de comprendre et de prévoir ces effets dans le cadre des modèles Jahn-Teller de bandes, ce qui donne des arguments supplémentaires en faveur de ce mécanisme

ON THE THERMAL AND ELECTRICAL CONDUCTIVITY OF V3Si AND V5Si3

Thermal conductivity and electrical resistivity measurements have been performed on V3Si and V5Si3 single crystals. Thermal conductivity anomalies are observed on both compounds and seem likely to be related to the resistivity "saturation" behaviour exhibited by these two compounds

Specific heat of the 90-K superconductor Tl₂Ba₂CuO₆ ("2201") prepared in high pressure Ar or He gas

We present high resolution heat capacity data in the temperature range 30–300 K for several homogeneous samples of the superconductor Tl2Ba2CuO6 (“2201”) with transition temperatures in the vicinity of 90 K. The specific heat peaks that appear close to the superconducting transition in some samples are identified as being due to gases trapped in the pores of the ceramics. The very small intrinsic anomaly at Tc, of the order of 1% of the total specific heat, is best observed in samples prepared under He atmosphere. The data are analysed using a model that includes a parametrized phonon spectrum, a mean-field jump at Tc, and a fluctuation contribution of the Lawrence-Doniach type. The fit yields an average coherence length of 7.5 Å (sample with Tc =92 K) and 8.2 Å (sample with Tc=87 K), and a vanishingly small mean-field jump. The homogeneity of the electronic subsystem is questioned. The data are compared with new results for Bi2Sr2CaCu2O8 (“2212”, Tc=93 K) and YBa2Cu3O7 (“123”, Tc=91 K). The fluctuation component is similar for all three, but YBa2Cu3O7 stands out with its relatively large mean-field jump

Tl2Ba2CuO6+x : from normal state to superconductivity

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Reversible magnetization below <i>Tc</i> in high-quality superconducting ceramics

We have investigated the reversible magnetization below Tc in high-quality YBa2Cu3O7−δ (Y-123), YBa2Cu4O8 (Y-124), Y2Ba4Cu7O15+δ (Y-247), and Bi2Sr2CaCu2O8+δ (Bi-2212), Tl2Ba2Cu1O6+δ (Tl-2201) and Tl2Ba2 Ca1Cu2O8+δ (Tl-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 μ0∂Hc2,c/∂T|Tc≈−4.3 T/K and an extrapolated μ0Hc2,c(0)≈280 T