H. Savary

Strong decrease of anisotropy by lead-substitution in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/

The in-plane ( rho /sub ab/) and out-of-plane ( rho /sub c/)/sub c/ resistivities were measured above the critical temperature, T/sub c/, in several lead-substituted single crystals of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ and compared to the results obtained for lead-free samples. It was found that the lead substitution induces an important decrease of rho /sub c/, resulting in a lowering of the anisotropy rho /sub c// rho /sub ab/ by two orders of magnitude near T/sub c/. Moreover, rho /sub c/ changes its behavior from a semiconducting type of behavior to a metallic one. These results are related to the role of lead, which was recently inferred from structural considerations. The possibility of improving the out-of-plane critical current by lead substitution is discussed.<<ETX>>

Resistivity anisotropy in lead-substituted Bi2Sr2CaCu2O8+y

We show that lead-substitution in single crystals of Bi2Sr2CaCu2O8+y induces a decrease of both the structural and resistivity anisotropies. In the first place we have found that lead substitution can reduce the resistivity anisotropy ρcρab by two orders of magnitude and that ρc changes its behaviour from a semiconductor-like to metallic-like. In the second place, the existence of a threshold value in the amount of lead-substitution, above which iodine intercalation is no longer possible is consistent with the occurence of a strengthening of the binding force between adjacent Bi-O planes.

Influence of lead substitution on the properties of BiSrCaCuO superconducting ceramics and crystals

Abstract We have examined systematically the influence of substitution of lead to bismuth in Bi2Sr2CanCun+1O2n+6, (2 2 n n+1), superconductors. The stabilisation of the (2223) phase is greatly promoted for substitution between 0.5 and 0.7 Bi in the above formula, and virtually single phase ceramics have been obtained, with zero resistivity at T=108K. In (2212) single crystals, we have observed an intermodulation of the incommensurate wave already present in the lead free compounds and the wave specific of lead doping. The wavelength of the latter modulation increases with the lead content. These results are discussed with respect to the increase of the strengh of the bonding between Bi-O layers, present in the structure of these materials, induced by the lead substitution.

Spin susceptibility behaviour in the Bi2Sr2Can−1CunO2n+4 compounds : from underdoped to overdoped regime

Abstract We have studied the thermal behaviour of the spin susceptibility, χ spin (T), of the CuO 2 planes in the normal phase. χ spin (T) was measured by 17 O and 43 Ca NMR in various compounds of the Bi 2 Sr 2 Ca n −1 Cu n O 2 n +4 family : a n=3 sample almost optimally doped, an overdoped n=2 sample and a more strongly doped n=1 sample. By contrast to the underdoped case, in the overdoped one, χ spin (T) increases when cooling. Furthermore, this behaviour is enhanced by increasing the doping. χ spin (T) is compared to the macroscopic susceptibility behaviour. On the other hand, our results are consistent with a decrease of the number of charge carriers per copper plane for increasing n.

Influence of lead substitution on the magnetic irreversibility line of 2212 superconducting single crystals

Abstract An experimental study of the magnetic irreversibility lines has been carried out on superconducting single crystals of Bi 2− x Pb x Sr 2 CaCuO 2 O 8 , with and without lead. The irreversibility field values B irr ( T ) are almost identical for x = 0 and x = 0.4. However, the power-law fits of B irr ( T )∼1/ T α at low temperatures provide different exponents α in both cases, α = 6 in the lead-free sample and α = 4 in the lead-substituted sample. At high temperatures a regime change is observed in both samples and B irr vanishes as ( T c − T ) β with β = 1 for x = 0 and β = 0.37 for x = 0.4. A discussion of these results is achieved within the TAFF and vortex solid melting models taking into account a possible “dimensional crossover” of the vortex system behavior.

Insulating materials for BiSrCaCuO-multilayers

Abstract We have deposited, by an in-situ laser ablation technique, multilayers composed of thin films of the (2212) bismuth cuprate phase alternating with films of either of two insulating solid solutions Sr 14−x Ca x Cu 24 O 41 and Ca x Sr 2−x CuO 3 . The chemical and structural compatibility of these compounds with the superconductor and their interest are discussed.

17O NMR investigation of the properties of the various types of CuO2 planes in (Bi,Pb)2Sr2Can−1CunOy

Abstract In the normal state of the (2223) phase of bismuth cuprates the 17 O Knight shift of the CuO 2 planes with pyramidal coordination for copper is nearly temperature independent except in the vicinity of Tc where it presents a steep decrease. The 17 O Knight shift of the CuO 2 planes with square plane coordination for copper is significantly lower and presents a continuous decrease with decreasing temperature which is already detected just below room-temperature. We discuss. these results on the basis of a recent theoretical prediction for the existence of an inhomogeneous charge carriers distribution among the two types of CuO 2 planes, and also on the basis of similar available data, for other high Tc superconductors.

Lead substitution effects on the hysteresis curves and magnetic irreversibility lines 2212 compounds

Abstract We discuss the nature of the magnetic irreversibility line for the Bi:2212 compounds with two different lead contents. A crossover is observed in both cases at two different temperatures. This cross-over is consistent with the dimensional change of the vortex ensemble predicted by Glazman and Koshelev. This last approach provides two values of the anisotropy ratio in full agreement with the anisotropy decrease by Lead. At the crossing of the IL, no rough change of the magnetization relaxation regime is observed.

Investigation of the CuO2 planes in the normal state of the 110K superconductor (Bi, Pb)2Sr2Ca2Cu3Oy by 170 NMR

Abstract 17O NMR spectra are obtained in a powdered sample of (Bi, Pb)2Sr2Ca2Cu3Oy, and resonance lines from oxygen sites in the two unequivalent CuO2 planes are clearly distinguished. The results presented here concerning the temperature dependence of the corresponding Knight shifts, i.e., of the local spin susceptibility of the two types of CuO2 planes, are consistent with the existence of an inhomogeneous charge carriers distribution among the two types of CuO2 planes.

Growth and characterization of YBa2Cu3O7-δ single crystals

Abstract We specify the growth conditions of single crystals of YBa2Cu3O7-x of dimensions up to 2×1.5×0.1mm3. These crystals display systematically the orthorhombic twinning whose features are investigated by means of polarized light reflected by the c-surface of the samples. Raman scattering measurements partly confirm the available ceramic data, and reveal additional lines in the spectrum of these substances.