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Study of the Influence of Impurities on the Properties of High-Tc Superconductors

Objective

This Action aimed to understand what role impurities play in superconducting materials. When the impurities act as "vortex pinning centres", the critical current of the superconducting material can be enhanced. To do so, a thorough theoretical understanding of the effects of impurities needs to be developed.
The effects of magnetic and nonmagnetic impurities on the properties of high temperature superconductors were studied. Impurities were introduced into bulk samples, single crystals and 20 to 200 um thick films.

Monophasic zinc and iron substituted yttrium barium copper oxide bulk material has been synthesized withless than 0.1% impurity phases. Structural and microscopic characterization have been made on this material and intentionally prepared multiphase material. Solubility limits, new phases, twin and tweed structures have been studied in relation to the substitution.

Point contact and tunnelling investigations of the gap have been made together with yttrium barium copper oxide (IR) studies allowing gap and temperature dependence studies.

Nuclear magnetic resonance (NMR) studies on zinc substituted yttrium barium copper oxide indicate that zinc impurities depress superconductivity without major modification of the hole doping and of the temperature dependence of the homogenous susceptibility, which is therefore not a direct manifestation of superconductivity. Local moments induced by zinc are evidenced and might play a role in the pair breaking phenomenon. This is correlated by our by our latest results on transport properties, Hall effect, magnetoresistance and zero field resistivity studied in relation to the zinc impurity substitution.

By inelastic neutron diffraction studies in the iron doped bismuth calcium copper oxide compounds, the coexistence of superconductivity and magentizm has been observed together with the appearance of a gap in the magnetic excitations spectrum.
APPROACH AND METHODS
The effect of magnetic and non-magnetic impurities was studied in various samples of polycrystalline material (prepared by powder and sol gel techniques) and single crystals (prepared by flux methods).
The modification of the crystal phases, stoichiometry, and the appearance of new superconducting phases were to be measured and correlated with the incorporation of impurities. Depending on their position in the superconducting phase, state of ionisation and electronic interaction with other elements present in the structure, measurements of fundamental superconducting properties can convey important information for improving the theoretical understanding of the underlying mechanisms of high-temperature superconductivity.
The samples were characterised with respect to:
-physico-chemical properties (stoichiometry, crystalline and microstructural characterisation) using various characterisation techniques
-superconducting properties in relation to electrical resistivity and critical current, FIR absorption, and nuclear magnetic resonance (NMR) absorption and relaxation studies.
PROGRESS AND RESULTS
Monophasic Zn and Fe substituted YBaCuO bulk material has been synthesized with less than 0.1% impurity phases. Structural and microscopic characterization have been made on this material and intentionally prepared multiphase material. Solubility limits, new phases, twin and tweed structures have been studied in relation to the substitution.
Point contact and tunnelling investigations of the gap have been made together with IR studies allowing gap and its temperature dependence studies.
NMR studies on Zn substituted YBCO indicate that Zn impurities depress superconductivity without major modification of the hole doping and of the T dependence og the homogeneous susceptibility, which is therefore not a direct manifestation of superconduct ivity. Local moments induced by Zn are evidenced and might play a role in the pair breaking phenomenon. This is correlated by our latest results on transport properties, Hall effect, magnetoresistance and zero field resistivity studied in relation to the Zinc impurity substitution.
By inelastic neutron diffraction studies in the Fe doped BiCaCuO compounds, the coexistence of superconductivity and magnetism has been observed together with the appearance of a gap in the magnetic excitations spectrum.
POTENTIAL
This Action is leading to the theoretical understanding of the role of impurities or clusters of impurities on the basic properties of high Tc suprconductors, and also of their role on the current transport and the mechanisms of dissipation. This knowledge will be used in the preparation of high critical current superconducting material.
Very good collaboration between the groups has developed, 10 final year student projects being carried out involving contacts between the three groups. In addition, two PhD theses have been carried out and two more are in preparation.

Coordinator

VRIJE UNIVERSITEIT BRUSSEL
Address
Bd. Du Triomphe, Campus Plaine Ulb, 210, 212
1050 Bruxelles
Belgium

Participants (3)

MAX-PLANCK-INSTITUT FUER FESTKOERPERFORSCHUNG
France
Address
25 Avenue Des Martyrs
38042 Grenoble
Université de Paris XI (Université Paris-Sud)
France
Address
Bâtiment 510 Centre Université D'orsay
91405 Orsay
VRIJE UNIVERSITEIT BRUSSEL
Belgium
Address
Avenue F.d. Roosevelt, 50, 122
1050 Bruxelles