Work Area: High-Tc Superconductivity
Keywords high Tc superconductivity
Start Date: 1 July 92 / Duration: 24 months / Status: running
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Abstract HTSC aims to determine the local structure of samples of high Tc superconductors. The aim is the establishment of meaningful relationships between the local structure, the synthesis procedures, and the fundamental physical properties. The ultimate goal will be to attain control of the local structure and to produce samples with a given local structure, which will allow meaningful physical measurements. The emphasis will be on the identification of flux pinning centres.
The scientific goal of the Working Group is the determination of the local structure of Cu oxide high Tc superconductors. Samples of Cu oxide superconductors contain superstructures, microwins, non stoichiometry in the anion sublattice, cation ordering, point and extended defects, and intergrowth.With such samples, if we limit ourselves to Bragg scattering, we can only determine the average structure. In order to approximate the local arrangement we are: (i) using more than one radiation (X-ray, neutrons, electrons); (ii) taking into account the diffuse scattering; (iii) carrying out high resolution electron microscopy; (iv) using absorption techniques (EXAFS and XANES).
Within the framework of the WG two workshops have been organised, the first at El Escorial (Spain) on 16-17 October 1992 and the second at Port Hydra (Greece) on 7-8 May 1993. The WG laboratories have access, via collaborations with Darmstadt and Ris_, to the large installations of a synchroton light source (Hasylab) and neutron scattering facility (Ris_). A round table about the future of the WG was held at the end of the meeting.
At Port Hydra the workshop comprised ten lectures given by members of the WG laboratories and eighteen posters. Dr. A. Bianconi (University of Rome) spoke about the problems of the local structure in Bi2Sr2CaCu2O8+x. Dr. D. Jefferson (Cambridge University) illustrated the microstructural irregularities in various oxide superconductors, and Dr. J.P. Locquet (IBM Zürich) spoke about a new growth mechanism of thin films of high Tc superconductors.
Some of the most important contributions made by the Working Group are:
The new oxide high Tc superconductors obtained by replacing Cu polyhedra by CO3-2, SO4-2, NO3-1 and BO3-3 groupings open up the possibility of changing the distances between the CuO2 superconducting layers on an ad hoc basis. The replaced Cu polyhedra are those which do not participate directly in the superconducting mechanisms.
The new Hg-bearings series, HgBa2Ca1-nCunO2n+2+d , is also very promising because its members exhibit the highest Tc's. For example, for n = 1, 2, and 3, Tc (onset) is 94, 126, and 134 K, respectively.
The technique of relating the twin structure of 123 samples with the critical current density, developed by the Darmstadt group, also has good potential.
The research on high Tc superconductivity carried out within the framework of the Working Group is published in international journals such as Physica C, Nature, Journal of Solid State Chemistry, Physical Review, etc. and presented at international meetings, where very often the members of the WG are invited speakers.
CNRS - F
Laboratoire de Cristallographie
F - 38042 GRENOBLE CEDEX 09
RUCA Antwerpen - B
ETH Zürich - CH
Technische Hochschule Darmstadt - D
Ris_ National Laboratory - DK
Università Complutense Madrid - E
CRISMAT CAEN - F
NCSR Demokritos - GR
Università di Parma - I
Trinity College Dublin - IRL
Universiteit Delft - NL
Istituto Superior Tecnico - P
University of Birmingham - UK
Dr. M. Marezio
tel +33/76 88 10 40
fax +33/76 88 10 38
HTSC - 6885, August 1994
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html version of synopsis by Nick Cook