Hg Cuprate Superconductors Technological and Conceptual Tools

Doctoral thesis, 1996

This work concerns superconductivity in the cuprates. It touches some of the problems that this field displays. As a representing case, the Hg based cuprates are chosen. The first part presents an experimental work related to the newly discovered mercury compounds with their record Tc and the remarkable increase of Tc under pressure which they show. The structural changes under pressure in these compounds were studied using synchrotron radiation and a diamond anvil cell. A reversible phase transition was discovered and the structural changes are discussed in relation to the ionic interactions in the system and its electronic structure. The second part is a summary of experimental work related to thin film deposition and single crystal growth, of the Hg based compounds. A record Tc of 125 K for superconducting films was achieved by means of multilayer deposition and ex-situ reaction in vacuum. High pressure synthesis using a new gas pressure system is reported. Liquid phase epitaxy was tried successfully. Synthesis of new compounds was obtained. Crystalline samples show monophasic structure and Tc of 133 K. The third part concerns the electronic structure of high temperature superconductors. The local and static properties of the cuprates are studied by the use of a cluster model. Dynamical and non dynamical correlations are taken into account explicitly. A new singlet state is identified and the nature of its coupling to the buffer ions is studied. The stability of that pairing as function of the chemical composition of the cluster is studied. Aspects of the cuprate phase diagram are treated. Paper 9 presents a possible treatment of the Fröhlich Hamiltonian through the two fluid model. The dispersion of the quasi particle spectrum at the Fermi level is identified as an important property. Paper 10 suggests a simplified understanding of BCS superconductivity by means of a standing waves representation.