High temperature superconductors
In less than ten years' time, the scientific community will celebrate the hundredth anniversary of the discovery of superconductivity - the observation of the resistanceless flow of electrical current through mercury cooled to very low temperatures. It took decades to develop a successful theory describing 'conventional' superconductivity in simple metals. Discovery of High Temperature Superconductivity (HTSC) in cuprates 15 years ago was not predicted by this theory, implying that a completely novel approach is needed to explain the nature of the phenomenon. The enormous technological potential of the HTSC materials - which now start to find real-life applications in the fields of energy and telecommunication - has initiated extensive efforts by experimentalists and theorists alike to understand what exactly makes cuprates so special. Despite this unparalleled research effort, the high Tc superconductors have kept their secrets guarded well into the new millenium.
The most important structural components of the typical cuprate Bi2Sr2CaCu2O8+d are the two CuO2 planes separated by a layer of Ca atoms as it's in these planes that the superconducting current is believed to flow without experiencing any resistance from the crystal lattice below a certain critical temperature (Tc). The number of such CuO2 layers varies between different cuprates and Tc is proportional to this number (as long as one doesn't have more than three). Layers of Sr-O and Bi-O are not directly involved in carrying the superconducting current and act as a charge reservoir for the transport layers.
How can the electrons move through this lattice without being scattered by the numerous atoms ? In the case of cuprates - nobody knows the answer. It is known that, as in conventional superconductors, the electrons cooperate with each other to form pairs in order to overcome the resistance. There formation of such pairs is driven by the interaction between electrons and phonons (lattice vibrations). In the high-Tc cuprates, there exists no consensus as to what is able to overcome the natural dislike the electrons have for each other (both are negatively charged and would otherwise repel each other strongly): the driving force behind the pairing is of yet unknown or controversial origin.IFF / Electronic and optical Properties || Last update 5. December 2006 || Author: S. Borisenko