Superconducivity is a phenomenon observed in elements and alloys by decreasing the temperature as low as some Kelvin. Below this temperature (called superconducting transition temperature, Tc) the systems transitions to a state with completely different physical properties such as electrical resistivity, magnetic and thermodynamic properties relative to the normal one (above Tc). This state has been explained to be a quantum macroscopic state of electron-pairs created from the attraction between the electrons due to the electron-phonon interaction (BCS theory, Noble prize in Physics 1972). The very low resistivity (unmeasurable and near absolute zero) and quantum tunneling of the pairs have been produced lots of applications of the superconducting materials in different parts of industry and medical instruments. Due to the low Tc and critical magnetic filed these applications have been limited to be commercialized well.
In 1986, a new kind of these materials, known as high-Tc superconductors, has been discovered. In opposite to the usual low temperature superconductors, these materials are Mott insulator in their ground states and by hole (or electron) doping becomes superconductor below the Tc, which is considerably high (more than 100 K) relative to the usual already superconductors (some K). The properties of these materials in both normal and superconducting sates are strongly different with the previous superconductors and other usual metal, known as Fermi-liquid metals. High Tc, the pseudogap in the normal state, antiferromagnetic ground state in the undoped system, d-wave asymmetry of the superconducting gap, charge and spin order called as stripes, strong microscopic inhomogeneities, strong electron-phonon interaction, strong electronic correlation, non Fermi liquid behaviors of the normal state properties, sign reversal of Hall coefficient, etc are among them.
The large varieties of the compounds which show this state and the unexpected behaviors have been produced difficulties to extend the BCS like mechanism for these materials even by very intense studies in the last two decades. However, fast evolving researches in this filed has produced lots of new concepts and developments in the physics of materials in the solid state. The research is continuing to discover other unexpected properties of these systems and to develop a theory to explain this state.
For more information and details see the following websites:
