Abstract

In recent years, there has been a tremendous expansion in our knowledge of the classes of materials which exhibit what is generally called ‘metallic behavior’ and in the subclasses of these which exhibit superconductivity. In fact, it is not a trivial matter to define what constitutes a ‘metal,’ given that a number of organic compounds with structures which contain various low-dimension features, including doped C₆₀ fullerenes, a wide variety of intermetallic compounds, many ceramic oxides and even suitably doped liquid ammonia are now considered to be metallic in character and even normally gaseous elements such as iodine and hydrogen have been made to become metallic at high pressures. A working definition of ‘metallic behavior’ is that the electrical resistivity (or at least one element of the resistivity tensor in anisotropic materials) remains finite, or falls to zero in the case of superconductors, as the temperature approaches absolute zero, as shown in Figure A2.3.1.