The Two Types of Superconductors

Type 1

    Type I superconductors consist of only one element, either a metal or metalloid.  These superconductors typically have the lowest critical temperatures for which they reach their superconducting state.  Elemental carbon has the highest critical temperature at 15 K.  Note that this is true only when carbon is configured in a single wall nano-tube.  Diamond and graphite does not exhibit superconductivity .  Type I superconductors usually exhibit slight conductivity at room temperature but have a rather sharp switch to the superconductivity state when brought to their critical temperature.  It is surprising to note that three of the best conductors known at room temperature, copper, silver, and gold, have not been found to have any superconducting properties. This is because a superconductor must have a strong acoustic coupling between the electrons and the lattice. Good normal conductors by contrast are good conductors at normal temperatures because they have a weak interaction between the electrons and the lattice. This weak interaction accounts mainly for their lack of superconductivity. 


     Many additional elements can be coaxed into a superconductive state with the application of high pressure. For example, lithium is the Type 1 element with the highest Tc. But, it requires compression pressures of 300,000 atmospheres to become superconductive and 480,000 atmospheres to reach a Tc of 20 K. The above list is for elements at normal (ambient) atmospheric pressure. See the periodic table below for all known elemental superconductors

    To reach the state of superconductivity, a metal or alloy must have an average of 2-8 valence electrons. Transition metals average 3, 4·7, or 6·4 valence electrons per atom seem particularly favorable. 2, 4 or 5·6 is unfavorable. It is also known that superconducting elements have relatively small atomic values.

Type II

Type II superconductors are, for the most part, comprised of metallic compounds and alloys.  This class of superconductors generally has a much higher critical temperature than those in Type I. They achieve a higher critical temperature than Type 1 superconductors by a mechanism that is still not completely understood. It is believed that it relates to the planar layering within the crystalline structure. The highest critical temperature  reached is currently 138 K. Debates still arise as to whether or not an upper limit exists for a critical temperature to be found.

Title Page
Introduction to Superconductors
Basic Conditions
The Resistance in a Superconductor
The Two Types of Superconductors
Applications of Superconductors