|
Contents:
Main page
Introduction
Historical
Background
Zero
electrical resistance
Superconducting
phase transition
Meissner
effect
Temperature
measurements
Glossary
|
|
In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature Tc. The value of this critical temperature varies from material to material. Conventional superconductors usually have critical temperatures ranging from less than 1K to around 20K. Solid mercury, for example, has a critical temperature of 4.2K. As of 2001, the highest critical temperature found for a conventional superconductor is 39K for magnesium boride (MgB2), although this material displays enough exotic properties that there is doubt about classifying it as a "conventional" superconductor. Cuprate superconductors can have much higher critical temperatures: YBa2Cu3O7, one of the first cuprate superconductors to be discovered, has a critical temperature of 92K, and mercury-based cuprates have been found with critical temperatures in excess of 130K. The explanation for these high critical temperatures remains unknown.(Electron pairing due to phonon exchanges explains superconductivity in conventional superconductors, while it does not explain superconductivity in the newer superconductors that have a very high Tc)
The onset of superconductivity is accompanied by abrupt changes in various physical properties, which is the hallmark of a phase transition. For example, the electronic heat capacity is proportional to the temperature in the normal (non-superconducting) regime. At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to be linear. At low temperatures, it varies instead as e-a/T for some constant a. (This exponential behavior is one of the pieces of evidence for the existence of the energy gap.)
Behavior of heat capacity (C) and resistivity (p) at the superconducting phase transition
The order of the superconducting phase transition is still a matter of debate. It had long been thought that the transition is second-order, meaning there is no latent heat. However, recent calculations have suggested that it may actually be weakly first-order due to the effect of long-range fluctuations in the electromagnetic field
|