Superconducting Transition Temperature

The temperature at which a superconductor loss electrical resistance is called its superconducting “transition temperature” or “critical temperature” and written as Tc, is different for each metal (Table 2.1). The transition is so sudden and complete that appears to different phase of matter. Above a critical temperature Tc the properties of metal are completely normal; below Tc superconducting properties are displayed, the most dramatic of which is the absence of any measurable DC electrical resistance[23].


In general the transition temperature is not very sensitive to small amounts of impurity, but the superconductivity of a few metals, such as iridium and molybdenum, which in the pure state have very low transition temperature, may be destroyed by presence of minute quantities of magnetic impurities. Such elements, therefore, only exhibit superconductivity if they are extremely pure, and specimens of these metals of normal commercial purity are not superconductors. Not all pure metals have been found to be superconductors; for example copper, iron and sodium have not shown superconductivity down to the lowest temperature to which they have so far been cooled.


Table 2.1 Values of Tc and Hc for the superconducting elements


Superconductivity is not a rare phenomenon; about half the metallic elements are known to be superconductors and in addition a large number of alloys are superconductors. For an alloy it is possible to be a superconductor, even it is composed of two metals which are not themselves superconductors, such as Bi- Pd.

And in general some superconductor alloys have advantageous properties for applications about critical temperature. For example niobium is the metallic element with the highest transition temperature (9.3 K), but some alloys and metallic compounds remain superconducting up to even higher temperatures . For example Nb3Sn has a transition temperature of about 18 K

The eventuation of transition temperature of superconductors may be shown differences if the sample is pure or not. The transition to the superconducting state may be extremely sharp if the specimen is pure and physically perfect on cooling. For example in a good gallium specimen, the transition has been observed to occur within a temperature range 10-5 degrees. Adversely, if the specimen is impure or has a disturbed crystal structure the transition may be considerably broadened.

Figure 2.4 -The transition temperature for pure and impure superconductor metals