Question

Define the critical temperature. Slow that in the superconducting state the material is perfectly diamagnetic. 

Answer 1

The critical temperature is the transition temperature at which the normal state of a conductor changes to superconducting state.
A superconducting material kept in a magnetic field expels the magnetic flux out its body when cooled below the critical temperature and exhibits perfect diamagnetism. This is called MEISSNER EFFECT.

⦁ It is found that as the temperature of the specimen is lowered to T_c, the magnetic flux is suddenly and completely expelled from it. The flux expulsion continues for T < T_c. The
effect is reversible. 

⦁ When the temperature is raised from below T_c . The flux density penetrates the specimen again at T = T_c and the material turns to the normal state.
⦁ For the normal state the magnetic induction inside the specimen is given by:

B = μ_o (H+M) = μO^(1+χ) H………………………………………..(1)

Here H is the applied magnetic field , m is the magnetization produced within the specimen , is χ the susceptibility of the material and μ_o  is the permeability of free space.

At T<T_c as seen above

B = 0 

Hence  equation (1) reduced to,

M = -H

And thus χ = `M/H `=-1

⦁ The specimen is therefore a perfect diamagnetic. The diamagnetism produces strong repulsion to the external magnets.
⦁ This effect is used to identify a superconductor , in levitation effect and suspension effect.