Lecture 4 - Superconducting Materials

Question 1

What is superconductivity?

Answer 1

The phenomenon whereby in certain pure metals, alloys and compounds, the resistance to current flow sharply decreases to zero at a particular temperature.

Question 2

What is the mechanism behind superconductivity?

Answer 2

The creation of Cooper pairs.

Quantised thermal energy, phonons, lead to oscillation of an ion core in a material. This core attracts two electrons on either side which have opposite spin and would normally repel eachother. The resultant cooper pairs can’t be scattered as normal conduction electrons, leading to zero resistance.

Question 3

How does a magnetic field affect a type I superconductor?

Answer 3

An applied magnetic field tends to align both electrons in cooper pairs to spin in the same direction. Thus the superconductive effect is opposed to the point of its destruction.

Question 4

What is the Meissner effect?

Answer 4

Magnetic field lines are excluded from a superconductor below Tc and an applied field is expelled from the interior of a material if it is field cooled when it reaches Tc.

Question 5

What are type I and type II superconductors?

Answer 5

Type I superconductor:
They exhibit total field exclusion up to Hc, followed by suppression of superconductivity where perfect diamagnetism (χ=-1) is lost.

Type II superconductor:
They exhibit field exclusion and perfect diamagnetism up to Hc1 -Lower critical field.

Then between Hc1 and an upper critical field Hc2, magnetic flux B is able to penetrate the superconductor in the form of flux quanta Φο which are screened from the superconductive matrix by a current vortex (screening current).

Question 6

Things to look at in this chapter in detail?

Answer 6

The mixed state
The Bean model
Applications
Consequence of higher PM fields (>2T)