Lecture 14 Flashcards
What is a superconductor?
A conductor who’s measured resistance goes to exactly zero at a given critical temperature.
What are the 3 ways that semiconductors can be formed?
- In bulk at low temperature and at ambient pressure
- In bulk at high pressure
- In modified form
Give 2 applications of superconductors
- MRI scanners: can provide fields up to 18T
- Quantum computing: superconducting qubit
Are ferromagnets superconductors?
No because superconductors require electrons to pair up
Which metals cannot be used as superconductors?
Metals with the highest room temperature conductivities. They are good conductors but not superconductors.
Which metals can be used as high Curie temperature superconductors?
Metals with the lowest room temperature conductivities.
What does the Curie Temperature of elements depend on?
The isotopic mass (A) and the lattice properties of the element
Describe the change in resistance of a normal metal with temperature
It normally decreases as temperature is reduced because the number of phonons in reduced.
For a pure metal this resistance tends to zero as temperature tends to zero but for one with impurities it tends to a finite resistance.
How can a metal be tested to see if it is a superconductor?
- Establish a current in a loop of a ‘superconductor’.
- Measure the current very precisely by measuring the magnetic field produced using NMR.
- If there is no decay of current over time then it is a superconductor as it has no resistance.
What is the decay time of a superconductor?
100,000 years
What is the critical current density of a superconductor?
The current density at (and above) which superconductivity is destroyed.
What is the critical field of a superconductor?
The magnetic field at (and above) which superconductivity is destroyed.
What is the critical flux density of a superconductor?
The flux density at (and above) which superconductivity is destroyed.
Give the equation for the critical flux density of a superconductor
B_c = critical flux density
T = temperature
T_c = Curie temperature
What is the Meissner effect?
If a superconductor is cooled in a magnetic field, all magnetic flux is ejected from the interior of the superconductor at the critical temperature. This is true when the magnetic field is smaller than the critical field.
B = 0 inside the superconductor
Give the equation for the change in magnetic field of a perfect conductor (one where E = 0)
Describe how a perfect conductor reacts to being cooled below the Curie temperature whilst a magnetic field is applied
a = no initial field
b = applied field
When the perfect conductor is cooled below the Curie temperature whilst in an applied magnetic field, the field becomes trapped. within the conductor even if the external magnetic field is removed.
Describe how a conductor reacts to being cooled below the Curie temperature whilst a magnetic field is applied
c = no initial field
d = applied field
When a semiconductor is cooled below the Curie temperature whilst in an applied magnetic field, the field is expelled from inside the conductor so cannot be trapped when the external magnetic field is removed. This is a quantum effect
Give the equation for the magnetic field in the bulk of a superconductor
B = magnetic field
M = magnetism
Give the equation for the magnetic susceptibility of a superconductor
χ = magnetic susceptibility
M = magnetisation
Describe the graph of -µM against H for both type I and type II superconductors
Describe a crystal in a mixed/vortex state
Give the equation for the change in Gibbs free energy of a system due to an applied flux
dG = change in Gibbs free energy
M = magnetism
B = applied flux
Give the equation for the Gibbs free energy of a superconductor
G_s = Gibbs free energy
B = magnetic flux
Give the equation for the difference in Gibbs free energy between a superconducting state and a normal state
G_s = Gibbs free energy
B = magnetic flux = 0
B_c = critical flux
