SEM 2 PT 4 Flashcards
Until when do Type 1 superconductors magnetic field remain zero until
critical field when superconductivity is destroyed
Until when do Type 2 superconductors magnetic field remain zero until
Lower critical fields
Are superconductors and ferromagnets compatable
no beacuse adding only a
few percent of magnetic impurities to non-magnetic metals usually destroys superconductivity.
Do the best room temperature conductors or metals with high room temperature resistivity become better superconductors
metals with high room temperature resistivity
What is the superconducting critical current density
As the
current density through a superconductor is
increased the critical temperature TC is reduced and
for a given temperature T below TC there is a critical
current density, jC (T) above which superconductivity
is destroyed. T
What are Type 1 superconductors
Exoel all flux form within the bulk material up to critical applied magnetic field above which superconductivity is destroyed
Below a certain critical magnetic field (π»πΆ), they show perfect diamagnetism.
What are Type 2 superconductors
Magnetic field in bulk of supercondutor remains 0 until a lower critical field.
Above which magnetic flix penetrated through a region of material which becomes normal leaving other regions in superconducting state
Have two critical fields (π»πΆ) and (π»πΆ2):
Below π»πΆ1: Perfect diamagnetism.
Between π»πΆ1 and HC2
β : Mixed state where magnetic flux penetrates in small regions, forming vortices of normal (non-superconducting) material.
What are superconductors
Materials that can conduct electricity without resistance below a certain temperature
What is meissner effect
Magnetic fields are expelled from inside of a superconductir
What are supercurrents
Whats the point
Supercondictors cannot suddenlt drop tp 0 so supercurrents form at surface
These generate magnetic fields that cancels external firkd so no magnetic field penetrated deep in the superconductor
What is penetration depth
distance of which the magnetic fields and supercurrent decrease
Why are superconductors a perfect diamagnetism
there is no magnetic field becomes surfece supercirrents perfectly cancels it - creating perfect diamagnetism
What is BCS Theoru
how electrons form pairs (called Cooper pairs) and how these pairs enable superconductivity.
What is phonon mediated attraction
Electron moves through metal lattice
Distorts lattice
Creates localised positive charge due to attraction between electron and +ve ion
Distortion moves slower compared to electrons movement
Elctron can be attracted to +ve distortion - creates indirect attrition mediated by lattice - describes electron emitting a phonon
Properties of cooper pairs
- Opposite momenta
- Opposite spins
- Zero total angular momentum
What is a BCS collective ground state
- All cooper pairs condense to single quantum state
- Paira behave like composure bison
What must be true for cooper paits to be scattered and broken
the energy gained by breaking the pair must be at least 2Ξ, where
Ξ is the binding energy of the Cooper pair.
How density of electrons / cooper pairs are affected
The density of normal electrons decreases.
The density of Cooper pairs increases.
What is Coherence Length (π)
The coherence length (
π
ΞΎ) is the shortest distance over which the density of Cooper pairs (or the supercurrent density) can change.
Why does the coherence length matter
The coherence length ensures that the transition from normal electrons to Cooper pairs is smooth and gradual over this distance.
avoiding abrupt changes that are physically unrealistic.
Fundamental difference between type 1 and 2 superconfuctors
superconductors is that for type I
superconductors π < π and for type II superconductors π > οΏ½
For type I superconductors this leads
to the surface energy of the superconductor in an applied field being positive
For type II
superconductors the surface energy is negative
