SEM 2 PT 4

Question 1

Until when do Type 1 superconductors magnetic field remain zero until

Answer 1

critical field when superconductivity is destroyed

Question 2

Until when do Type 2 superconductors magnetic field remain zero until

Answer 2

Lower critical fields

Question 3

Are superconductors and ferromagnets compatable

Answer 3

no beacuse adding only a
few percent of magnetic impurities to non-magnetic metals usually destroys superconductivity.

Question 4

Do the best room temperature conductors or metals with high room temperature resistivity become better superconductors

Answer 4

metals with high room temperature resistivity

Question 5

What is the superconducting critical current density

Answer 5

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

Question 6

What are Type 1 superconductors

Answer 6

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 (𝐻𝐶), they show perfect diamagnetism.

Question 7

What are Type 2 superconductors

Answer 7

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 (𝐻𝐶) and (𝐻𝐶2):
Below 𝐻𝐶1: Perfect diamagnetism.
Between 𝐻𝐶1 and HC2
​ : Mixed state where magnetic flux penetrates in small regions, forming vortices of normal (non-superconducting) material.

Question 8

What are superconductors

Answer 8

Materials that can conduct electricity without resistance below a certain temperature

Question 9

What is meissner effect

Answer 9

Magnetic fields are expelled from inside of a superconductir

Question 10

What are supercurrents
Whats the point

Answer 10

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

Question 11

What is penetration depth

Answer 11

distance of which the magnetic fields and supercurrent decrease

Question 12

Why are superconductors a perfect diamagnetism

Answer 12

there is no magnetic field becomes surfece supercirrents perfectly cancels it - creating perfect diamagnetism

Question 13

What is BCS Theoru

Answer 13

how electrons form pairs (called Cooper pairs) and how these pairs enable superconductivity.

Question 14

What is phonon mediated attraction

Answer 14

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

Question 15

Properties of cooper pairs

Answer 15

• Opposite momenta
• Opposite spins
• Zero total angular momentum

Question 16

What is a BCS collective ground state

Answer 16

• All cooper pairs condense to single quantum state
• Paira behave like composure bison

Question 17

What must be true for cooper paits to be scattered and broken

Answer 17

the energy gained by breaking the pair must be at least 2Δ, where
Δ is the binding energy of the Cooper pair.

Question 18

How density of electrons / cooper pairs are affected

Answer 18

The density of normal electrons decreases.
The density of Cooper pairs increases.

Question 19

What is Coherence Length (𝜉)

Answer 19

The coherence length (
𝜉
ξ) is the shortest distance over which the density of Cooper pairs (or the supercurrent density) can change.

Question 20

Why does the coherence length matter

Answer 20

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.

Question 21

Fundamental difference between type 1 and 2 superconfuctors

Answer 21

superconductors is that for type I
superconductors 𝜆 < 𝜉 and for type II superconductors 𝜆 > �

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