BCS (1 Viewer)

[screwuhsc]

BCS theory
Atoms in a crystal lattice constantly vibrate. This is because they are
all connected and these vibrating atoms create waves throughout the
metal called phonons. The more the atoms are vibrating the larger the phonons. In superconductors (at low temperatures) the phonons are small, and any distortion caused by the electrons is reflected in phonons. These phonons can attract electrons to form cooper pairs.

According to the Barden-Cooper-Schrieffer theory, atoms of metals in the superconducting state will have electrons that lock and travel together throughout the lattice. This pair of electrons causes the lattice to distort as the electrons move unimpeded. This is because their strong negative charge will repel any positive atoms that it gets close to, thus no collisions occur and electron flow is not resisted. As explained by the BSC theory, when a superconductor is cooled below its critical temperature electrons begin to pair known as a cooper pair. A → Cooper pair consists of two electrons that are a considerable distance apart. The attractive force between the two electrons is provided by the exchange of
phonons (lattice vibrations).

 

[seventhroot]

feed back:
- write BCS; don't need to waste your time
- fairly unclear
- not really accurate
- too long

out of 5 maybe 2 or 3 being generous

 

[seventhroot]

the key points you should be talking about:

- how it related to superconductivity
- when a super conductor is at T_c; electrons passing through cause +ve ions to be attracted to it
- this distorts the lattice
- this electron now 'emits' a phonon which creates a +ve area behind it; which attracts another electron
- the 2 electrons form a cooper pair and they move unimpededly through the lattice

if there are a few more marks; maybe mention some applications, benefits, limitations, etc

 

[screwuhsc]

again really ahhaha

 

[screwuhsc]

cheers bro

 

[seventhroot]

it's a bit rusty but that's the gist of it

 

[QZP]

The BCS theory explains superconductivity using Copper pairs. For a superconductor below critical temperature, there is minimal lattice vibrations to allow the formation of cooper pairs. An unimpeded electron influenced by an electric field flows through the crystal, distorting the metal cation lattice via electrostatic attraction and thereby forming phonons. A second electron utilises the energy of the phonons to overcome the electrostatic repulsion of the 1st electron, forming a cooper pair. Thus, the 1st electron guides the 2nd electron along an unimpeded path through the crystal lattice as they continually exchanging phonons. Hence, electric current is able to flow with zero resistance (superconductivity). The BCS theory is limited as it is only able to explain type 1 superconductors but not type 2 superconductors (whose critical temperature is predicted not to allow the formation of cooper pairs).

My understanding of phonons is incomplete sorry. I believe the HSC doesn't require you to know about it (I've seen an alternate layman explanation where 1st electron distorts metal cation lattice to create a dense area of positive charge which attracts and thus pulls along a second electron)

 

[photastic]

the bcs theory explains superconductivity using copper pairs. For a superconductor below critical temperature, there is minimal lattice vibrations to allow the formation of cooper pairs. An unimpeded electron influenced by an electric field flows through the crystal, distorting the metal cation lattice via electrostatic attraction and thereby forming phonons. A second electron utilises the energy of the phonons to overcome the electrostatic repulsion of the 1st electron, forming a cooper pair. Thus, the 1st electron guides the 2nd electron along an unimpeded path through the crystal lattice as they continually exchanging phonons. Hence, electric current is able to flow with zero resistance (superconductivity). The bcs theory is limited as it is only able to explain type 1 superconductors but not type 2 superconductors (whose critical temperature is predicted not to allow the formation of cooper pairs).

My understanding of phonons is incomplete sorry. I believe the hsc doesn't require you to know about it (i've seen an alternate layman explanation where 1st electron distorts metal cation lattice to create a dense area of positive charge which attracts and thus pulls along a second electron)

copper pairs ftw.

 

[seventhroot]

tbh; tl;dr

you won't have time to write that in an exam

copper pairs ftw.

lel

 

[QZP]

True. Does anyone know what the HSC wants?

 

[QZP]

copper pairs ftw.

Hahahaha that was funny Did so much chem today

 

[seventhroot]

True. Does anyone know what the HSC wants?

a nice clear succinct answer

 

[QZP]

I mean in terms of knowledge (phonons or incorrect BCS theory which is the dense positive charge one)

 

[Fade1233]

I mean in terms of knowledge (phonons or incorrect BCS theory which is the dense positive charge one)

No. Dont need.

 

[QZP]

No. Dont need.

No need to know BCS theory? Hooray -_-

 

[Fade1233]

No need to know BCS theory? Hooray -_-

I meant dont delve much into phonon interactions.

 

[purplemoon]

lame question, but I've read that phonons are the actual vibrations of the lattice, and I've also read that they're quanta released by the electron...could someone please clarify the concept of a phonon and which ones correct?

 

[Fizzy_Cyst]

Most recent HSC exam which asked about BCS -- no need for phonons (I think 2012?) whereas earlier exam it was in the marking scheme (I think ~2005) as our understanding of super conductivity has changed (flux pinning / quantum trapping) in the interim.

So, u would expect that you would not need to mention it in your responses, but I would anyway, just in case they revert back to wanting it in again! I really doubt that a question relating to BCS will be in there though, simply due to the recent changes in understanding.

 

[Fizzy_Cyst]

lame question, but I've read that phonons are the actual vibrations of the lattice, and I've also read that they're quanta released by the electron...could someone please clarify the concept of a phonon and which ones correct?

Easy way to think of it -- at equilibrium, ions in lattice are all arranged uniformly, electron comes through, causes lattice to distort (thus changing the energy in the lattice) -- this energy must come from somewhere -- it arises due to the electron releasing a phonon which is then transferred to the lattice. The phonon enables the vibration of the lattice, which then attracts a second electron (second electron 'absorbs' the phonon) and due to it being absorbed, the lattice relaxes back to equilibrium

 

[purplemoon]

Thank you so much! That actually makes sense