Phonon - seriously confused (1 Viewer)

[study-freak]

I know that phonons are quanta of vibrational energy, but what role does it play in the BCS theory?
I have seen Jacaranda Physics going on about exchange of phonons between electrons but I don't really get it.

Can someone explain me the role of phonon in BCS theory in a way that will be accepted in the HSC if I use it? (coz I know that most markers won't know the real physics)

Thanks a lot

 

[the-derivative]

I know that phonons are quanta of vibrational energy, but what role does it play in the BCS theory?
I have seen Jacaranda Physics going on about exchange of phonons between electrons but I don't really get it.

Can someone explain me the role of phonon in BCS theory in a way that will be accepted in the HSC if I use it? (coz I know that most markers won't know the real physics)

Thanks a lot

I'm a bit confused with this BCS crap as well. The way I explain it (I'm pretty sure I'm correct) is that as one negative charged electron passes by the positive charged ions in the lattice, it distorts. This causes phonons to be emitted from a trough of positive charge around the electron.

Therefore, as a result of the distortion, there is increased positive charge, therefore before the first electron passes by, a second electron is drawn into the tough and this allows for the formation of cooper pairs.

I'm not sure if I'm correct though, and if anyone has a better answer, please do share.

 

[Uncle]

lol there are no mention of Phonons in my Physics for Scientists and Engineers textbook.

 

[study-freak]

I'm a bit confused with this BCS crap as well. The way I explain it (I'm pretty sure I'm correct) is that as one negative charged electron passes by the positive charged ions in the lattice, it distorts. This causes phonons to be emitted from a trough of positive charge around the electron.

Therefore, as a result of the distortion, there is increased positive charge, therefore before the first electron passes by, a second electron is drawn into the tough and this allows for the formation of cooper pairs.

I'm not sure if I'm correct though, and if anyone has a better answer, please do share.

Then what happens to the phonon?

 

[the-derivative]

Then what happens to the phonon?

From my understanding, as phonons are emitted, they continue to vibrate. However, it is through the interactions of phonons that bind together the electrons allowing for cooper pairs to stay together.

I don't even think you need much on phonons. To be honest, the BCS theory is a load of bullshit. The only reason why people care about it is because it's in the syllabus. =)

 

[study-freak]

From my understanding, as phonons are emitted, they continue to vibrate. However, it is through the interactions of phonons that bind together the electrons allowing for cooper pairs to stay together.

I don't even think you need much on phonons. To be honest, the BCS theory is a load of bullshit. The only reason why people care about it is because it's in the syllabus. =)

lol.. That's quite an unsatisfactory explanation though..

Thanks anyways

 

[untouchablecuz]

Discuss the BCS theory.
Þ[FONT=&quot] [/FONT]The BSC theory is a quantum mechanical explanation of superconductivity.
Þ[FONT=&quot] [/FONT]At sufficiently low temperatures, electrons moving through superconducting materials induce vibrations, phonons, in the lattice of positive ions.
Þ[FONT=&quot] [/FONT]These phonons distort the lattice, moving the positive ions towards the electrons as they pass.
Þ[FONT=&quot] [/FONT]This distortion results in a momentary concentration of positive charge behind the electrons.
Þ[FONT=&quot] [/FONT]This positive region attracts a second electron, accelerating it towards the electron in front.
Þ[FONT=&quot] [/FONT]Whilst both the electrons are negatively charged, they attract with phonon-mediated attraction, travelling through the lattice as a Cooper pair.
Þ[FONT=&quot] [/FONT]As these pairs move through the lattice, phonons that are constantly emitted by the electron in front are absorbed by the second electron. This exchange continually distorts the lattice, maintaining electron attraction.
Þ[FONT=&quot] [/FONT]These Cooper pairs move unimpeded by the lattice, causing superconductivity.
Þ[FONT=&quot] [/FONT]As the temperature of the conducting material increases above its critical temperature, the lattice vibrations increase and diminish these Cooper pairs, causing a loss of superconductivity.


from my notes, does that help?

 

[annabackwards]

^
Thank you soooooooooooooooooooooo much. You are a lifesaver! If only i knew this before my assignment was due ><"

 

[untouchablecuz]

nooooooo worries habib

 

[study-freak]

Discuss the BCS theory.
ÞThe BSC theory is a quantum mechanical explanation of superconductivity.
ÞAt sufficiently low temperatures, electrons moving through superconducting materials induce vibrations, phonons, in the lattice of positive ions.
ÞThese phonons distort the lattice, moving the positive ions towards the electrons as they pass.
ÞThis distortion results in a momentary concentration of positive charge behind the electrons.
ÞThis positive region attracts a second electron, accelerating it towards the electron in front.
ÞWhilst both the electrons are negatively charged, they attract with phonon-mediated attraction, travelling through the lattice as a Cooper pair.
ÞAs these pairs move through the lattice, phonons that are constantly emitted by the electron in front are absorbed by the second electron. This exchange continually distorts the lattice, maintaining electron attraction.
ÞThese Cooper pairs move unimpeded by the lattice, causing superconductivity.
ÞAs the temperature of the conducting material increases above its critical temperature, the lattice vibrations increase and diminish these Cooper pairs, causing a loss of superconductivity.


from my notes, does that help?

Thanks, it helps, but exactly how is phonon absorbed by the 2nd electron?

 

[Al-Ghazali]

lol there are no mention of Phonons in my Physics for Scientists and Engineers textbook.

Thats b cos the BCS theory is not even true, Hsc teaches you shit

 

[Aerath]

Is it even necessary to discuss the phonon in the BCS theory?

 

[Bernie Mac]

The phonon is insignificant for the course actually, but the knowledge would help describing the BSC theory and the formation of Cooper pairs.

Anyways the phonon is actually a positively charged trough which is formed due to the crystal lattice bending in (Let the crystal lattice be the positive charge), due to electron 1. This trough forms behind electron 1 because negative attracts positive charges.
Electron 2, behind electron 1 comes along and is attracted to the positive trough and therefore forms a bond. This is a Cooper Pair

So phonon is actually a positive trough, or field.
This is my understanding from the Jacaranda text.
Do most of you have to do your own learning? Cos we have to and its a dam mission sometimes...:mad1:

 

[rama_v]

Thanks, it helps, but exactly how is phonon absorbed by the 2nd electron?

This is the problem with reducing BCS theory to a bunch of words. The phonon-mediated attraction depends on coupling of vibrational modes within the crystal (i.e., the phonons) with the electron (which have a wavevector, usually denoted by the letter k). The theory is mathematical, complicated and most certainly not something that is understandable at the HSC level. But, its a dotpoint, so what to do?

Simple. Just memorise the post by untouchablecuz. Write that and you will get the marks. But to actually "know/understand" the BCS theory - well, I wouldn't go there. At least not until 3rd/4th year uni.

 

[youngminii]

I thought a phonon was a quanta of vibrational energy that comes out of the lattice due to the lattice's movement. Untouchable's notes say that the phonons distort the lattice, but I thought that the moving electron distorts the lattice by attracting it, causing phonons (vibrational energy) to be released, some of which are absorbed by another electron (with opposite spin) to overcome the Coulomb forces of repulsion.
Oh and the other electron is there because of that whole positive area crap.

At least that's how I was taught..

 

[the-derivative]

Is it even necessary to discuss the phonon in the BCS theory?

I've looked at the marking criteria for CSSA and Independent Trial Exams and non of their explanations mention phonons.

I've asked my physics teacher and she said in the HSC they'll accept most reasonable explanations since it varies so much in the textbook.

 

[youngminii]

Might as well shove in the word to make it seem as if you know your stuff then
What if there's a bias marker though, and he/she wants to read stuff about photons..
God I wish our exams were marked by some super intelligent computer or something.

 

[helper]

I've asked my physics teacher and she said in the HSC they'll accept most reasonable explanations since it varies so much in the textbook.

With the HSC question a few years ago on BCS theory, the term Phonon was expected. However, it didn't need to be explained. So a statement like a phonon transfer energy between the two electrons via the crystal lattice would be fine. As long as you explain the other interactions between the lattice and electron.