mikey11 said:
Thanx rama v, i checked out csu but i was wondering if neone else had nething different coz i dont unserstand the whole concept
also this question in the cssa trial was a doozy about the bcs theory
the bcs thoery states that in superconducting materials electrons move through the lattice in cooper pairs
using a diagram as a part of ur response, discuss bcs theory by considering a material above and below critical temp.
i got 3 out of 5 for this question but i honetly had no idea and was just wondering what others wrote
The concept is quite simple. Basically, put it this way: When an electron in the valence band moves into the conduction band, it (obviously) leaves behind a 'hole', i.e. a region of empty space. Previously, this empty space was occupied by an electron (i.e. negative charge), but since there is an
absence of negative charge we can say that it is a theoretical positive charge - a
positive hole. So electrons are now free to move to this positive hole. Now you may ask why they would want to do this - good question. Electrons naturally have kinetic energy, so they do random things like jump up and down energy shells. But when a potential difference (i.e. voltage) is applied across a semiconductor (or any conductor for that matter) this random movement becomes controlled - the electron will want to move into the positive hole in an attempt to get from an area of higher potential (the negative side) to an area of lower potential (the positive side). By using the empty positive holes, the electrons are able to achieve this.
Therefore, an electron jumps into this positive hole, and this in turn leaves a region of empty space where it originated from. This creates another positive hole. The process continues, and it looks as if electrons are moving one direction and the positive holes are moving in the other direction. This represents current.
With the BCS theory, I strongly advise reading the Jacaranda Book's explanation and diagram - basically for a superconductor below critical temperature, as an electron moves through the crystal lattice, the lattice 'bends' slightly towards the electron, as the lattice is made of positive ions, while the electrons are made of negative ions. Don’t forget that when something with a mass moves, it creates vibrations. This is true with electrons also. The electron moves through the lattice, and the lattice bends slightly towards it. This has the effect of creating a dense positive charge just behind the electron. If you think about it this makes sense, because the electron would have moved slightly by the time the lattice has bent. This dense region of positive charge 'pulls' another electron towards it. Thus there are now two electrons travelling together - a cooper pair. The pair is free to move through the lattice without any obstruction from the crystal lattice.
When a superconductor is above the critical temperature however, the pairs are not able to form as the lattice is constantly vibrating and knocking back electrons, producing resistance. Thus when no pairs are forming the electrons do not have a 'free path' which they can use to move through the lattice unobstructed…
