THE BCS THEORY OF SUPERCONDUCTIVITY:
In 1957, John Bardeen, Leon Cooper and John Schrieffer of the USA proposed a detailed quantum mechanical theory, now known as BCS Theory, to explain superconductivity. The predictions of this theory are in excellent agreement with experimental results for low temperature superconductors. Some adjustments or perhaps even a new theory are necessary in order to fully explain high temperature superconductivity.
BCS Theory suggests that superconductors have zero electrical resistance below their critical temperatures because at such temperatures the electrons pass unimpeded through the crystal lattice and therefore lose no energy. The theory states that the supercurrent in a superconductor is carried by many millions of bound electron pairs, called Cooper pairs. These pairs form when one electron passing between adjacent positive ions in the lattice attracts the ions, causing them to move slightly inwards and to create a region of increased positive charge density. Due to the elastic properties of the lattice, this region of increased positive charge density propagates through the lattice as a wave. A second electron passing through the lattice is attracted into this moving region of increased positive charge density and is effectively swept along by the lattice wave created by the first electron. Thus, by pairing off two by two, the electrons pass more smoothly through the lattice.
Cooper pairs continually form, break and re-form. Since random lattice vibrations break up Cooper pairs, the temperature needs to be low enough to keep such vibrations to a minimum.
Note that the description of BCS Theory given above has been greatly over-simplified. A more detailed description is way beyond the scope of this course.
Source:
http://webs.mn.catholic.edu.au/physics/emery/hsc_ideas_implementation.htm
