the superconductivity
Superconductivity is the complete disappearance of all electrical resistance at low temperatures . We’ll relate superconductivity to the structure and energy-band model of a solid .
Although superconductivity was discovered in 1911 , it was not well understood on a theoretical basis until 1957 . In that year , the American physicists John Bardeen , Leon Cooper , and Robert Schrieffer published the theory of superconductivity , now called the BCS theory , that was to earn them the Nobel Prize in physics in 1972 . (it was Bardeen’s second Nobel Prize ; he shared his first for his work on the development of the transistor) . The key to the BCS theory is an interaction between pairs of conduction electrons , called Cooper pairs , caused by an interaction with the positive ions of the crystal . Here’s a rough qualitative picture of what happens . A free electron exerts attractive forces on nearby positive ions , pulling them slightly closer together . The resulting slight concentration of positive charge then exerts an attractive force on another free electron with momentum opposite to the first . At ordinary temperatures this electron-pair interaction is very small in comparison to energies of thermal motion , but at very low temperatures it becomes significant .
Bound together this way , the pairs of electrons cannot individually gain or lose very small amounts of energy , as they would ordinarily be able to do in a partly filled conduction band . Their pairing gives an energy gap in the allowed electron quantum levels , and at low temperatures there is not enough collision energy to jump this gap . Therefore the electrons can move freely through the crystal without any energy exchange through collisions , that is , with zero resistance .
Researchers have not yet reached a consensus on whether some modification of the BCS theory can explain the properties of the high-Tc superconductors that have been discovered since 1986 . There is evidence for pairing , but possibly of holes rather than electrons . Furthermore , the original pairing mechanism of the BCS theory seems too weak to explain the high transition temperatures and critical fields of these new superconductors .
Sources :-
1- University physics , 11th edition , young and freedman .
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