Lecture Note:PH100 ENGINEERING PHYSICS (Super Conductivity)

Super Conductivity

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon.

INTRODUCTION

Superconductors have often been called the bridge between theoretical physics and practical application. As the knowledge of superconducting materials is ever increasing, the possibility of a high temperature superconductor is nearly realized. The applications of this phenomenon are relevant in nearly every technological field and the medical implications relating to magnetic resonance imaging and brain mapping are astounding. This paper presents an overview of some AC and DC applications of superconductors (specifically high temperature). However, In order to better understand and appreciate these applications of superconductors, it is first necessary to gain an understanding of the history and basic theory of superconductors.
In a superconductor, the Cooper pairs, mentioned earlier, act as bosons. This means that the pairs can exist simultaneously in ultra-low energy states. These pairs form closely to the top of the collection of energy levels (also called the Fermi level) by interacting with the crystal lattice. The slight lattice vibrations attract these Cooper pairs, leaving an energy gap. Because the pairs are not subject to Pauli Exclusion Principle due to their combined integer spin, they can simultaneously occupy the same energy state. This attraction is termed the phonon interaction. The energy gap is then the sort of “tunnel” through which resistance free traveling can occur in the case that the thermal energy is less than the band gap.Where in normal circumstances, collisions would occur causing normal resistivity, at low temperatures, the thermal energy drops to a value less than the band gap, leaving the band gap open for tunneling. This theory of functionality for superconducting principles is known as the Bardeen-Coorper-Schrieffer Theory (or BCS theory). [7] [5] The BCS theory not only substantiates the microscopic theory of superconductivity,but also predicts a bandgap based on the critical temperature.The equation for this prediction can be seen below where Eg is the predicted bandgap and Tc is the critical temperature.