Superconducting Gap Structure and Magnetic Penetration Depth

Abstract

The BCS theory proposed by J. Bardeen, L. N. Cooper, and J. R. Schrieffer in 1957 is the first microscopic theory of superconductivity. In this theory, Fermi surface becomes gapped to avoid the instability caused even by an infinitely small interaction if it is attractive, which drives the system into the superconducting condensate state at low temperatures. The wave function that describes the electron pair, which serves as the order parameter, is related to the superconducting energy gap. Therefore, identifying the detailed superconducting gap structure is a major step toward clarifying the interactions that produce the pairing. Since the magnetic penetration depth is directly connected to the superfluid density, the measurement of the penetration depth is a powerful method to elucidate the superconducting gap structure, particularly the presence or absence of nodes. In this chapter, we will give a brief account of superconductivity and magnetic penetration depth.