2 The Characteristic Length Scales of Superconductivity

In this chapter the definitions of the characteristic superconducting length scales ($\lambda$ and $\xi$) in the London, BCS and GL theories are introduced. Both the linear and the nonlinear response of a superconductor to a static external magnetic field are discussed. The effect of nonlocal electrodynamics on the behaviour of the length scales is also considered in connection with a d_x²-y²-wave superconductor. The chapter concludes with a brief summary of how one can measure $\lambda$ and $\xi$.