The Meissner effect, which has long been used to characterize conventional superconductors, is also readily observed in high temperature superconductors. When these compounds are cooled below a critical temperature Tc in the presence of an externally applied magnetic field, all of the magnetic flux is expelled from their interior in the absence of flux pinning. When the external field exceeds a critical value (Hc), the superconductor returns to its normal state. Superconductors which obey the magnetization curve depicted in Fig. 2.1 are called type-I superconductors. The critical field Hc (T)which separates the superconducting and normal phases is a temperature dependent quantity.