Superconductors cannot be penetrated by magnetic flux lines (Meissner–Ochsenfeld effect). This Meissner state breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs. In Type-I superconductors, superconductivity is abruptly destroyed via a first order phase transition when the strength of the applied field rises above a critical value Hc. As such, they have only a single critical magnetic field at which the material ceases to superconduct, becoming resistive. Depending on the geometry of the sample, one may obtain an intermediate state described first by Lev Landau [1] consisting of a baroque pattern[2] of macroscopically large regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. Elementary superconductors, such as aluminium and lead are typical Type-I superconductors. The origin of their superconductivity is explained by BCS theory. This type of superconductivity is normally exhibited by pure metals, e.g. aluminium, lead or mercury.