The origin of the Meissner effect in new and old superconductors
  It is generally believed that superconducting materials are divided into two classes: `conventional' and `unconventional'. Conventional superconductors (the elements and thousands of compounds including $MgB_2$) are described by conventional London-BCS-Eliashberg electron-phonon theory. There is no general agreement as to what mechanism or mechanisms describe `unconventional' superconductors such as the heavy fermions, organics, cuprate and pnictide families. However all superconductors, whether `conventional' or `unconventional', exhibit the Meissner effect. I argue that there is a single mechanism of superconductivity for all materials, that explains the Meissner effect and differs from the conventional mechanism in several fundamental aspects: it says that superconductivity is driven by lowering of kinetic rather than potential energy of the charge carriers, it requires conduction by holes rather than electrons in the normal state, and it predicts a non-homogeneous rigid charge distribution and an electric field in the interior of superconductors. Furthermore I argue that neither the conventional mechanism nor any of the other proposed unconventional mechanisms can explain the Meissner effect. Superconductivity in materials is discussed in the light of these concepts, some experimental predictions, connections to Dirac's theory, and connections to the superfluidity of $^4He$. 
J. E. Hirsch
cond-mat.supr-con