  As shown in former papers, the nonadiabatic Heisenberg model presents a novel
mechanism of Cooper pair formation which is not the result of an attractive
electron-electron interaction but can be described in terms of quantum
mechanical constraining forces. This mechanism operates in narrow, roughly
half-filled superconducting bands of special symmetry and is evidently
responsible for the formation of Cooper pairs in all superconductors. Here we
consider this new mechanism within an outer magnetic field. We show that in the
magnetic field the constraining forces produce Cooper pairs of non-vanishing
total momentum with the consequence that an electric current flows within the
superconductor. This current satisfies the London equations and, consequently,
leads to the Meissner effect. This theoretical result is confirmed by the
experimental observation that all superconductors, whether conventional or
unconventional, exhibit the Meissner effect.
