  Analyzing the exchange energy of two conduction electrons in a crystal we
find that the exchange energy may be negative and, thus, a singlet state may be
favorable. A full overlap in real space of wave functions of two conduction
electrons leads to a deeper exchange energy. Thus, the exchange interaction
causes a bond between two conduction electrons in real space. The singlet bond
is possible because the singlet electrons can simultaneously and permanently
occupy one spatial ground state, so the average energy of paired electrons is
lower than the energy of unpaired electrons. Thus, the pairing is a result of
the Pauli Exclusion Principle. If conduction electrons, before pairing, are put
on the Fermi surface in the momentum space, then every pair may exist
permanently in time. The motion of conduction electrons in the crystal may
prevent the formation of Cooper pairs, because the kinetic energy of the motion
is usually larger than the binding energy in the pair. Conduction electrons as
standing waves are local and have zero momenta, hence their momenta are
synchronous; therefore, weak singlet bonds are stable despite the large kinetic
energy on the Fermi surface. The local pairing of standing waves explains the
inverse isotope effect, Tc-dome, insulator-superconductor transitions and many
other facts about superconductors. The electron pairs, as bosons, can form a
macroscopically coherent Bose-Einstein-Condensate and, thus, become
non-dissipative and non-local.
