ABSTRACT
In the superconducting phase, the single-particle excitation is radically different from that of normal metals. When proper account is taken of residual interactions conventionally neglected in the description of the normal state, the "pairing correlations" leading to superconductivity emerge in a natural manner. In superconductors a minimum energy, called the energy gap, is required to make a single-particle excitation from the ground state. The energy gap and most of the observed properties of the superconducting phase would be absent were it not for strong correlations between the pairs. Actual superconductors differ in a fundamental manner from a bound pair model in which the pairs are either well separated in space and/or weakly interacting. The bound pairs would presumably be capable of translational motion relative to the other pairs and one would obtain a continuum of Bose–Einstein excitations above the ground state without an energy gap, in contrast with the pairing theory.
